Rates Equations & Rates Terms
- The rate of reaction can be found by:
- Measuring the decrease in the concentration of a reactant over time
- Measuring the increase in the concentration of a product over time
- The units for rate of reaction are mol dm-3 s-1
Rate of Reaction
- The following general reaction will be used as an example to study the rate of reaction
D (aq) → E (aq) + F (g)
- The rate of reaction at different concentrations of D is measured and tabulated
Rate of reactions table
- A directly proportional relationship between the rate of reaction and concentration of D is observed when the results are plotted on a graph:
Rate of reaction over various concentrations of D
- This leads to a very common rate expression:
Rate ∝ [D] or Rate = k[D]
- This rate expression means that if the concentration of D is doubled, then the rate doubles
- Equally, if the concentration of D halves, then the rate halves
Rate Equations
- The following reaction will be used to discuss rate equations:
A (aq) + B (aq) → C (aq) + D (g)
- The rate equation for this reaction is:
Rate of reaction = k [A]m [B]n
- Rate equations can only be determined experimentally and cannot be found from the stoichiometric equations
- In the above rate equation:
- [A] and [B] are the concentrations of the reactants
- m and n are orders with respect to each reactant involved in the reaction
- Products and catalysts may feature in rate equations
- Intermediates do not feature in rate equations
Order of reaction
- The order of a reactant shows how the concentration of a chemical, typically a reactant, affects the rate of reaction
- It is the power to which the concentration of that reactant is raised in the rate equation
- The order can be a positive, negative or fractional value
- Orders that are a fraction suggest that the reaction involves multiple steps
- When the order of reaction with respect to a chemical is 0
- Changing the concentration of the chemical has no effect on the rate of the reaction
- Therefore, it is not included in the rate equation
- When the order of reaction with respect to a chemical is 1
- The concentration of the chemical is directly proportional to the rate of reaction, e.g. doubling the concentration of the chemical doubles the rate of reaction
- The chemical is included in the rate equation
- When the order of reaction with respect to a chemical is 2
- The rate is directly proportional to the square of the concentration of that chemical, e.g. doubling the concentration of the chemical increases the rate of reaction by a factor of four
- The chemical is included in the rate equation (appearing as a squared term)
- The overall order of reaction is the sum of the powers of the reactants in a rate equation
Worked example
The chemical equation for the thermal decomposition of dinitrogen pentoxide is:
2N2O5 (g) → 4NO2 (g) + O2 (g)
The rate equation for this reaction is:
Rate = k[N2O5 (g)]
- State the order of the reaction with respect to dinitrogen pentoxide
- Deduce the effect on the rate of reaction if the concentration of dinitrogen pentoxide is tripled
Answers
Answer 1:
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- Dinitrogen pentoxide features in the rate equation, therefore, it cannot be order zero / 0
- The dinitrogen pentoxide is not raised to a power, which means that it cannot be order 2 / second order
- Therefore, the order with respect to dinitrogen pentoxide must be order 1 / first order
Answer 2:
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- Since the reaction is first order, the concentration of dinitrogen pentoxide is directly proportional to the rate
- This means that if the concentration of the dinitrogen pentoxide is tripled, then the rate of reaction will also triple
Worked example
The following equation represents the oxidation of bromide ions in acidic solution
BrO3- (aq) + 5Br- (aq) + 6H+ (aq) → 3Br2 (l) + 3H2O (l)
The rate equation for this reaction is:
Rate = k[BrO3- (aq)][Br- (aq)][H+ (aq)]
- State the overall order of the reaction
- Deduce the effect on the rate of reaction if the concentration of bromate ions is doubled and the concentration of bromide ions is halved
Answers
Answer 1:
-
- All three reactants feature in the rate equation but they are not raised to a power, this means that the order with respect to each reactant is order 1 / first order.
- The overall order of the reaction is 1 + 1 + 1 = 3 or third order.
Answer 2:
-
- Since each reactant is first order, the concentration of each reactant is directly proportional to the effect that it has on rate
- If the concentration of the bromate ion is doubled, then the rate of reaction will also double
- If the concentration of the bromide ion is halved then the rate will also halve
- Therefore, there is no overall effect on the rate of reaction - one change doubles the rate and the other change halves it