MODULE 5: Rates of Reactions

Question 1

Name the two equations for rate of reaction?

Answer 1

rate = quantity/time, rate = change in concentration/change in time

Question 2

What is the order of reaction?

Answer 2

The relationship between the rate of reaction and the change in concentration of the product(s)

Question 3

What is the rate of reaction proportional to?

Answer 3

The concentration of the reactant(s) to the power of the order (rate = k[A]ⁿ)

Question 4

What is zero order?

Answer 4

Concentration does not have an effect on rate

Question 5

What is first order?

Answer 5

When concentration is doubled, rate is doubled

Question 6

What is second order?

Answer 6

When concentration is doubled, rate is quadrupled

Question 7

What is the rate equation that shows the relationship between concentrations of reactants and rate?

Answer 7

rate = k[A]ⁿ[B]ⁿ

Question 8

What is the overall order?

Answer 8

The sum of all of the orders of reactants in the rate equation

Question 9

How do you determine the units of the rate constant?

Answer 9

Rearrange the equation so you are finding ‘k’, add units to the equation, cross out common units on the denominator and numerator

Question 10

What is continuous and discontinuous monitoring?

Answer 10

Continually measured and only measured at the beginning and end

Question 11

What are some methods of continuous monitoring?

Answer 11

Monitoring by gas production, mass loss, absorbance change with a colourimeter

Question 12

What are the shapes of each order in a CONCENTRATION-TIME graph?

Answer 12

Zero = straight line on a negative gradient
First = downward curve with decreasing gradient over time
Second = steeper downward curve than first order, tails off more slowly

Question 13

What is half-life?

Answer 13

Time taken for half of a reactant to be used up (t1/2) (exponential decay)

Question 14

What two equations can you use to calculate rate constant? (k)

Answer 14

Rearranged rate equation (k = rate/[A]), or half-life equation k = In2/t1/2

Question 15

What are the shapes of each order in a RATE-CONCENTRATION graph?

Answer 15

Zero = horizontal line with no gradient
First = straight line through the origin
Second = upward curve with increasing gradient over time

Question 16

What is initial rate?

Answer 16

Instantaneous rate at the beginning of a reaction (when t=0) which is proportional 1/t

Question 17

What is a clock reaction?

Answer 17

A reaction where the initial rate can be obtained by taking a single measurement, usually time taken for an observable or visual change (colour/precipitate). This reaction is repeated over several concentrations so 1/t are calculated for each run

Question 18

Describe an iodine clock

Answer 18

Aqueous iodine is orange-brown, and when sodium thiosulfate is added it delays the colour change of iodine. Once all of the sodium thiosulfate has all reacted, the colour disappears, the iodine can form a starch-iodine complex that turns the solution from colourless to blue-black

Question 19

What is the rate-determining step?

Answer 19

In a multi-step reaction, the rate determining step is the slowest step

Question 20

How do you know if the reaction mechanism you have predicted is correct?

Answer 20

The rate equation only involved reacting species involved in the rate-determining step, and the orders in the rate equation match the number of species reacting

Question 21

What effects k (rate constant)?

Answer 21

Temperature

Question 22

Why does temperature effect k?

Answer 22

Increasing temperature increases the proportion of molecules with equal or more than the activation energy, and causes an increase in kinetic energy which causes particles to move faster and increases rate of successful collision

Question 23

What is the Arrhenius equation?

Answer 23

k = A e^-Ea/RT
k = rate constant
A = pre-exponential factor (frequency)
e = exponential factor
Ea = activation energy
T = Temperature
R = gas constant

Question 24

What do ‘A’ and ‘e^-Ea/RT’ in the Arrhenius equation represent?

Answer 24

A = frequency of collisions with correct orientation (successful)
e^-Ea/RT = proportion of molecules that exceed the activation energy