Rate Equation ( A-level )

Question 1

what is rate of a reaction?

Answer 1

change in concentration of products/reactants in a given time

Question 2

at a given instant, how could you calculate the rate of reaction?

Answer 2

rate of reaction = change in concentration/ change in time

Question 3

how could you measure the rate of reaction experimentally?

Answer 3

• colorimeter to see if there is a change in colour
• gas syringe to measure volume of gas
• titration to measure concentration of a product
• weighing scale to measure change in mass

Question 4

how could you determine the rate constant and the rate expression of a reaction?

Answer 4

only experimentally

Question 5

what affects the value of the rate constant for a given reaction?

Answer 5

only temperature

Question 6

define the term order of a reaction with respect to a given product

Answer 6

the power to which a species concentration is raised in the rate equation

Question 7

define the term overall order of a reaction

Answer 7

the sum of the orders of reaction of all species in the rate expression

Question 8

how would you calculate the units of the rate constant?

Answer 8

-units of rate are mol dm-3s-1 and units of concentration are mol dm-3
-rearrange rate equation to get k=
-sub in the units and cancel them out

Question 9

how could you find the rate expression using the initial rate method?

Answer 9

• do a series of experiments, during which you vary concentrations
• plot a graph of concentration against time for each reactant and draw a tangent at t=0 to find the initial rate of reaction
• compare rates and concentrations between each experiment to find order of reactants and overall rate equation

Question 10

how can you calculate the rate of reaction from data from a iodine clock reaction?

Answer 10

-record time taken for colour change to occur
- use rate = 1/time

Question 11

what is the Arrhenius equation? what does each term mean?

Answer 11

k=Ae−Ea/RT
k = rate constant
A = pre-expotential factor ( number of collisons between reactant molecules
e = mathematical quantity
R = gas constant
T = temperature in kelvin
Ea = activation energy in joules

Question 12

how could you convert the arrhenius equation into useful form of plotting a graph?

Answer 12

In k = -Ea/RT + In A
Graph of In k against 1/T is a straight line: gradient = -Ea/R and y intercept is In a

Question 13

what is the rate determining step?

Answer 13

the slowest step in a reaction mechanism, which determines the overall rate of reaction