1.9 Rate Equations

Question 1

Define the term rate of reaction.

Answer 1

Change in concentration (of any reactant or product) 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 (different methods) ?

Answer 3

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

Question 4

How would you measure reaction rate for really fast reaction?

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 sin the chemical equation may be excluded and species not in the chemical equations e.g. 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 species’ concentration is raised in the rate equation.

Question 10

Define the term overall order of reaction

Answer 10

The sum of the orders of reaction of all species in the rate expression e.g. (from earlier), total order = x + y

Question 11

How would you calculate the units of the rate constant?

Answer 11

Units of rate are moldm^-3 s^-1 and units of concentration are mold dm^-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 concentration graph for zero order reactant.

Answer 13

In a zero order reaction the reactant has no effect on the rate of reaction. This means that as the concentration changes, the rate stays the same. A graph of rate of reaction against concentration is therefore a straight, horizontal line.

Question 14

Draw a rate concentration graph for a first order reactant

Answer 14

In a first order reaction the reaction rate is proportional to the concentration of the reactant. So a graph of rate of reaction against concentration is a straight line with a positive gradient

Question 15

Draw a rate concentration graph from a second order reactant.

Answer 15

For a second-order reaction, a plot of the inverse of the concentration of a reactant versus time is a straight line with a slope of k.

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 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 17

What must you add to react with 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 Na2S203 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 the 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 (concentration 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

In k = -Ea/RT + In A

Graph of ink against 1/T us a straight line; gradient = -Ea/R and y intercept is InA.

Question 24

What is the rate determining step?

Answer 24

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

Question 25

How does the rate determining step link to the species involved in the rate expression?

Answer 25

Any species involved in the rate determining step may appear in the rate expression. Species only involved after the rate determining step do not appear in the rate expression.