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Edexcel_IAL_Chemistry_WCH14-WCH15_Revision_Notes
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PearsonREVISION GUIDE
Edexcel
IAL
CHEMISTRY
WCH14/WCH15
,How fast? - rates
1 TERMINOLOGY
Rate of reaction: The rate with which the concentration of a product increases or the rate
with which the concentration of a reactant decreases.
Rate equation: An expression relating the rate of a reaction to the concentrations of the
reactants present in the reaction mixture.
Order of reaction: The way in which the rate of reaction varies with the concentrations of
the reacting substances.
Rate constant: The proportionality constant between the rate of reaction and the products
of concentrations that determine how the rate varies with concentration.
Half-life: The time taken for the concentration of a reactant to decrease to half its original
value.
Rate-determining step: The slowest step in a chemical reaction.
Activation energy: The minimum energy required to start a reaction by breaking bonds.
Heterogeneous catalyst: A catalyst in a different phase from the reactants.
Homogeneous catalyst: A catalyst in the same phase as the reactants.
2 THE RATE OF REACTION
𝑑[𝑝𝑟𝑜𝑑𝑢𝑐𝑡] 𝑑[𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡]
𝑟𝑎𝑡𝑒 = =−
𝑑𝑡 𝑑𝑡
[]→ “concentration”
Rate is measured in units of concentration per unit time, and the most common unit is mol
dm-3. The negative sign in the second expression reflects the fact that the concentration of
the reactant is decreasing and therefore gives a positive value for the rate.
3 METHODS OF MEASURING THE RATE OF REACTION
Any methods used to find out rates of reactions must employ a technique which measures
either the decreasing concentration of reactants or the increasing concentration of products
with respect to time.
Production of a gas: Either measure the volume produced at different times collected over
water or in a gas syringe or the time to produce a fixed volume of gas. This is a suitable
method for most of the gas-producing reactions, such as the decomposition of hydrogen
peroxide.
1
,→ Clock reactions: In a clock reaction, the reactants are mixed and the time taken to
produce a fixed amount of product is measured. This gives the initial rate of the reaction.
Production of a solid: The time taken to produce enough solid to hide a cross on a piece of
paper under the reaction apparatus or on the side of the apparatus can be measured. This
method is used to measure the decomposition of sodium thiosulfate by acid.
Loss of mass: Measure the mass of the total reaction mixture as the reaction progresses, and
see how quickly it decreases. This is suitable when considering the rate of reaction between
calcium carbonate and hydrochloric acid, because as carbon dioxide escapes, mass
decreases.
Colour change: The change in colour of either a reactant or a product is measured using a
colorimeter. A reaction that can be studied in this way is the reaction between iodine and
propanone. The decrease in the intensity of the colour of iodine can be measured with a
colorimeter as a function of time.
Conductivity: It involves measuring the conductivity changes in a reaction mixture over time.
These reflect the changes in the ions present in the solution.
Titrimetric analysis: This involves removing small portions (aliquots) of the reaction mixture
at regular intervals. These aliquots are usually added to another reagent, which immediately
stops or quenches the reaction. There are no further changes to the concentrations in the
reacting mixture. The quenched aliquots are then titrated to find the concentrations of
known compounds in them.
Conductimetric analysis: The number and type of ions in a solution affect its electrical
conductivity. As some chemical reactions take place in solution, the ionic balance changes
and the resulting change in conductivity can be used to measure the rate of the reaction.
These reflect the changes in the ions present in the solution and so can be used to measure
the changes in concentration of the various components of the mixture.
4 RATE EQUATIONS, RATE CONSTANTS AND THE ORDER OF A
REACTION
A rate equation shows how changes in the concentration of reactants affect the rate of a
reaction.
A + B → products
The equation that describes how the rate varies with the concentrations of the reactants
takes this form:
rate = k[A]m[B]n , where k is the rate constant.
The values m and n represent the reaction orders and often are whole numbers or equal to
zero. Fractions are uncommon.
Overall order of the reaction = m + n
2
, • Every reaction has its own particular rate equation and its own rate constant.
• Rate constants will only change their value with a change in temperature.
The order of a reaction describes the way in which the rate of reaction varies with the
concentrations of the reacting substances. The usual relationship is that the rate of reaction
is directly proportional to the concentration. However, in some cases the rate of reaction
might be directly proportional to the square of the concentration or the rate may not
increase at all for increasing concentrations.
Rate equations for different orders of reaction
• Zero order rate equation: rate = k[A]0 or rate = k
• First order rate equation: rate = k[A]
• Second order rate equation: rate = k[A]2
If two or more reactants are involved, then it is possible to have a third order rate equation:
• Third order rate equation: rate = k[A]2[B]
Units of the rate constant
The units of the rate constant differ for different reaction orders. These units can be worked
out from the rate equations.
Order Unit
Zero mol dm-3 s-1
First s-1
Second dm3 mol-1 s-1
Third dm6 mol-2 s-1
1. First order reactions
A reaction is first order with respect to a reactant if the rate of reaction is proportional to the
concentration of that reactant. This means that doubling the concentration of the reactant
leads to a doubling of the rate of reaction. Hence, a plot of rate against concentration gives a
straight line passing through the origin, whereas a graph of concentration of a reactant and
time is a decreasing exponential graph as shown:
3
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