Rate Of Reactions

Question 1

How can a colorimeter be used to measure rate?

Answer 1

• A colorimeter measures the absorbance of light by a coloured sample.
• The more concentrated, the darker the colour
• more light is absorbed.
• Then plot a calibration curve
• known concentrations of (iodine) on x-axis.
• The absorption is measured for sample
• results are plotted
• then use curve and absorbance to find out [Iodine]

Question 2

What is the initial rate?

Answer 2

the rate right at the start of the reaction when T=0

Question 3

Describe a clock reaction

Answer 3

• You time how long it takes for a reaction to occur
• is used to simplify the initial rate method
• sit the reaction vessel on a paper with a cross
• Time how long is takes until you cant see the cross through the beaker
• There is a colour change
• This is the end point
• The quicker the clock reaction, the faster the initial rate is

Question 4

What are 3 assumptions of the clock experiments?

Answer 4

• Temperature of the reaction remains constant
• no significant change in conc of reactants during the reaction
• Reaction has not proceeded too far when the end point is seen.

Question 5

Describe the iodine clock experiment

Answer 5

• Add sodium thiosulfate and starch (indicator) to excess hydrogen peroxide + iodide ions in sol
• H2O2 + 2I- + 2H+ = 2H20 + I2
• Sodium thiosulfate reacts immediately with the Iodine produced
• 2S2O3(2-) + I2 = 2I- + S4O6(2-)
• iodine from first reaction used up in second
• when ST used up, some iodine left in solution
• starch indicator = blue/black
• end of clock reaction

Question 6

What is an order?

Answer 6

• An order is the power to which a concentration is raised to in the rate equation
• Tells us how the concentration of the substance affects the rate

Question 7

What is zero order on a rate-conc graph?

Answer 7

• Changes in concentration has no effect on rate
• If [A] doubles then rate doesn’t change .
• produces a horizontal straight line with no gradient

Question 8

What is first order on a rate-conc graph?

Answer 8

• Changes in concentration has a proportional change to rate
• If [A] doubles then rate doubles.
• A positive line passing through the origin.

Question 9

What is second order on a rate-conc graph?

Answer 9

• Changes in concentration has a squared proportional change on rate.
• If [A] double then rate x4
• Forms an exponential curve

Question 10

How to get rate from second order graph?

Answer 10

• Rate-conc graph is a curve
• Plot a second graph of rate against the conc squared.
• Results in a straight line through the origin.
• The gradient of the straight line is equal to the rate constant k.

Question 11

What does the value of ‘k’ link to rate mean?

Answer 11

• Larger the value of ‘k’ the faster the rate of reaction

Question 12

Describe zero order on a rate-time graph

Answer 12

• straight line with a negative gradient
• reaction rate doesn’t change during the reaction
• gradient = k

Question 13

Describe first order on a rate-time graph

Answer 13

• downward curve with a decreasing gradient
• time for the conc of the reactant to halve is constant
• half life = k

Question 14

Describe second order on a rate-time graph

Answer 14

• downward curve like 1st

- a lot steeper at the start.

Question 15

What is half life?

Answer 15

• time taken for half of a reactant to be used up.

Question 16

How to calculate k for a first order reaction?

Answer 16

k = ln2 / t(1/2)

Question 17

what is the rate determining step?

Answer 17

• the slowest step in a multi-step reaction.

- substances that aren’t in the rate equation can’t be in the rate determining step.

Question 18

What is the arrhenius equation?

Answer 18

k = Ae^-Ea/RT
k = rate constant 
Ea = activation energy (J mol -1) 
T = temperature (K) 
R = gas constant (8.31) 
A = pre exponential factor

Question 19

What is the relationship between Ea and k?

Answer 19

• As Ea gets smaller, k gets bigger.
• Bc as Ea drops, ROR increases as particles have enough energy to react when they collide.
• large Ea = slow rate

Question 20

Effect of increase in temp on k?

Answer 20

• Increase in temperature, increases k
• particles have more kinetic energy
  more likely to collide with at least the Ea
• ROR increases.

Question 21

What is the log version of the Arrhenius equation

Answer 21

lnk = lnA - Ea / RT
Ea = (lnA - lnk) x RT

Question 22

How to use Arrhenius equation when given T and k table?

Answer 22

-Ea/R = gradient
lnA = y intercept
- first convert to 1/T and lnk using values in the table.
- Plot a graph of lnk(y) against 1/T(x)
- calculate the Ea/R from the -gradient of the straight line
- calc Ea by multiplying -gradient by R(8.31)
- Calculate lnA by extrapolating your line back to T=0/y - intercept.
- To calculate A use inverse of ln = e^lnA

Question 23

what is ROR

- ways to measure

Answer 23

• change in the amount of reactants/products per unit time
• measure volume of gas given off
• measure loss in mass as gas is evolved
• colorimetry to measure colour change of a Rt
• pH change of a reaction