5.1.1 How Fast?

Question 1

What is the rate of reaction?

Answer 1

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

Question 2

What is the order?

Answer 2

• with respect to a reactant is the power to which the concentration of the reactant is raised in the rate equation

Question 3

What is the overall order?

Answer 3

• the sum of the individual orders

Question 4

What is the rate constant?

Answer 4

• k
• the constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation

Question 5

What is zero order?

Answer 5

• rate is unaffected by changing the concentration of A
• not present in rate equation

Question 6

What is first order?

Answer 6

• rate is directly proportional to the concentration

Question 7

What is second order?

Answer 7

• the change in rate will be equal to the change in concentration squared

Question 8

What is the rate equation?

Answer 8

• rate = k[A][B][C]

Question 9

What is the shape of concentration-rate graphs for different orders?

Answer 9

• 0 order - straight line horizontally
• 1 order - directly proportional
• 2 order - exponential (as conc x2, rate x4)

Question 10

What is the shape of concentration-time graphs for different concentrations?

Answer 10

• 0 order - graph decreases at constant rate (straight line)
• 1 order - concentration halves at equal time intervals (constant half life)
• 2 order - concentration decreases rapidly (half life increases)

Question 11

What is the rate determining step of a reaction?

Answer 11

• the slowest step in a reaction mechanism of a multi step reaction

Question 12

What does each component in the Arrhenius equation mean?

Answer 12

• k = rate constant
• A = pre-exponential factor (same units as k)
• Ea = activation energy Jmol-1
• R = universal gas constant (8.314) Jmol-1k-1
• T = temperature in Kelvin

Question 13

What is the effect of temperature on rate constants?

Answer 13

• as temperature increases, the rate constant increases and the rate of reaction increases

Question 14

What is half-life?

Answer 14

• the time taken for the initial concentration of the reactants to decrease by half
• t|1/2

Question 15

What is the half-life for a first order reaction?

Answer 15

• the half life is constant throughout the reaction

Question 16

What experimental techniques can be used to obtain rate data for reactions?

Answer 16

• measuring change in reactant mass or concentration over time
• measuring change in product mass or concentration over time

Question 17

How can mass change produce rate data?

Answer 17

• if gas if produced, mass of mixture will decrease
• plotting mass-time graph and drawing a tangent to curve can find initial r of r

Question 18

How can volume of gas obtain rate data?

Answer 18

• measuring volume of gas produced and plotting graph of volume against time
• gas cylinder used to collect gas

Question 19

How can titration be used to obtain rate data?

Answer 19

• small samples of reaction mixture can be removed at regular intervals throughout a reaction
• samples can be titrated to determine the conc of a given reactant or product
• a concentration-time graph can then be plotted

Question 20

How can colorimetry be used to obtain rate data?

Answer 20

• amount of light absorbed by colorimeter is proportional to concentration of coloured species
• calibration curve can be generated (colorimeter measures absorbance of solutions with known concentrations)

Question 21

Describe the iodination of propanone (iodine clock)

Answer 21

CH3COCH3 + I2 -> CH3COCH2I + H+ + I-
- initial solution brown due to iodine present
- as iodine is used up, solution changes from brown to orange to yellow and finally to colourless
- the concentration of iodine can be found by continually taking samples of reaction mixture and measuring absorbance using a colorimeter

Question 22

What is the relationship between the rate constant (k) and constant half life?

Answer 22

• k = ln 2 / t 1/2