Rate Equations

Question 1

define the term rate of reaction

Answer 1

change in concentration per unit time

Question 2

at a given instant how could you calculate rate of reaction

Answer 2

rate of reaction = change in conc. / change in time

Question 3

how could you measure the rate of reaction experimentally

Answer 3

• use a colorimeter at a suitable intervals if there is a colour change
• if gas is evolved, use a gas syringe to collect volume of gas evolved, use a gas syringe to collect volume of gas evolved, or measure the change in mass of the reaction mixture

Question 4

how would you measure reaction rate for really fast reactions

Answer 4

use a flash of light to break bonds, use probe flashes to record amount of light absorbed by a species; this can show its concentration. first used for ClO2 –> ClO* + O*
can now monitor reactions that occur in times as fast as 10-12 seconds

Question 5

how can you determine the rate constant and rate expression for a reaction

Answer 5

only experimentally

Question 6

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

Answer 6

temperature, nothing else

Question 7

write a generic rate expression and state what each term means

Answer 7

Rate = k [X]^x [Y]^y
k = rate constant for the reaction
[X] and [Y] are concentrations of species X and Y respectively x and y are the orders of reaction with respect to X and Y

Question 8

do species need to be in the chemical equation to be in the rate expression

Answer 8

no - species in the chemical equation may be excluded and species may not in the chemical equation e.g. catalysts, may be included

Question 9

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

Answer 9

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

Question 10

define the term overall order of reaction

Answer 10

sum of the orders of reaction of all species in the rate expression e.g. total order = x+y

Question 11

how would you draw a rate concentration graph

Answer 11

plot [A] against time, draw tangents at different values –> draw a secondary graph of rate against [A]

Question 12

draw a rate concentration graph for a zero order reactant

Answer 12

straight horizontal line

Question 13

draw a rate concentration graph for a first order reactant

Answer 13

directly proportional

Question 14

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

Answer 14

• do a series of experiments, during which you vary concentrations, so the concentration of just one reactant changes each time
• plot a graph of concentration against time for each reactant and use 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 15

when does the starch turn a blue-black colour in an iodine clock reaction and why

Answer 15

when all of the Na2S2O3 has been used up and so I2 is produced, which reacts with starch leading to a blue black colour

Question 16

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

Answer 16

record time taken for colour change to occur
use rate = 1/t
effectively the initial rate

Question 17

what is the effect of a 10K temperature increase on the rate of reaction, roughly

Answer 17

doubles rate of reaction

Question 18

what is true of the half life of a first order reactant

Answer 18

half life is constant

Question 19

what is the Arrhenius equation

Answer 19

k = Ae -Ea/RT
k = rate constant for reaction
A = pre-exponential factor
e = mathematical quantity
R = gas constant
T = temperature in kelvin
Ea = activation energy for reaction in Joules

Question 20

how can you convert the Arrhenius equation into a useful form for plotting a graph

Answer 20

ln k = -Ea/RT + ln A

graph of lnk against 1/T is a straight line:
gradient = -Ea/R and y intercept is lnA

Question 21

what is the rate determining step

Answer 21

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