Home CHEMISTRY TOPIC 7: CHEMICAL KINETICS, EQUILIBRIUM AND ENERGETICS | CHEMISTRY FORM 3

TOPIC 7: CHEMICAL KINETICS, EQUILIBRIUM AND ENERGETICS | CHEMISTRY FORM 3

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The Rate of Chemical Reactions
Copmarison between the Rates of Chemical Reactions
Compare the rates of chemical reactions
Chemical reactions take place at different rates. Some are fast whereas others are very slow. Let us consider the following reactions:
  1. Addition of sodium metal to water: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) The reaction takes place immediately and violently. It is therefore a fast reaction.
  2. The rusting of iron in the presence of air and water giving hydrated iron (III) oxide, F2O3.XH2O: This is an extremely slow reaction.
These two reactions could be taken as representative examples of extremely fast and extremely slow reactions, respectively.There are, however, other reactions which proceed at rates intermediate between these two extremes. Rates of some of these reactions can be measured.
The rate of a chemical reaction can be measured in various ways. Let us
consider the reaction between zinc and sulphuric acid to produce zinc
sulphate and hydrogen gas:
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
When zinc is added to dilute sulphuric acid in a flask, they react together. The zinc slowly disappears and the gas (H2) bubbles off. After sometime, the bubbles of a gas form less quickly.
The reaction is slowing down. Finally, no more bubbles appear. The
reaction is over, because all the acid has been used up. Some zinc
remains behind in a beaker.
In this reaction both zinc and sulphuric acid get used up in the reaction.
At the same time, zinc sulphate and hydrogen form. The rate of this
reaction could be determined by measuring any of the following:
  • the amount of zinc used up per unit of time;
  • the amount of sulphuric acid used up per unit of time;
  • the amount of zinc sulphate produced per unit of time; or
  • the amount of hydrogen produced per unit of time.
In general, the rate of a chemical reaction is determined by measuring the amount of reactant used up per unit of time or the amount of product produced per unit of time. Therefore, the rate of a chemical reaction simply
refers to the amount of reaction which occurs in a unit time.
Experiments to Measure the Rates of Chemical Reactions
Perform experiments to measure the rates of chemical reactions
For the reaction described above, it is easiest to measure the amount of
hydrogen produced per minute. The hydrogen can be collected as it
bubbles off and its volume can then be measured as shown in figure

Apparatus for measuring the production of gas
An
experiment may be designed to measure the volume of hydrogen produced
after every twenty seconds or so and then recording the data in a
notebook.The table below shows sample results from such an experiment.
Time (s) Volume of hydrogen gas (cm3)
0 0
20 13
40 22
60 30
80 37
100 41
120 44
140 46
160 47
180 47
200 47
Questions from the experiment
Use these data to draw a graph of time (horizontal axis) against volume of hydrogen (vertical axis).
There are also other ways by which rates of chemical reactions can be measured. These include measuring the:
  1. change in intensity of colour:Many
    chemical reactions involve a change in colour. Potassium permanganate,
    for example, when it reacts with sulphur dioxide it changes from purple
    to colourless. The rate of such a reaction could be determined by
    measuring the rate at which the colour changes.
  2. formation or disappearance of a precipitate:The
    reaction between hydrochloric acid and sodium thiosulphate produce a
    yellow precipitate of sulphur. The rate at which this precipitate forms
    is a measure of the rate of a reaction.
The Concept of Endothermic and Exothermic Reactions
Explain the concept of endothermic and exothermic reactions
You
have met many different chemical reactions so far in chemistry. But
they all have one thing in common, that is, they involve an energy
change. The great majority of chemical reactions are accompanied by a
marked heat change.
During
chemical reactions as reactants form products, there is a change in
heat content. This is referred to as the enthalpy changeand is always
expressed in kilojoules per mole (kJmol-1). Two types of heat
change are distinguished. Those reactions that are accompanied by
evolution of heat to the surroundings are termed as exothermic reactions while those that are accompanied by absorption of heat from the surroundings are endothermic reactions.
  • An exothermic reaction is one during which heat is liberated to the surroundings.
  • An endothermic reaction is one during which heat is absorbed from the surroundings
When magnesium is burnt in air heat is evolved.
2Mg(s) + O2(g) → 2MgO(s)+ heat
The same case applies to the burning of coal in air.
C(s)+ O2(g) → CO2(g) + heat
Mixing sulphur nitrate and sodium chloride solutions gives a white precipitate of silver chloride and a temperature rise.
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
When
ammonium nitrate is dissolved in water, there is a fall in temperature.
Also adding a mixture of citric acid and sodium bicarbonate to water
produces bubbles and a fall in temperature. In both reactions, the
temperature of the water falls because the reactions take heat energy
from it. These reactions are therefore endothermic.
The
heat changes that occur during any chemical reaction represent changes
in the energy content of the whole system. The energy content may
increase or decrease depending upon whether heat is absorbed or evolved.
Energy Level Diagrams for Exothermic and Endothermic Reactions
Draw energy level diagrams for exothermic and endothermic reactions
For exothermic reactions,
the enthalpy changeis conventionally assigned a negative value. For
example, when pellets of sodium hydroxide or concentrated sulphuric acid
dissolve in water, heat is evolved and the system loses heat to the
surrounding.

Energy level diagram for exothermic reaction
For endothermic reactions,
the enthalpy changeis assigned a positive value. For example, when
potassium iodide or ammonium chloride dissolves in water, heat is
absorbed from the surroundings.

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