1.9 Rate equations

Question 1

Define the term rate of reaction

Answer 1

Change in concentration per unit time. State what is being monitored (usually production of a product)

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

Use a colorimeter at suitable intervals if there is a colour change
If gas is 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 species; this can show its concentration

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 value

Answer 6

Temperature, nothing else

Question 7

Write a generic rate expression and state what each term meand

Answer 7

Rate = k[A]^m[B]^n
k=rate constant for the reaction
[A] and [B] are concentrations of species A and B
m and n are the orders of reaction with respect to A and B

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 not in the chemical equation eg catalysts may be included

Question 9

Define the term order of a 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 a reaction

Answer 10

The sum of the orders of reaction of all species in the rate expression, total order = m + n

Question 11

How would you calculate the units of the rate constant

Answer 11

Units of rate are moldm^-3s^-1 and units of concentration are moldm^-3
Rearrange rate equation to get k=
Sub in units and cancel them out

Question 12

How would you draw a rate concentration graph

Answer 12

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

Question 13

Draw a rate conc graph for a zero order reactant

Answer 13

Google

Question 14

Draw a rate conc graph for a first order reactant

Answer 14

Google

Question 15

Draw a rate conc graph for a second order reactant

Answer 15

Google

Question 16

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

Answer 16

Do a series of experiments, during which you vary concs, so the conc 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 concs between each experiment to find order of reactants and overall rate equation

Question 17

What must you add to react the I2 as it is produced for an iodine clock reaction (equation)

Answer 17

Known moles of sodium thiosulfate and a little starch. Reacts with I2 in 1:2 ratio
I2 + 2S2O3^2- –> S4O6^2- +2I-

Question 18

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

Answer 18

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 19

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

Answer 19

Record time taken for colour change to occur. Use rate = 1/t
This is effectively initial rate

Question 20

What is the effect of a 10k temperature increase on the rate of reaction roughly

Answer 20

Doubles rate of reaction

Question 21

What is true of the half life of a first order reactant (conc against time graph)

Answer 21

Half life is constant

Question 22

What is the Arrhenius equation
What does each term mean

Answer 22

k = Ae -EA/RT
k = rate constant for reaction
A= pre-exponential factor (number of collisions between reactant molecules)
e= mathematical quantity
R = gas constant
T = temperature in Kelvin
Ea= activation energy for reaction in Joules

Question 23

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

Answer 23

lnk= -EA/RT + lnA

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

Question 24

What is the rate determining step

Answer 24

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