Chapter 18: Rates of Reaction

Question 1

What can you measure of time do calculate the rate of a reaction?

Answer 1

1) quantity reacted/produced

2) change in concentration

Question 2

How is rate and concentration related?

Answer 2

Rate is proportional to the concentration of a reactant raised to a power.
The power is the order of the reaction in respect to that reactant

Question 3

What is zero order?

Answer 3

When the concentration of that reactant does not affect the rate at all.

Question 4

What is first order?

Answer 4

When the concentration of the reactant is directly proportionate to the rate and so the rate is increased by the same factor the concentration is increased.

Question 5

What is second order?

Answer 5

When the rate is increased by the factor the concentration is increased squared

Question 6

Give the rate equation.

Answer 6

rate = k [a]^n [b]^m

Question 7

What is k in the rate equation?

Answer 7

the rate constant = number that mathematically converts between rate of reaction and concentration and orders.

Question 8

What is the overall order?

Answer 8

The sum of all the orders with respect to each reactant

Question 9

How do you work out the unit of k (rate constant)?

Answer 9

rate/concentrations with orders

=moldm-3s-1/ concentrations (moldm-3)

Question 10

How do you work out orders of a reaction?

Answer 10

You cant determine them directly, you carry out an experiment changing and keeping certain concentrations of the reactants and compare the effects they had on the reaction rates

Question 11

Give methods of continous monitoring of rate?

Answer 11

• gas collection

- mass loss

Question 12

Give a method of non-continous monitoring of rater?

Answer 12

Colorimeter

Question 13

How can you determine orders form a CONC against TIME graph?

Answer 13

decreasing straight line = 0 order
decreasing curve = 1st order
steeper decreasing curve = 2nd (but u dont need to know)

Question 14

How can you calculate the rate constant from a CONC against TIME graph?

Answer 14

0 order = straight line gradient is equal to rate constant
1st order = k=ln2/t0.5 OR rearrant the rate equation using a point on the curve the values of its exact concentration and rate reading off the point

Question 15

What is a half life?

Answer 15

Time it takes for the concentration of a reactant to halve.

Question 16

How can you confirm that a reactant has an order of 1?

Answer 16

Calculate its half lifes, if the first few successive half lives are the same then it is 1st order.

Question 17

How can you determine orders form a RATE against CONC graph?

Answer 17

0 order = straight horizontal line
1st order = straight increasing line
2nd order = increasing curve

Question 18

How do you calculate rate constants from a RATE against CONC graph?

Answer 18

0 order = the y intercept
1st order = the gradient of the line
2nd order = plot a rate against conc SQUARED graph and it should give straight line - this gradient.

Question 19

What is a clock reaction?

Answer 19

An initial rates method:
It is assumed that the initial rate is the average rate of the reaction.
Initial rate is proportional to 1/t
You carry out experiments changing certain reactions at once each time and you normally time how long it takes for a precipitate to appear and change colour or to disappear. This time over 1 is proportional to initial rate and so when you plot a graph of 1/t against conc = conc against rate graph and you can deduce the orders from the line shape

Question 20

What is a rate determining step?

Answer 20

The slowest step in the sequence of a mechanism

Question 21

Why is it likely that some reactions are involved in a multi-step reaction?

Answer 21

When there are2 mols of a reactant reacting with 1 mol of the other reactant, it is unlikely that 2 molecules and 1 molecule of the other will collide stimultaneously. More likely to occur in a series of steps.

Question 22

What does the rate equation only include?

Answer 22

The concentrations of the reacting species inolved in the rate determining step.

AND the orders in the rate equation equal the number of species involved in the RDS

Question 23

How do you propose a multi step mechanism from a rate equation?

Answer 23

Use the species in the rate equation as your first RDS reactants and the other reactants from the overall equation for your next step and fill in the gaps.

Question 24

Why does the rate constant and rate increase when temperature is increased?

Answer 24

• shifts the boltzmann distribution to the right increasing the number of particles that exceed the activation energy
• particles have increased kinetic energy so there are moving faster and collide more frequently. (THIS HAS LESS OF AN EFFECT COMPARED TO THE BOLTZMANN REASON )

Question 25

What is the arrhenius equation arranged to the y-mx+c format?

Answer 25

lnk = (-Ea/R x 1/T) + lnA

intercept = lnA
gradient = -Ea/R

Question 26

Arrhenius equation ( k = Ae^-Ea/RT)
What does the e^-Ea/RT represent?

Answer 26

The proportion of molecules that exceed Ea and have sufficient energy.

Question 27

What does the A take into account?

Answer 27

The frequency of collisions with the correct orientation.
It may increase very slightly with temperature but it is essentially constant over small temperature ranges.
It therefore gives the rate if there was no activation energy.