Module 5: rates

Question 1

How to calculate rate:

Answer 1

rate= change in concentration/change in time OR rate=quantity reacted or produced/time

Question 2

What is rate of reaction proportional to?

Answer 2

Rate is proportional to the concentration of a particular reactant raised to a power e.g [A]^2

Question 3

What does the power represent in [A]^n

Answer 3

Represents the order of reaction

Question 4

What is a zero order reaction?

Answer 4

Concentration of a reactant has no effect on the rate

Question 5

What is a first order reaction?

Answer 5

If concentration is doubled, rate of reaction also doubles

Question 6

What is a second order reaction?

Answer 6

Rate increases by the square of the concentration increase; when concentration is doubled, rate of reaction quadruples

Question 7

What is the rate equation?

Answer 7

Rate=k[A]^n[B]^n

Question 8

What is k in the rate equation?

Answer 8

Rate constant

Question 9

How to calculate the overall order of a reaction?

Answer 9

overall order= sum of orders of each reactant

Question 10

Rules when writing rate equation:

Answer 10

Zero order reactants are not represented

Don’t need to write power of 1 when representing first order reactants

Question 11

How to calculate units for rate constant?

Answer 11

1. Rearrange equation to make k the subject
2. Substitute units into expression for k
3. Cancel common units and show final units on a single line
   Remember to swap powers if units are on the bottom of the equation!

Question 12

How can orders of reaction be found from experimental results?

Answer 12

• By monitoring physical quantity changes over time

- This can be done either through a continuous monitoring method or initial rate method

Question 13

How can continuous monitoring be carried out?

Answer 13

• Colorimeter
• Measure volume of gas produced over time
• Change in pH

Question 14

Rate of reaction definition:

Answer 14

Change in the amount of reactants or products per unit time

Question 15

How can initial rate be measured?

Answer 15

• Iodine clock

- Track formation of a precipitate

Question 16

What shape is a zero order time-concentration graph?

Answer 16

Straight line with negative gradient

Question 17

What shape is a first order time-concentration graph?

Answer 17

Downward curve with a decreasing gradient over time

Question 18

What shape is a second order time-concentration graph?

Answer 18

Also a downward curve, steeper at the start, but tailing off more slowly

Question 19

What is half-life?

Answer 19

Time taken for the concentration of a reactant to half

Question 20

How to calculate the rate constant from the rate?

Answer 20

1. Draw tangent to the curve on the concentration-time graph is drawn at a particular concentration
2. Gradient of tangent calculated, giving the rate of reaction
3. Rearrange equation to find k, and input rate and concentration values

Question 21

What shape is a zero order rate-concentration graph?

Answer 21

Horizontal straight-line with zero gradient

Question 22

What shape is a first order rate-concentration graph?

Answer 22

Linear line through the origin

Question 23

What shape is a second order rate-concentration graph?

Answer 23

Upward curve with increasing gradient

Question 24

How can rate constant be calculate from iodine clock practical?

Answer 24

• Solution is colourless at the start and the time is measured for the black blue colour of the iodine to appear
• Experiment repeated at varying concentrations
• Initial rate is proportional to 1/t
• A graph of rate against concentration plotted
• Shape of graph identified to represent order in respect to one reactant
• Rate equation can be written and rate constant calculated

Question 25

What is the rate-determining step?

Answer 25

Slowest step in a reaction that requires multiple steps

Question 26

Why might some reactions occur in more than one step?

Answer 26

Reactions with many reactants will occur in more than one step as it is unlikely for them all to collide together at the same time, and in the correct orientation

Question 27

How can you check your reaction mechanism is correct?

Answer 27

• The rate equation only includes species in the rate determining step
• The orders involved in the rate equation match the number of species involved in the rate determining step

Question 28

How can you work out the mechanism of a reaction?

Answer 28

If you know the rate determining step of a reaction, then you can figure out the mechanism

Question 29

What is the effect of temperature on the rate constant?

Answer 29

As temperature increases, the value of k will also increase

Question 30

What is the Arrhenius equation?

Answer 30

Equation that can be used to calculate the effect of both temperature and activation energy on the value of k

Question 31

What is the relationship between first order reactions and half life?

Answer 31

First order reactants have constant half lifes

Question 32

What is the equation used to determine rate constant using half life in a first order reaction?

Answer 32

k=In/half life

Question 33

For a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate determining step?

Answer 33

The order of reactant

Question 34

What is the effect of a 10*C temperature rise on the rate of reaction (roughly)?

Answer 34

Doubles rate of reaction

Question 35

How to rearrange Arrhenius equation into useful form for plotting a graph?

Answer 35

In K= -Ea/RT + In A