CHEMISTRY 2b – SUMMARY
Items in ITALLICS are HIGHER TIER ONLY
C2.4.1 RATES OF REACTION
Speeding up, or slowing down, chemical reactions is important in everyday life and in industry
The rate of a chemical reaction can be found by measuring the amount of product formed or
the amount of reactant used.
You should be able to: interpret graphs showing the amount of product formed (or reactant
used up) with time, in terms of the rate of reaction.
The speed (rate) of a chemical reaction increases:
 if the temperature increases;
 if the concentration of dissolved reactants increases;
 if solid reactants are in smaller pieces (greater surface area);
 if a catalyst is used;
 if the pressure of gases increases.
Collision theory: For a reaction to take place the reacting particles must collide with each
other. Kinetic theory tells us that the particles are always moving around. To increase the
rate of reaction we must increase the number of collisions.
Activation energy: Not all collisions result in a reaction, only those with sufficient energy.
This is because energy is required for the reaction to occur and not all collisions provide
sufficient energy. This energy is called the activation energy.
Increasing the energy of the collisions will increase the number of collisions with sufficient
energy and hence the rate of reaction.
Effect of concentration:
Increased concentration increases the rate of reaction. Increasing the concentration means
there are more particles and so more collisions. This also explains why the rate of reaction
is fastest at the start - as the reaction proceeds the particles are used up and so the
concentration decreases.
Effect of temperature
Increased temperature increases the rate. The particles move faster as they are heated
and this has two effects:a) they are travelling faster, will go further in a given time, and so be involved in
more collisions.
b) when they do collide the increased speed means they have more energy and are
therefore more likely to be successful (exceed the activation energy).
Effect of pressure
Increasing the pressure of reacting gases also increases the frequency of collisions and so
increases the rate of reaction. (it pushes the particles closer together)
Effect of a catalyst
A catalyst increases the rate of a chemical reaction but it is not used up during the reaction.
It can be used over and over again to speed up the conversion of reactants to products.
Different reactions need different catalysts - some work better than others.
Increasing the rates of chemical reactions is important in industry because it helps to
reduce costs. Lots of industrial reactions use catalysts.
Effect of surface area
With solids the reaction occurs on the surface. Increasing the surface area (breaking the
solid up) gives more places for the reaction to occur and so it is faster.
C2.5.1 ENERGY IN REACTIONS
Chemical reactions involve energy. This is most noticeable when a fuel burns - the energy is
given off as heat.
Whenever chemical reactions occur, energy is usually transferred to or from the
surroundings.
If there is a rise in temperature (it feels hot) energy is being given out. This is an
exothermic reaction. e.g. combustion, neutralisation and many oxidation reactions.
This is used in things like hand warmers and self heating cans.
If there is a fall in temperature (it feels cold) energy is being taken in. This is an
endothermic reaction. Examples are thermal decompositions and sports injury cold packs.
If a reversible reaction is exothermic in one direction it is endothermic in the other. The
same amount of energy is transferred in each case.
For example if you heat blue copper sulphate it turns white and gives off water. If you add
water to white copper sulphate it turns blue and gives off heat.
C2.6.1 Neutralisation and making salts
You Should Be Able to: suggest a method to make a named soluble salt.
1) By mixing an acid and alkali and using an indicator to show when they are neutral (all
the acid and alkali reacted). You have to measure the quantities accurately and then
mix the exact amount of acid and alkali without the indicator (a burette is useful). You
can then crystallise the salt by evaporating the water.
2)
Acid plus base (insoluble) e.g. copper oxide. Since the base is insoluble, excess can
be added (to make sure all the acid is used) and then filtered. The solution can be
crystallised by evaporation. An example of this is adding black copper oxide to
sulphuric acid. Copper sulphate (a blue solution) is made.
COPPER OXIDE
CuO
+ SULPHURIC ACID
+
H2SO4


COPPPER SULPHATE + WATER
CuSO4
+
H2O
3) Acid plus metal – needs care as some metals are too reactive (and some not reactive
enough). Excess metal is used and then filtered. A salt and Hydrogen gas (squeaky pop
test) are produced.
MAGNESIUM + HYDROCHLORIC ACID
Mg
+
2HCL
MAGNESIUM CHLORIDE + HYDROGEN

MgCl2

+
H2
Precipitation reactions to make insoluble salts
Insoluble salts can be made by precipitation reactions. Two solutions are mixed and if one of
the products is insoluble it will form a precipitate. This technique can be used to remove
unwanted ions from waste or drinking water as the ions, once trapped as a solid, can be
filtered and removed.
You Should Be Able to: name the substances needed to make a named insoluble salt. (i.e.
what you would need to mix to make Lead Iodide)
Worth Remembering: all nitrates, sodium and potassium salts are soluble
C2.6.2 Acids and bases
Metal oxides and hydroxides are bases (the opposite of acids). Alkalis are soluble bases
Salts are compounds and can be made by reacting acids and bases
This reaction is called neutralisation.
ACID + ALKALI (BASE)
NEUTRAL SALT SOLUTION

+
WATER
The particular salt produced in any reaction between an alkali and an acid depends on:
 the metal in the alkali or base;
 the acid used.
Neutralising hydrochloric acid produces chlorides.
Neutralising nitric acid produces nitrates.
Neutralising sulphuric acid produces sulphates.
e.g.
SODIUM HYDROXIDE
NaOH
COPPER OXIDE
CuO
+
+
+
+
KOH
HCl
SULPHURIC ACID
POTASSIUM HYDROXIDE

HYDROCHLORIC ACID
H2SO4
+
+
NaOH



NITRIC ACID
HNO3
SODIUM CHLORIDE + WATER
COPPER SULPHATE
CuSO4


+
+
+
POTASSIUM NITRATE
KNO3
H 2O
WATER
H 2O
+
+
WATER
H2O
Although it is not a metal, ammonia also dissolves in water to form an alkali solution. This can
be neutralised with an acid to produce an ammonium salt. Ammonium salts are important as
fertilisers.
e.g.
AMMONIUM HYDROXIDE
+
NITRIC ACID

AMMONIUM NITRATE
+ WATER
NH4OH
+
HNO3

NH4NO3
H 2O
+
What makes an acid an acid?
Hydrogen ions H
+
(aq)
make solutions acidic
Hydroxide ions OH-(aq) make solutions alkaline
(aq)
is a state symbol - it means aqueous or dissolved in water.
Other state symbols are (s) for solid; (l) for liquid; and (g) for gas.
Neutralisation reactions can be summarised as follows: The hydrogen ions react with
hydroxide ions to produce water.
H
+
-
(aq)
+ OH (aq) 
H2O(l)
The pH scale is a measure of the acidity or alkalinity of a solution.
0-6 is acid;
7 is neutral;
8 – 14 is alkali
C2.7.1 ELECTROLYSIS
Electrolysis will only work if the substance is made of ions (charged particles).
The ionic substance must be melted or dissolved in water so the ions are free to move.
Passing an electric current through ionic substances that are molten or in solution breaks
them down into elements. This process is called electrolysis (it is sort of reversing bonding).
The substance that is broken down is called the electrolyte.
During electrolysis, positively charged ions (metal ions) move to the negative electrode, and
negatively charged ions (non metals) move to the positive electrode.
In molten solutions there is only one set of ions (e.g. sodium and chloride ions in sodium
chloride) so they are the ions attracted and released as atoms.
In solutions there are Hydrogen (H+) and Hydroxide (OH-) ions as well from the water. The
product formed depends on the reactivity of the elements.
At the positive electrode: get oxygen released unless a halogen (Gp 7) is present.
At the negative electrode: get hydrogen unless there is an unreactive metal. (usually silver or
copper but see the reactivity series on the back of the periodic table)
At the negative electrode, positively charged ions gain electrons (reduction).
At the positive electrode negatively charged ions lose electrons. (oxidation).
Note: Oxidation and reduction does not just involve oxygen.
Reactions at the electrodes can be shown as ionic half equations e.g.
At the positive electrode:
2Cl -  Cl2 + 2e- or 2Cl - - 2e -  Cl2
At the negative electrode:
2Na+ + 2e-  2Na
You may be expected to complete or balance these.
The electrolysis of sodium chloride solution (brine) is an important industrial process.
Chlorine gas is formed at the positive electrode and hydrogen gas at the negative electrode.
A solution of sodium hydroxide is also formed. These are important reagents for the
chemical industry. E.g. Sodium hydroxide is used to make soap; Chlorine to make bleach and
plastics;
Aluminium is made by the electrolysis of a molten mixture of aluminium oxide and cryolite.
(cryolite lowers the melting point and so saves energy). Aluminium forms at the negative
electrode and oxygen at the positive one. The positive electrode is made of carbon which
reacts with the oxygen to produce carbon dioxide.
Electrolysis is used to electroplate items, often copper or silver plating. The item to be
plated is used as the negative electrode and so the positive metal ions are attracted to it
and stick.