WJEC AS Chemistry- Kinetics

Complete set of notes for WJEC Chemistry Kinetics

Covers: energy profiles, activation energy, catalysts, enzymes and more

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  • Created on: 08-04-13 12:08
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Rate of Reaction
Is the change in concentration of a reactant or product per unit time.
Rate= change in concentration/time
Units: moldm-3s-1
Rate is fastest at the start of a reaction since each reactant has its greatest
Rate slows down as the reaction proceeds since the concentration of the reactant
Rate becomes zero when the reaction stops, i.e. when one of the reactants has
been used up.
Measuring Rates of Reaction
Change in gas volume: in a reaction in which gas is formed, the volume of gas can
be recorded using a gas syringe at various times.
Mg (s) + 2HCl (aq) MgCl2 (aq) + H2
Change in gas pressure: reactions between gases involve a change in the number
of moles of gas. The change in pressure (at constant volume) at various times can
be followed using a manometer.
PCl5 (g) PCl3 (g) + Cl2 (g)
Change in mass: if a gas forms in a reaction and is allowed to escape, the change
in mass at various times can be followed using weighing scales.
CaCO3 (s) + 2HCl (aq) CaCl2 (aq) + H2O (l) + CO2 (g)
Change in Colour: the intensity of the colour of the iodine can be monitored over
time by using a colorimeter and hence its change in concentration can be
CH3COCH3 (aq) + I2 CH3COCH2I (aq) + HI (aq)
Sampling: Samples of the reaction mixture are removed at various times. The
reaction in each sample taken is slowed down significantly (quenched) by diluting
in ice-cold water. Each sample is titrated against standard alkali and the
concentration of ethanoic acid is calculated.
CH3CO2C2H5 (aq) + H20 (l) CH3CO2H (aq) + C2H5OH (aq)

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Calculating initial rates of reaction
Is done by measuring the concentration of a reactant over a period of time. The
results obtained are plotted to give a graph. To find the initial rate, it is necessary
to find the initial slope (gradient) of the line. The graph will always be a straight line
to begin with.
Rate= change in
1) Draw a line from any
convenient point on the straight
line (from P to M)
2) Calculate vertical distance
between N and M.…read more

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Collision Theory
Reactions occur by molecules colliding. However only a small fraction of these
collisions lead to a reaction. For a collision to be effective molecules must collide
in the correct orientation and must have sufficient energy. The minimum energy
needed is called the activation energy.
Energy Profiles
The activation energy is shown on an energy profile diagram. These compare the
enthalpy of the reactants with the enthalpy of the products.…read more

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A heterogeneous catalyst is in a different phase from the reactants, i.e. a different form
(solid, liquid, gas.) Many industrially heterogeneous catalysts are transition metals, and
are used to absorb gases onto the metal surface and react. The larger the surface area
the better the catalyst. Examples are iron in the Haber process and Vanadium (V) Oxide in
the contact process for sulphuric acid.
A homogeneous catalyst is in the same phase as the reactants.…read more


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