Physics Electricity --> Physics- Electricity
Circuits, voltage and energy.
Current is the amount of charge flowing per second. We measure current using an ammeter.
An ammeter is always connected in series with the components in the part of the circuit where the current is being measured. An ammeter has to have very low resistance so that it does not affect the flow of electric charge in the circuit.
The chemical energy in the battery is first converted to electrical energy in the battery. This energy is then transported around the circuit as an electric current. As the current flows through each component in a circuit (such as lamps. buzzers. motors etc...) the electrical energy can be converted to other types of energy like heat, light and sound amongst others. Electric current (or electrical energy) is not used up but is converted to other types of energy. Energy is always conserved.
Resistors actually try to resist (or slow down) the flow of electrical charge in a circuit.
A resistor can be a piece of wire, a component like a light bulb or a special resistor made for the purpose of opposing the flow of charge.
When resistors have charge flowing through them they get warm. Sometimes this is a good thing just like in the case of an electrical fire. However, it is often a bad thing because unless you want to lose energy through heating then it
means you are losing energy from your circuit.
Voltage is measured using a voltmeter. The voltmeter tells us what the voltage (potential difference) across a component in a circuit is. A voltmeter is always connected in parallel with the component being tested.
We have also said that as resistance gets higher in a circuit the current gets lower. We call this an inverse relationship.
Voltage is the driving force that pushes the electric charge around a circuit.
Usually if the voltage is increased so is the current.
This is because there is larger driving force behind the current.
The voltage is a measure of how much energy the electric charge is carrying around a circuit.
Charge is measured in Coulombs. When one Coulomb of charge passes through a component in a circuit the amount of energy it transfers to the component is called the voltage (or potential difference).
We have already seen that voltage is the amount of energy transferred by electric charges to a component in a circuit and that current is the flow of charge around a circuit. If we combine both of these ideas we can work out how quickly energy is transferred to components in a circuit.
This is called power. Power is the rate that a component converts energy from one form to another.
Power is measured in Joules per second. Another name for this is Watts. Power ratings tell you how quickly electrical energy is used and can also give you an idea of how much they cost to run.
The idea that positive charge flows around a circuit is called conventional current
Electrons carry charge in metals because there is a sea of electrons in metal which can move around freely. They are not held tightly by the nuclei of the atoms making up the metal.
These electrons are called 'free electrons' and all electrons are negatively charged. In non-metals there is no sea of electrons and therefore electrons cannot carry around charge. These means most non-metal are insulators.
The motion of the electrons around a circuit determines whether we say the current is direct or alternating.
If the electrons start out at the negative side of the circuit and move steadily around the circuit to the positive side without changing direction then we call this 'direct current'.
If the electrons continually change direction backwards and forwards because the positive and negative sides of the circuit keep changing for some reason then we call this 'alternating current'. Alternating current has a frequency. This is how many times the current charges direction every second. In the UK this is 50 Hz (50 cycles per second)
The live wire (brown) in the domestic mains supply alternates between being a high positive and negative voltage. This gives rise to mains electricity being alternating current.
The neutral (blue) wire never changes and is always at zero volts.
Electric current normally flows in and out of these two wires.
The earth (Yellow and green) wire works in conjunction with the fuse and/or a circuit breaker to provide a safety mechanism in the event that there is a fault in the circuit.
The end of the earth wire which is not connected to the plug is connected to any exposed metal parts of the appliance. If the appliance has a metal casing this is where the earth wire is attached.
If there is no metal casing then there will be no earth wire and the appliance is said to be double insulated.
If for some reason the live wire were to become detached inside the appliance and touch the metal casing of an appliance this could be potentially dangerous if someone was to touch the case.
Instead the earth wire carries this electricity down to earth. In the process of doing this a very large current is produced in the live wire.
If a fuse is fitted the fuse will get hot because of this high current and will melt. Because the fuse forms part of the circuit from the live wire the electricity will stop flowing when the fuse melts as the circuit will be broken. This then removes the danger of the case being a danger to anyone touching it.
Circuit breakers work in a similar way but they monitor the amount of current flowing in the earth wire.
If this reaches above a pre-set amount then this indicates there is a fault. The circuit breaker will switch off the mains supply.
A circuit breaker can be part of the domestic electricity installation at the distribution board or can be a separate unit that is plugged into a mains socket between the socket and the appliance being used.
We mentioned earlier that when something like a resistor opposes the flow of electrons in an electric current then the resistor gets warm or even hot.
Some times this is useful to us and is used in electric fires, showers (for heating water), electric cookers, hair dryers, water heaters, kettles etc... This heating effect also produces enough heat in a filament light bulb for it to give out light.
It is because there are positively charged protons and negatively charged electrons in an atom that we can talk about electric charge.
It is the movement of electrons that causes all electric current and electrostatic phenomena. Only the negative electrons move not the positive protons in the centre of the atoms.
Static electricity is exacts what its name suggests. Static refers to something that is not moving and that is exacts what static electricity is, electricity that does not move around.
Lightning is probably the most spectacular demonstration of a build up of static charge which is then quickly discharged as a bright spark of electric charge giving out intense light and heat. The expansion of the heated air around this huge spark gives the thunder that we hear.
When objects are rubbed together electrons are rubbed from one of the materials to the other. If the two materials are insulators the electrons cannot be conducted and instead they just build up in one place. This is a build up of electric charge.
Electrostatic charges can be useful as well as dangerous.
Positive and negative charges will attract each other whilst similar charges always repel each other.
This can be used in a number of ways. Painting objects can be made easier if the object is charged with one type of charge and the paint is charged with the opposite charge. This causes the paint to stick more evenly to the object being painted. This principle is also used in precipitators. These are devices that remove smoke particles from the gases discharged from factories and power stations. The particles pass through a charged wire mesh causing then to be charged and they are then attracted to another object such as a metal plate which carries the opposite charge further up the chimney stack. In this way less smoke particles are discharged into the atmosphere. Photocopiers and laser printers also use electrostatic charge to produce the image being created on a special printing drum.
However in the wrong place electrostatic charge can be a serious hazard. When aeroplanes are fuelled the movement of the liquid fuel in the pipeline from the fuelling truck to the aeroplane causes a build-up of static charge. If this charge was to become discharged between the truck and aeroplane the resulting spark may cause an explosion of the fuel. Lightning is also dangerous itself. It can cause damage to buildings as well as fires and can kill if it strikes a person.
Charge is measured in Coulombs. Current is measured in Amperes (Amps). Time is measured in seconds.
The amount of charge that passes through a circuit is determined by how much current flows in the circuit and the amount of time that the current is flowing for.
In metals electric current is the flow of free electrons. In general, only metals have these free electrons that can move around and this makes most metals good conductors of electric current.
In liquids (during electrolysis) electric current is the flow of both positive and negative ions. Ions are charged particles and in liquids both positive and negative ions can move. Positive ions are attracted to the negative electrode and negative electrons attracted to the positive electrode. The positive electrode is called the anode and the negative electrode is the cathode.
When an electric current flows through a wire it produces a magnetic field. If the wire is then placed near to a magnet which is also producing a magnetic field then the two fields will interact.
There will be forces acting on both the wire and the magnet. Whether the wire and magnet attract or repel each other depends on which magnetic pole generated by the wire is nearest to the magnet. This is called the motor effect.
When a conductor like a metal wire moves through a magnetic field an electric current is made to flow (induced) in the conductor. This only happens when the conductor is moving in the magnetic field and is cutting across the lines of magnetic force. If the conductor is moving along the lines of force or is not moving then no current is induced in the conductor.
An a.c. generator works by causing a conductor to be moved through and cut the lines of force in a magnetic field. Either a coil can be rotated in a magnetic field or a magnet can be rotated in the region of a coil. As the magnet or coil rotates an electric current is induced in the coil first traveling in one direction and then in the other. This alternating of the current flow continues as long as the generator is operating.
A transformer is used to step-up or step-down the voltage in a circuit. Transformers work using the principle that if an electric current is passed through a coil it will produce a magnetic field. If a second coil is then placed near to the first coil the magnetic field from the first coil will induce a current in the second coil. For this to work the magnetic field has to be constantly changing (as with the a.c. generator) otherwise no current is induced in the second coil. Therefore transformers only work with a.c. current.
There is a very simple relationship between the number of turns of wire on the coil in a transformer and the voltage that is induced in the coils. The coil on the input side is called the primary coil. The coil on the output side is called the secondary coil.
If there are twice as many turns of wire on the secondary coil as on the primary coil then the output voltage of the secondary coil will be twice that of the input voltage. This would be a step-up transformer. In short the turns ratio equals the voltage ratio in a transformer.
Mains electricity is generated in power station which are spread out all over the country. In each power station there is a source of energy which is eventually converted to electrical energy. Most power stations use fossil fuels (coal. oil and gas) as their fuels but some use nuclear energy and others use moving water as a source of energy. NOTE: Not much detail here on generators, mtors etc so use other guides too!
Chemistry-Metals --> Metals
Atoms consist of a nucleus surrounded by a cloud of electrons orbitting around it. However, the nucleus itself is composed of two particles, neutrons and protons. All particles are light so scientists measure their masses in atomic mass unit instead of grams.
Protons and electrons carry a charge; neutrons do not.
The different energy levels for the electrons are called energy shells. Each shell can hold a limited number of electrons.
Particle in atom: Mass: Charge:
proton 1 unit positive(+1)
neutron 1 unit neutral
electron negligible negative (-1)
Each element is given a symbol, a sort of shorthand version of its name, for example Sodium has the symbol Na. Next to the symbol are two numbers, the top number (the larger of the two), is known as the mass number, the lower number (smallest of the two) is the proton number.
The periodic table is arranged in order of proton number, a hydrogen atom having the smallest.
The horizontal rows of the periodic table are periods. You read across the rows from left to right for increasing atomic number. The vertical columns are groups. Groups are numbered in Roman numerals from 1 to 0 (8) left to right. This is because the 8th column represents elements which have a full outer shell of eight electrons and no electrons in a shell that is incomplete hence 0 or 8.
Elements in the same group react in a similar way. This is because how any element reacts is governed by the number of outer shell electrons. As all elements in a group have the same number of outer shell electrons they all react in a similar way.
In groups 1-3 the elements get more reactive as they go further down the group. This is because these metals form compounds by losing electrons. Further down the group the electrons that can be lost are in shells which are further from the nucleus of the atom and are more easily lost. This makes elements at the bottom of the group more reactive.
In groups 5-7 the elements are more reactive towards the top of the group. This is because these elements form compounds by gaining electrons. The shells which have space for an electron are closer to the nucleus at the top of the group rather than the bottom and this allows the nucleus to have a greater effect on electrons which may possibly react with the element and form a compound.
The noble gases are very important. They have full outer shells and therefore have no interest in reacting with anything. They are very unreactive. As such they are useful for mixing with oxygen for divers, for preventing the filament of a light bulb from just burning out in a second or so (argon) and for neon lighting.
The Alkali Metals form Group 1 of The Periodic Table, and called so because they form oxides and hydroxides that dissolve in water to give alkaline solutions.
• Typical metallic properties: good conductors of heat and electricity, high boiling points, silvery grey surface (but rapidly tarnished by air oxidation).
• When an alkali metal atom reacts, it loses an electron (oxidation) to form a singly positively charged ion eg Na ==> Na+ + e-. In terms of electrons 2.8.1 ==> 2.8 and so forming a stable ion with a noble gas electron arrangement.
• They tend to react mainly with non-metals to form ionic compounds which are usually soluble white solids.
• Untypical metallic properties: low melting points, low density (first three float on water), very soft (easily squashed, extremely malleable) and so they have little material strength.
• Important trends down the group with increase in atomic number ...
• the melting point and boiling point generally decrease* (see data table below)
• the element gets more reactive
• the atoms get bigger (as more electron shells are added
• generally the density increases (although the atom gets bigger, their is greater proportional increase in the atomic mass.
• generally the hardness decreases
They possess all the normal physical properties of metals and are very soft. They all have a single outer shell electron which they are desperate to lose and this makes them very reactive. They react with water. Their reaction with water produces hydroxide and hydrogen. They become more reactive as you go down the group.
2Na(s) + 2H2O(l) > NaOH(aq) + H2(g)
They react with oxygen to form oxide.
4Na(s) +O2(g) > 2Na2O(s)
Metal oxides are basic (alkaline), they react with acids to form a salt and water.
Na2O(s) + 2HCl(aq) > 2NaCl (aq) + H2O(l)
Electrolysis of NaCl gives sodium and chlorine. The electrolysis of NaCl in solution gives hydrogen, chlorine and sodium hydroxide. NaCl (salt) is used in cooking.
NaOH (Sodium Hydroxide) is a strong alkali. It can be used to neutralise acids to form salt and water (see above) and is used in the production of soap, detergents, bleach etc....
The halogens have 7 electrons in the outer shell and are quite reactive, more so higher up in the group for reasons explained in a previous section.
At room temperature Fluorine (F) and Chlorine (Cl) are gases, Bromine (Br) is a liquid and Iodine (I) a grey solid. In displacement reactions the more reactive halogens displace the less reactive ones, for example:
chlorine + potassium bromide > potassium chloride + bromine
Note the change of name, 'ine' becomes 'ide'.
Fluorine is used in toothpaste, Chlorine in bleach, antiseptic, water purification, pesticides, Bromine in photography and medicines and Iodine in antiseptic.
Sodium Chloride is the most commonly know compound involving a halogen and an alkali metal.
The transition metals are a bit odd because they do not fall into a group. They don't quite follow the rules and quite frankly they can be confusing. You only need to concentrate on a few important ones and not worry about the properties of the others too much.
Transition metals are in a block between groups 2 and 3. Although they are not in a group they do all have similar properties. They transition metals include the well known metals, iron, copper, zinc, gold, and silver.
They are generally hard, dense and shiny. Three of the transition metals are magnetic. The transition metals are less reactive than metals in groups 1 and 2 and are often coloured.
The transition metals can form compounds that have more than one formula. CuO and Cu2O.
Uses of the transition metals include iron and steel, copper pipes and wires, jewellery, catalytic converters (platinum) and other catalysts in industry. Transition metals and their compounds all make good catalysts.
Look at the magnesium atom diagram again. It has 12 protons. This fact is used to identify an atom since it is specific to them.
For example: only a magnesium atom has 12 protons, only a sodium atom has 11 protons!
The magnesium atom has 12 electrons. The number of electrons for an atom always equals the number of protons. This means that their opposite and equal charge cancel one another out. Atoms are neutral!
The electrons in an atom have negligible mass. So the mass number only takes into account the number of protons and neutrons.
The mass no. = the no. of protons + the no. of neutrons.
Since we already know the number of protons, as given by the proton number, we can calculate the number of neutrons.
For a magnesium atom:
Mass number = 24
Proton number =12
24 = 12 + the number of neutrons
Therefore, number of neutrons in a magnesium atom = 24 - 12 = 12 neutrons.
The atoms of an element are not always the same. Although they may contain the same number of protons their neutron numbers may differ from atom to atom.
.The formation of compounds
Most elements form compounds.
For example: A reaction between sodium and chlorine gives the compound sodium chloride (salt) quite readily.
The noble gases do not usually form compounds. They are different from other elements, since their atoms are described as stable or unreactive. They are stable because their outer electron shell is full. A full outer shell makes an atom more stable.
Only the noble gases have full outer shells. This is why they are stable.
Other elements react with each other in order to obtain full outer shells, this makes them more stable.
How atoms lose and gain electrons
Depending on their electronic configurations, atoms lose or gain electrons in order to achieve a full outer shell.
The sodium atom has one electron in its outer shell. If it loses this one electron it will achieve a full outer shell. By losing the one electron to another atom, it becomes a sodium ion.
The sodium ion still has 11 protons but by losing one electron it has only 10 electrons compared to the atom. Hence, its overall charge is +1.
This +1 charge is due to the ion having one more proton than electron.
In naming ions, you take the symbol Na and assign a positive charge. This gives us the sodium ion Na+.
A chlorine atom has seven electrons in its outer shell. It can reach a full outer shell by gaining one electron. It will then become the chloride ion, Cl-.
A negative charge is assigned to the ion to signify that the ion contains one more electron than proton.
Any atom can become an ion if it gains or loses electrons.
An ion is a charged particle. It is charged due to an unequal number of electrons and protons.
Example 1: Reaction between sodium and chlorine
A sodium atom loses one electron to achieve a full outer shell and chlorine gains one electron to complete a full outer shell. So when a sodium atom reacts with a chlorine atom, the sodium atom loses its one electron to chlorine. The two ions formed are a sodium ion, Na+ and a chlorine ion Cl-.
The two ions have opposite charges, they attract one another. The force of attraction between them is an electrostatic one. This type of attraction is strong. It is called an ionic or electrovalent bond
Example 2: Reaction between magnesium and oxygen
Other metals and non-metals react together to form ionic compounds This is because metals tend to lose electrons, whereas non-metals tend to gain electrons.
A magnesium atom has two electrons in its outer shell, whereas oxygen has six electrons. This means that magnesium wants to lose two (to oxygen) and oxygen wants to gain two (from magnesium) so that they can have full outer shells
Atoms are the smallest basic unit of an element. When these atoms combine new substances are formed that may or may not posses any of the properties of the elements that make them up. These new substances are called compounds.
A new substance is formed when atoms combine. This combining of atoms is formed by a chemical bond. The chemical bond is the result of atoms sharing or transferring electrons. Atoms react in order to achieve a full outer shell. The first shell of an atom has 2 electrons when full, and the next 2 shells both have 8 electrons when full. The way in which atoms react with each other depends on the number of outer shell electrons.
Atoms which have 1, 2 or 3 outer shell electrons are all metal (apart from hydrogen). These all lose electrons when bonding and form positive ions. Metals do not share electrons.
Atoms that have 4, 5, 6 or 7 outer electrons gain or share electrons to fill their shells. Hydrogen can lose or share electrons.
Atoms with full outer shells (i.e. 8 electrons) do not react. These are the noble gases.
Ionic bonding occurs when electrons are transferred from one atom to another. The atoms become ions. Atoms which lose electrons become positive ions. Atoms that gain electrons become negative ions. The ions are strongly attracted to each other because of their opposite charges. They form a chemical bond due to this, i.e. ionic bonding.
Only non-metals form covalent bonds. The atoms involved share some of each others outer shell electrons. For example, atoms in elements such as oxygen, chlorine and hydrogen for covalent molecules O2, Cl2, H2. Compounds for example are CO2 ,CH4 ,HCl ,H2O.
The molecules above are examples of simple covalent bonding. Covalent bonds are stronger than ionic bonds. Simple covalent molecules have strong intramolecular forces but weak intermolecular forces. This means they have low boiling and melting points.
Carbon forms giant covalent molecules, diamond, graphite and Buckminster Fullerene. All these have high boiling and melting points because of strong intramolecular forces. These make diamond hard. The weak intermolecular forces between the layers of graphite make it soft and slippery.
When two non-metals react together, they both need to gain electrons to complete full outer shells. The only way this can be achieved is if they share their outer electrons.
Hydrogen: Each hydrogen atom has only one electron and needs one more to complete its first shell. When two hydrogen atoms get close together their shells can overlap and then they can share their electron.
Since, electrons are shared, there is a strong force of attraction between them. This force is a covalent bond.
The bonded atoms form molecules. Hydrogen's molecular formula is H2.
Oxygen: Each oxygen atom requires a share of two electrons.
There are a vast number of compounds that exist as molecules.
Water: In each molecule, H2O, one oxygen atom shares electrons with two hydrogen atoms
The four types:
4. Giant molecular
In a metal, the atoms are very tightly packed, leaving little space between them. Due to this tight packing, the outer electrons become delocalised from their atoms. This results in a 'sea' of electrons around a lattice of ions or 'pseudo' cations.
Ionic solids are made up of a lattice composed of oppositely charged ions. One of the most common ionic solids is sodium chloride. Sodium chloride is made up of sodium and chloride ions packed in a regular pattern - a lattice. The ions are held by electrostatic charges in an ionic bond. Properties of ionic solids
1. High melting points and boiling points due to strong ionic bonds. Most are solids at room temp.
2. They are brittle - will shatter with a hammer.
3. Usually soluble in water. Insoluble in non-polar solvents.
4. Do not conduct electricity in solid state. They do conduct when molten or dissolved in water since the ions are free to carry the charges as the ionic bonds do not hold them firmly in the liquid state.
In a molecular solid, the molecules are held together by weak Van Der Waal's force, but packed in a regular pattern. Iodine is an example of a molecular solid. Each iodine molecule is made up of 2 iodine atoms, held together by a strong covalent bond. Each iodine molecule is held to another by weak Van Der Waal's forces.
Acids and Alkalis
Properties of acids
1. They are liquids
2. They are solutions of compounds in water.
3. If concentrated they can be corrosive.
4. Acids taste sour (for example, vinegar).
5. Turn blue litmus paper red - this is an easy test for an acid!
6. Usually react with metals to form salts.
7. Acids contain hydrogen ions.
8. Turn Universal Indicator from green to red, and have a pH less than 7.
Examples of acids: are vinegar (ethanoic acid) and lemon juice (citric acid)
Properties of alkalis
2. They are soluble bases.
3. Like acids, they can burn the skin.
4. They turn red litmus blue
5. Alkalis contain hydroxide ions (OH-).
6. They taste bitter.
7. Turns Universal Indicator from green to blue or purple.
Dissociate- the cations (+) and anions (-) become fully ionized, and are free to move and conduct electricity.
Acids are non-electrolytes when pure but become electrolytes when dissolved in water. Aqueous solutions of acids undergo the following:
They react with metal oxides to produce salts and water
They react with the more reactive metals to produce salts and hydrogen
They react with carbonates to produce salts, carbon dioxide and water
Compounds which neutralise acids are called bases. Not all bases are soluble in water. A base which is soluble in water is called an alkali. Aqueous solutions of alkalis are electrolytes. They change the colour of indicators.
An acid dissociates to give hydrogen ions- H+. Bases dissociate to in water to produce hydroxide ions, OH-. A theory says that acid is a substance that donates protons, and a base is a substance that accepts protons.
When an acid reacts with an alkali it produces a salt and water.
How to extract iron from its ore
Three substances are needed to enable to extraction of iron from its ore. The combined mixture is called the charge:
Iron ore, haematite - often contains sand with iron oxide, Fe2O3.
Limestone (calcium carbonate).
Coke - mainly carbon.
The charge is placed a giant chimney called a blast furnace. The blast furnace is around 30 metres high and lined with fireproof bricks. Hot air is blasted through the bottom. Several reactions take place before the iron is finally produced.
Oxygen in the air reacts with coke to give carbon dioxide:
The limestone breaks down to form carbon dioxide.
Carbon dioxide produced in 1 + 2 react with more coke to produce carbon monoxide:
The carbon monoxide reduces the iron in the ore to give molten iron:
The limestone from 2, reacts with the sand to form slag (calcium silicate):
Both the slag and iron are drained from the bottom of the furnace.
The slag is mainly used to build roads.
The iron whilst molten is poured into moulds and left to solidify - this is called cast iron and is used to make railings and storage tanks.
The rest of the iron is used to make steel.
The solid mixture of haematite ore, coke and limestone is continuously fed into the top of the blast furnace.
The coke is ignited at the base and hot air blown in to burn the coke (carbon) to form carbon dioxide in an oxidation reaction (C gains O).
The heat energy is needed from this very exothermic reaction to raise the temperature of the blast furnace to over 1000oC to effect the ore reduction. The furnace contents must be he
carbon + oxygen ==> carbon dioxide
C(s) + O2(g) ==> CO2(g)
at high temperature the carbon dioxide formed, reacts with more coke (carbon) to form carbon monoxide
carbon dioxide + carbon ==> carbon monoxide
CO2(g) + C(s) ==> 2CO(g)
(note: CO2 reduced by O loss, C is oxidised by O gain)
The carbon monoxide is the molecule that actually removes the oxygen from the iron oxide ore. This a reduction reaction (Fe2O3 loses its O, or Fe3+ gains three electrons to form Fe) and the CO is known as the reducing agent (the O remover and gets oxidised in the process).
This frees the iron, which is molten at the high blast furnace temperature, and trickles down to the base of the blast furnace. The main reduction reaction is ...
iron(III) oxide + carbon monoxide ==> iron + carbon dioxide
(Fe2O3(s) + 3CO(g) ==> 2Fe(l) + 3CO2(g))
note, as in the two reactions above, oxidation and reduction always go together!
Other ore reduction reactions are ...
Fe2O3(s) + 3C(g) ==> 2Fe(l) + 3CO(g)
2Fe2O3(s) + 3C(g) ==> 4Fe(l) + 3CO2(g)
The original ore contains acidic mineral impurities such as silica (SiO2, silicon dioxide). These react with the calcium carbonate (limestone) to form a molten slag of e.g. calcium silicate.
calcium carbonate + silica ==> calcium silicate + carbon dioxide
CaCO3 + SiO2 ==> CaSiO3 + CO2
this is sometimes shown in two stages:
CaCO3 ==> CaO + CO2
CaO + SiO2 ==> CaSiO3
The molten slag forms a layer above the more dense molten iron and they can be both separately, and regularly, drained away. The iron is cooled and cast into pig iron ingots OR transferred directly to a steel producing furnace.
Iron from a blast furnace is ok for very hard cast iron objects BUT is too brittle for many applications due to too high a carbon content from the coke. So it is converted into steel alloys for a wide range of uses.
The waste slag is used for road construction or filling in quarries which can then be landscaped.
• Iron Ore eg haematite ore [iron(III) oxide, Fe2O3]
• coke (carbon, C)
• hot air (for the O2 in it)
• limestone (calcium carbonate, CaCO3)
The electrolysis of bauxite
How to extract aluminium from its ore
The bauxite (red-brown solid) - aluminium oxide mixed with impurities - is extracted from the earth.
The extracted aluminium oxide is then treated with alkali, to remove the impurities. This results in a white solid called aluminium oxide or alumina.
The alumina is then transported to huge tanks. The tanks are lined with graphite, this acts as the cathode. Also blocks of graphite hang in the middle of the tank, and acts as anodes.
The alumina is then dissolved in molten cryolite - this lowers the melting point - saves money!
Electricity is passed and electrolysis begins. Electrolysis is the decomposition of a compound using electricity.
When dissolved, the aluminium ions and oxide ions in the alumina can move.
At the cathode:
Here the aluminium ions receive electrons to become atoms again:
At the anode:
The oxide ions lose electrons to become oxygen molecules, O2:
Uses of aluminium:
1. Shiny metal - used as jewellery.
2. Low density - used to make aeroplanes and trains.
3. Non-toxic - used in drink cans.
The corrosion of iron
When a metal is attacked by water, air or acids in their environment, they corrode. Corrosion results in the metal become weaker and brittle. The corrosion of iron and steel is specifically called rusting due to the red-brown substance called rust that forms in the presence of water and oxygen.
How to prevent rusting
1. Paint or grease
This prevents water or oxygen reaching the iron. However, this is only a temporary step since paint can flake off and grease can be rubbed off quite easily. Bikes are often painted of greased to prevent rust, since this is the cheapest method of prevention.
Plastic is cheap and acts as a cover for the iron, for instance, it stops water or oxygen reaching the metal surface. Garden chairs are often made from iron coated in plastic.
This involves the iron been covered, usually in the form of a paint, by zinc. Since zinc is more reactive than iron, air and water react with the zinc rather than the iron. Outside structures, such as bridges are often galvanised.
4. Chromium plating
Works for the same reason as galvanising. Chromium is a more reactive metal than iron. Car bumpers are often chrome-plated.
Both chromium plating and galvanising are examples of sacrificial protection. Zinc and chromium are sacrificed for the iron.
When a reactant breaks down to give two or more products, we call this type of reaction decomposition.
Decomposition caused by heat is called thermal decomposition.
Decomposition can also be caused by light.
The reverse to decomposition - combination involves often two reactants reacting to form just one product.
When acids react with bases, they neutralise each other the products of a neutralisation reaction are neither acids nor bases.
The products of neutralisation are a salt and water
This reaction involves the decomposition of a compound by electricity.
Natural organisms, such as yeast can cause decomposition to occur. Yeast breaks down glucose, a sugar, into alcohol.
This reaction is important to the yeast cells since it produces the energy they require to multiply. This reaction is used in the making of beer and wines.
This reaction is also used in breadmaking
When a reaction involving two solutions produces an insoluble product. The product appears as a precipitate. This reaction is known as precipitation.
In this reaction it is the barium sulphate that appears as the precipitate.
This reaction involves the reaction of a substance with oxygen in the air. Sometimes the word burning is used instead of combustion.
The substance that reacts with oxygen is said to be oxidised. The result is a product called an oxide.
This is an example of an exothermic reaction, one that gives out heat energy.
Oxidation and reduction:
If a substance loses oxygen during a reaction it is reduced.
If a substance gains oxygen during a reaction it is oxidised.
Reduction and oxidation always take place at the same time.
Remember, when a reaction takes place bonds break (endothermic) then bonds are made (exothermic).
Overall, the reaction will be exothermic if more energy is released into the surroundings than was absorbed. An endothermic reaction will occur overall if, more energy is absorbed from the surroundings than is released.
Bond energy: This is the energy required to break one mole of bonds. The bond energy is also the energy given out when a mole of bonds is formed.
Activation energy: This is the minimum amount of energy required to break bonds to start the reaction off.
The diagram above shows how the oppositely charged ions are attracted to oppositely charged electrodes.
Cations (positive ions - metal ions and hydrogen) travel to the negative electrode, the cathode.
Anions (negative ions - non-metal ions) travel to the positive electrode, the anode.
Cations are positive so the go to the negative electrode, the cathode.
Anions are negative so go to the positive electrode, the anode.
The electrolysis of other compounds
Summary of electrolysis:
1. All ionic compounds when molten can be decomposed when electricity is passed through using electrolysis.
2. The metal and hydrogen always forms at the cathode.
3. Non-metal always forms at the anode.
4. Cations travel to the cathode.
5. Anions travel to the anode.
6. The electrodes are made from inert material such as graphite, so that they do not involve themselves with the reaction.
7. The molten substance been electrolysed is called the electrolyte.
The electrolysis of solutions
When a salt is dissolved in water, its ions become mobile.
Hence, the solution can be electrolysed. However, the products from the salt solution will be different to the molten solution because of the presence of the water, which itself produces ions.
During electrolysis, these ions compete with the metal and non-metal ions from the dissolved salts, to receive or give up electrons. At the cathode:
The more reactive a metal is the more it prefers being ions.
Therefore, if a reactive metal such as zinc or magnesium is present it will remain as the ions. The H+ ions will accept the electrons and hydrogen gas will be given off at the cathode.
If a less reactive metal, such as copper or silver is present it would rather accept the electrons than H+.
Hence, the metal forms at the cathode.
At the anode:
If halide ions are present, Cl-, Br-, I-, they will give up there electrons to become molecules of Cl2, Br2 and I2 respectively.
If no halogen is present, OH- will give up electrons more readily than other non-metal ions, and oxygen forms.