Unit 1: Section 7 - Rate Equations and Kp

Question 1

What is the definition of 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

1. Use a colorimeter at suitable intervals if there is a colour change
2. If a 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 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 expression and state what each term means?

Answer 7

Rate = k[A]^x[B]^y
k - rate constant for the reaction
[A] and [B] are concentrations of species A and B respectively
x and y 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, 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 a species’ concentration is raised in the rate equation

Question 10

How do you find the overall order of reaction?

Answer 10

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

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

What does a rate concentration graph for a zero order reactant look like?

Answer 13

A straight horizontal line

Question 14

What does a rate concentration graph look like for a first order reactant?

Answer 14

A straight line graph y=x

Question 15

What does a rate concentration graph from a second order plus reactant?

Answer 15

A curve starting at (0,0) and increasing exponentially

Question 16

How could you confirm that this reactant was second order?

Answer 16

Graph of rate against concentration squared would be a straight line y=x through the origin

Question 17

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

Answer 17

• Do a series of experiments, during which you vary concentrations, so the concentration of just one reactant changes over time
• Plot a graph od 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 reactants and overall rate equation

Question 18

What must you add to react with the I2 as it is produced for an iodine clock reaction?

Answer 18

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

Question 19

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

Answer 19

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 20

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

Answer 20

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

Question 21

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

Answer 21

Doubles rate of reaction

Question 22

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

Answer 22

Half life is constant

Question 23

What does each term in the Arrhenius equation mean?

Answer 23

k - rate constant for reaction (same units as k)
A - Arrhenius constant (same units as k)
e - mathematical quantity
R - gas constant
T - temperature in Kelvin
Ea - activation energy for reaction in Joules

Question 24

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

Answer 24

• Graph of lnk against 1/T is a straight line
• gradient - Ea/R
• y intercept - lnA

Question 25

What is the rate determining step?

Answer 25

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

Question 26

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

Answer 26

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