Y13 Rates

Question 1

Rate of reaction definition

Answer 1

The change in concentration of a reactant or product per unit time

Question 2

What are the units of rate?

Answer 2

mol dm-3 s-1

Question 3

How can absorbance readings be used to plot a concentration-time graph (3 steps)?

Answer 3

1. Measure the absorbance readings of standard solutions of known concentrations and plot a calibration curve
2. Do the reaction and take absorbance readings at regular time intervals
3. Use the calibration curve to convert absorbance into concentration

Question 4

What does zero order mean?

Answer 4

Rate is NOT affected by [A]

e.g. if [A] = 4, rate = 1

Question 5

What does first order mean?

Answer 5

Rate is DIRECTLY PROPORTIONAL to [A]

e.g. if [A] = 4, rate = 4

Question 6

What does second order mean?

Answer 6

Rate is proportional to [A]^2

e.g. if [A] = 4, rate = 16

Question 7

What is K in the rate equation (Rate = k [A][B]^2)?

Answer 7

Rate constant

Question 8

What does it mean if a reactant is not in the rate equation?

Answer 8

It is zero order

Question 9

What can orders NOT be determined by?

Answer 9

Molar ratios

Question 10

How is the rate equation rearranged to get the value of K?

Answer 10

K = rate / [A][B]

Question 11

What is used to calculate rate on a conc-time graph?

Answer 11

Gradient

Question 12

What does zero order look like on a CONC-TIME graph?

Answer 12

[A] doesn’t affect rate so rate is CONSTANT so gradient is CONSTANT = straight diagonal line: \

Question 13

What does first order look like on a CONC-TIME graph?

Answer 13

[A] DOES affect rate, as [A] decreases rate decreases so gradient becomes less steep = curved downwards line

Question 14

What does second order look like on a CONC-TIME graph?

Answer 14

Second order has greater effect than 1st = steeper downwards curve

Question 15

Half-life definition

Answer 15

Time taken for the CONCENTRATION of the reaction to reduce by half

Question 16

How can half-life be used to prove a reaction is first order?

Answer 16

If half-life is CONSTANT, the reaction is FIRST order

Question 17

What is the equation involving half-life used to find K?

Answer 17

K = ln2 / t 1/2

Question 18

What does zero order look like on a RATE-CONC graph?

Answer 18

Straight line across —

Question 19

What does first order look like on a RATE-CONC graph?

Answer 19

Straight line through the origin /

Question 20

What does second order look like on a RATE-CONC graph?

Answer 20

Curved line going upwards

Question 21

How can you prove a reaction is second order after plotting a rate-conc graph?

Answer 21

Plot a rate-conc^2 graph, if the line is now straight it is second order

Question 22

Which graph do initial rates allow to be plotted?

Answer 22

Rate-conc (shape used to find order of reaction)

Question 23

What is the initial rates method (7 steps)?

Answer 23

1. Time how long it takes for a visual change to occur (colour change/precipitate)
2. Run experiment with set concentrations of A and B
3. Repeat but change [A] whilst keeping [B] constant
4. Repeat again, but change [B] whilst keeping [A] constant (only change one at a time)
5. Keep total volume and temp. the same (keeps all other reactants’ concs the same)
6. Rate = 1 / t
7. Plot RATE-CONC of each reactant and deduce order from shape

Question 24

What is the rate-determining step?

Answer 24

Slowest step in the reaction mechanism of a multi-step reaction (usually the FIRST step)

Question 25

What will the rate equation include which helps to determine the rate-determining step?

Answer 25

All reacting species in (or before) the rate-determining step

Question 26

How do you deduce a mechanism from the rate equation (5 steps)?

Answer 26

1. Left hand side of rate-determining step from rate equation (assume 1st step)
2. Put remaining reactants into step 2
3. Put one product into step 1, others into step 2
4. Balance the rate-determining step with an intermediate (never single atom)
5. Add up steps to check it makes the overall equation

Question 27

Which step is the rate-determining step?

Rate = K [A]^2[B]
1. A + B –> AB
2. AB + A –> C
3. C + B –> D

Answer 27

Rate-determining step is step 2 because there are 2 moles of A (2nd order) and 1 mole of B (1st order) in steps 1 and 2 COLLECTIVELY

Question 28

What is the form of Arrhenius equation which must be learnt?

Answer 28

lnK = - Ea/R x 1/T + lnA

y = m x + c

Question 29

How is Ea calculated from a lnK-1/T graph?

Answer 29

Gradient = m = - Ea/R so…

Ea = gradient x -R

Question 30

How do you find A from an Arrhenius plot?

Answer 30

y-intercept = lnA so…

A = e^lnA

Question 31

What should you do if the x-axis doesn’t start at 0?

Answer 31

1. Rearrange the Arrhenius equation:

y = mx + c —–> c = y - mx

2. Plug in x and y from ONE point on the best fit line and the gradient (m)