Rates of Reaction

Question 1

Suggest what is meant by rate of reaction

Answer 1

change in concentration of reactants or products over time

Question 2

Describe how to calculate rate of reaction at an instant graphically

Answer 2

determine gradient of tangent to curve at point

Question 3

Overall reaction order

Answer 3

sum of reaction orders of reactants in rate question

Question 4

Suggest what is meant by order with respect to

Answer 4

power of concentration term in rate equation

Question 5

Under what conditions does rate constant, k change

Answer 5

change in temperature

Question 6

Suggest methods of determining orders of reactions experimentally

Answer 6

carry our series of experiments at constant TEMPERATURE, each time only varying concentration of one species

Method 1

• plot concentration of species against time (gradient of tangent=ROR)
• derive graph of rate against concentration of species (horizontal line = zero order, straight line= first order, curved line=other so concentration should be squared or cubed to form straight line graph)

Method 2

• plot graph of concentration of species against time
• draw tangent at t=0
• compare ratios of species with rate of reaction to determine order of reaction with respect to each species, hence overall order of reaction

Question 7

Rate determining step

Answer 7

step in mechanism of reaction which is slowest hence determines rate of reaction

Question 8

Define each term in k=Ae^-Ea/RT

Answer 8

k = rate constant 
A = frequency of collisions between reactants molecules 
e^-Ea/RT = fraction of collisions with sufficient energy to react

Question 9

Give equation of graph to plot based on Arrhenius equation in order to determine activation energy for a reaction

Answer 9

lnk = lnA - Ea/RT
where lnk plotted against 1/T
gradient = -Ea/R so solve for Ea

Question 10

Suggest ways to improve accuracy in iodine clock experiment

Answer 10

• place flask in water bath to maintain a constant temperature since rate of reaction affected by temperature
• colorimeter to give more accurate time for colour change (eliminates human error)

Question 11

Justify why this step is rate determining step based on steps shown in mechanism

Answer 11

same ratio (stating ratio) as in rate equation

Question 12

Suggest why the order with respect to a species is zero

Answer 12

• species does not affect rate of reaction

- reaction with species is not part of rate determining step

Question 13

Explain why doubling temperature has a much greater effect on rate of reaction than doubling concentration of reaction (first order)

Answer 13

• reactions occurs when molecules have E>Ea
• doubling T means many molecules have this E
• doubling concentration (first order) only doubles