- Messages
- 671
- Reaction score
- 457
- Points
- 73
Equations, formulae and moles
• If asked for a word equation, do not write a symbol equation. A word equation tests
knowledge of chemical names. Although a correct symbol equation is often accepted this is
not guaranteed and if you make an error, you won’t get the mark
• A common error is to think that a nitrate ion has a 2- charge. The formula for the nitrate ion is
NO3-
.This makes the formula for nitric acid HNO3
[(1.9), (3.4(a)(i))]
• The charge on a silver ion is 1+. A common mistake is to think that silver has a 2+ charge
• When working out formulae, don’t be confused by oxidation numbers. A common mistake is
to think that the formula for lead(IV) oxide is PbO4
or that lead(II) nitrate is Pb2(NO3).
In a formula you have to balance the positive and negative charges. Lead(IV) = 4+, lead(II) = 2+,
oxide = 2- and nitrate = 1-. So lead(IV) oxide is PbO2, and lead(II) nitrate is Pb(NO3) 2 (1.9)
• If asked to name a salt formed in a particular reaction, don’t put down any other product or
you will lose a mark (2.1(f)(ii))
• When explaining redox reactions, make sure you understand exactly what is being asked,
especially if the question says ‘use the equation…’. Don’t just give a definition of redox in
terms of electron loss or gain. If a question says ‘use the equation to explain why the iron
oxide is reduced’, you must refer to the species in the equation in your answer, e.g. ‘the iron
oxide loses its oxygen’. ‘Iron oxide gains electrons’ is incorrect (2.6(b)), (3.1(b)(iii))
• When calculating moles, if you are given an equation such as:
Mg + 2CH3CO2H → (CH3CO2)2Mg + H2
• ignore the 2 in the equation when calculating the molar mass of ethanoic acid. The molar
mass of ethanoic acid is 60, not 120. However, remember when calculating reacting masses
that the 2 needs to be taken into account
• Experimental methods • If you are distilling an aqueous solution of a salt, the salt itself does not evaporate as it has
too high a boiling point. Only the water evaporates (1.30)
Organic chemistry
• Examiners are often very particular. One way to please them is to use the word ‘only’ in the
definition of a hydrocarbon i.e. the answer ‘a compound containing only carbon and
hydrogen’. (2.4(d)(i))
• Only one compound is formed in the reaction of ethene with steam. Remember, this is a
simple addition reaction (one compound formed from two or more substances) – a common
error is to say that hydrogen is also formed (1.40)(2.2(b)(v))
• When trying to identify ‘cracking’ reactions from a set a reactions given, look out for one
molecule of reactant forming two or more molecules of product. Remember that cracking
does not involve oxygen
• ‘Clear’ does not mean ‘colourless’; when bromine is added to an alkene the colour change is
red-brown to colourless, not red-brown to clear (3.7(c)(ii))
Periodic table and metals
• Don’t confuse the properties of elements with those of their compounds (especially when they
appear in the same question). For example, if asked about the properties of the element
oxygen, don’t give the properties of an oxide (1.5)
• The properties of transition elements often cause problems. Remember that transition
elements themselves are NOT coloured, it is their compounds that are coloured
• When trying to distinguish between a transition metal and a non-transition metal, information
on boiling points is more important than information on density. Some non-transition elements
(such as lead) are very dense (1.25)
• If asked about the specific properties of transition metals, don’t list general properties of
metals, such as ‘shiny’, ‘malleable’, etc.
• In questions about sacrificial protection, remember that the more reactive metal of the pair
will corrode. To answer this sort of question, know the order of common metals in the
reactivity series (1.32)
• ‘Corrosive’ and ‘corrosion’ are often confused. ‘Corrosive’ means that a chemical ‘eats away’
another substance – acids and alkalis are corrosive. ‘Corrosion’ is the process of ‘eating
away’. A statement such as ‘iron is corrosive’ is therefore incorrect (2.6(d)(v))
• The source of an element is where it is found (i.e. a particular place or in a particular
substance) – a source of sulphur is the southern USA, or petrol. It does not mean the process
of extraction. Don’t write vague statements such as ‘underground’ (3.2(a)(i))
• Sulphur dioxide is not used ‘to make wood pulp’, it is used to bleach wood pulp (3.2(a)(ii)) Qualitative analysis
• Look out for phrases such as ‘chemical test’ or ‘physical test’ – don’t just focus on the word
‘test’. For example, a chemical test for water could be ‘turns anhydrous copper sulphate blue’
(the word ‘anhydrous’ is essential). A physical test for water could be ‘a boiling point of
100oC’, using the correct units (2.2(c)(i)
• When testing hydrogen chloride gas with litmus paper, many students think that the litmus
paper is bleached first and then goes red. Remember that chlorine does this, not hydrogen
chloride (2.1(f)(ii)
• The tests for ammonium and nitrate ions are commonly confused. Both require heating with
sodium hydroxide, but to test for nitrate you need to add aluminium, as you need to remove
the oxygen (reduce the nitrate) to make the ammonia. You don’t need to do this for the
ammonium ion as it has no oxygen (3.6(b)(i))
• Tests for aluminium ions and zinc ions are also often confused. Remember PANDA
(precipitate of aluminium (hydroxide) does not dissolve in ammonia). Both zinc and aluminium
ions form a white precipitate with sodium hydroxide, which re-dissolves in excess, but in
ammonia only the zinc precipitate re-dissolves (1.19)
• Questions involving the height of precipitates when sodium hydroxide is added to a solution
of metal ions often cause problems. Remember, as you add more hydroxide to a solution of
suitable metal ions (e.g. iron(II) ions) there will be more precipitate until all the metal ions are
used up. However, with excess sodium hydroxide, some hydroxides re-dissolve e.g.
aluminium hydroxide. In these cases the height of the precipitate will then decrease as you
add more hydroxide (3.4(b)(iii))
Rates of reaction and equilibrium
• Many students have difficulty explaining what is meant by rate of reaction. Remember two
points: it is the change in volume or mass etc over a fixed period of time. Time is often
omitted (2.3(a))
• Remember that the total volume of gas released by the same amount of metal is always the
same. A common error is to think that powdered metal, when reacted with acid, gives off
more gas than larger lumps of the same amount of metal (1.17)
• The total volume of gas released by a catalysed reaction is exactly the same as for an
uncatalysed reaction. The same amount of reactants is the important factor (3.3(c)(iii))
• In rate questions, when asked to analyse graphs of volume of gas against time for the
reaction of an acid with a metal or carbonate, a common error is to state the volume is
increasing and not mention the rate. Remember that the rate is getting less and less with time
because rate is the difference in volume divided by time (3.3(c)(i))
• A common mistake is to say that in an equilibrium reaction, a catalyst increases the rate of
the forward reaction more than the back reaction. One of the characteristics of equilibrium is
that the backward and forward reactions go at the same speed. This applies to catalysed as
well as uncatalysed reactions (3.2(a)(v))
Solubility
• A common error is to think that calcium hydroxide is insoluble in water. Remember that
limewater is a solution of calcium hydroxide, so it must at least be slightly soluble (1.20)
• If you are asked to explain what the symbol (aq) means, write down more than ‘aqueous’. An
answer such as ‘dissolved in water’ is needed (3.3(a))
• If asked for a word equation, do not write a symbol equation. A word equation tests
knowledge of chemical names. Although a correct symbol equation is often accepted this is
not guaranteed and if you make an error, you won’t get the mark
• A common error is to think that a nitrate ion has a 2- charge. The formula for the nitrate ion is
NO3-
.This makes the formula for nitric acid HNO3
[(1.9), (3.4(a)(i))]
• The charge on a silver ion is 1+. A common mistake is to think that silver has a 2+ charge
• When working out formulae, don’t be confused by oxidation numbers. A common mistake is
to think that the formula for lead(IV) oxide is PbO4
or that lead(II) nitrate is Pb2(NO3).
In a formula you have to balance the positive and negative charges. Lead(IV) = 4+, lead(II) = 2+,
oxide = 2- and nitrate = 1-. So lead(IV) oxide is PbO2, and lead(II) nitrate is Pb(NO3) 2 (1.9)
• If asked to name a salt formed in a particular reaction, don’t put down any other product or
you will lose a mark (2.1(f)(ii))
• When explaining redox reactions, make sure you understand exactly what is being asked,
especially if the question says ‘use the equation…’. Don’t just give a definition of redox in
terms of electron loss or gain. If a question says ‘use the equation to explain why the iron
oxide is reduced’, you must refer to the species in the equation in your answer, e.g. ‘the iron
oxide loses its oxygen’. ‘Iron oxide gains electrons’ is incorrect (2.6(b)), (3.1(b)(iii))
• When calculating moles, if you are given an equation such as:
Mg + 2CH3CO2H → (CH3CO2)2Mg + H2
• ignore the 2 in the equation when calculating the molar mass of ethanoic acid. The molar
mass of ethanoic acid is 60, not 120. However, remember when calculating reacting masses
that the 2 needs to be taken into account
• Experimental methods • If you are distilling an aqueous solution of a salt, the salt itself does not evaporate as it has
too high a boiling point. Only the water evaporates (1.30)
Organic chemistry
• Examiners are often very particular. One way to please them is to use the word ‘only’ in the
definition of a hydrocarbon i.e. the answer ‘a compound containing only carbon and
hydrogen’. (2.4(d)(i))
• Only one compound is formed in the reaction of ethene with steam. Remember, this is a
simple addition reaction (one compound formed from two or more substances) – a common
error is to say that hydrogen is also formed (1.40)(2.2(b)(v))
• When trying to identify ‘cracking’ reactions from a set a reactions given, look out for one
molecule of reactant forming two or more molecules of product. Remember that cracking
does not involve oxygen
• ‘Clear’ does not mean ‘colourless’; when bromine is added to an alkene the colour change is
red-brown to colourless, not red-brown to clear (3.7(c)(ii))
Periodic table and metals
• Don’t confuse the properties of elements with those of their compounds (especially when they
appear in the same question). For example, if asked about the properties of the element
oxygen, don’t give the properties of an oxide (1.5)
• The properties of transition elements often cause problems. Remember that transition
elements themselves are NOT coloured, it is their compounds that are coloured
• When trying to distinguish between a transition metal and a non-transition metal, information
on boiling points is more important than information on density. Some non-transition elements
(such as lead) are very dense (1.25)
• If asked about the specific properties of transition metals, don’t list general properties of
metals, such as ‘shiny’, ‘malleable’, etc.
• In questions about sacrificial protection, remember that the more reactive metal of the pair
will corrode. To answer this sort of question, know the order of common metals in the
reactivity series (1.32)
• ‘Corrosive’ and ‘corrosion’ are often confused. ‘Corrosive’ means that a chemical ‘eats away’
another substance – acids and alkalis are corrosive. ‘Corrosion’ is the process of ‘eating
away’. A statement such as ‘iron is corrosive’ is therefore incorrect (2.6(d)(v))
• The source of an element is where it is found (i.e. a particular place or in a particular
substance) – a source of sulphur is the southern USA, or petrol. It does not mean the process
of extraction. Don’t write vague statements such as ‘underground’ (3.2(a)(i))
• Sulphur dioxide is not used ‘to make wood pulp’, it is used to bleach wood pulp (3.2(a)(ii)) Qualitative analysis
• Look out for phrases such as ‘chemical test’ or ‘physical test’ – don’t just focus on the word
‘test’. For example, a chemical test for water could be ‘turns anhydrous copper sulphate blue’
(the word ‘anhydrous’ is essential). A physical test for water could be ‘a boiling point of
100oC’, using the correct units (2.2(c)(i)
• When testing hydrogen chloride gas with litmus paper, many students think that the litmus
paper is bleached first and then goes red. Remember that chlorine does this, not hydrogen
chloride (2.1(f)(ii)
• The tests for ammonium and nitrate ions are commonly confused. Both require heating with
sodium hydroxide, but to test for nitrate you need to add aluminium, as you need to remove
the oxygen (reduce the nitrate) to make the ammonia. You don’t need to do this for the
ammonium ion as it has no oxygen (3.6(b)(i))
• Tests for aluminium ions and zinc ions are also often confused. Remember PANDA
(precipitate of aluminium (hydroxide) does not dissolve in ammonia). Both zinc and aluminium
ions form a white precipitate with sodium hydroxide, which re-dissolves in excess, but in
ammonia only the zinc precipitate re-dissolves (1.19)
• Questions involving the height of precipitates when sodium hydroxide is added to a solution
of metal ions often cause problems. Remember, as you add more hydroxide to a solution of
suitable metal ions (e.g. iron(II) ions) there will be more precipitate until all the metal ions are
used up. However, with excess sodium hydroxide, some hydroxides re-dissolve e.g.
aluminium hydroxide. In these cases the height of the precipitate will then decrease as you
add more hydroxide (3.4(b)(iii))
Rates of reaction and equilibrium
• Many students have difficulty explaining what is meant by rate of reaction. Remember two
points: it is the change in volume or mass etc over a fixed period of time. Time is often
omitted (2.3(a))
• Remember that the total volume of gas released by the same amount of metal is always the
same. A common error is to think that powdered metal, when reacted with acid, gives off
more gas than larger lumps of the same amount of metal (1.17)
• The total volume of gas released by a catalysed reaction is exactly the same as for an
uncatalysed reaction. The same amount of reactants is the important factor (3.3(c)(iii))
• In rate questions, when asked to analyse graphs of volume of gas against time for the
reaction of an acid with a metal or carbonate, a common error is to state the volume is
increasing and not mention the rate. Remember that the rate is getting less and less with time
because rate is the difference in volume divided by time (3.3(c)(i))
• A common mistake is to say that in an equilibrium reaction, a catalyst increases the rate of
the forward reaction more than the back reaction. One of the characteristics of equilibrium is
that the backward and forward reactions go at the same speed. This applies to catalysed as
well as uncatalysed reactions (3.2(a)(v))
Solubility
• A common error is to think that calcium hydroxide is insoluble in water. Remember that
limewater is a solution of calcium hydroxide, so it must at least be slightly soluble (1.20)
• If you are asked to explain what the symbol (aq) means, write down more than ‘aqueous’. An
answer such as ‘dissolved in water’ is needed (3.3(a))
Thanx for creating it 