18 Rates of Reaction

Question 1

First order of reaction

Answer 1

A reaction is first order with respect to a reactant when the rate is directly proportional to its concentration.

Question 2

How do you determine orders from a concentration-time graph?

Answer 2

• Zero order: straight line with a negative gradient.
• First order: curve with constant half-life.
• Second order: downwards curve, does not have constant half-life.

Question 3

Define half-life.

Answer 3

Time taken for half of the reactant to be used up.

Question 4

How do you find the initial rate of reaction from a concentration-time graph?

Answer 4

Find the gradient at t=0.

Question 5

Monitoring rate of reaction using a colorimeter.

Answer 5

Colorimeter measures intensity of light passing through sample.

• Create a calibration curve using standard solutions.
• Carry out reaction and measure absorbance at timed intervals.
• Use the calibration curve to find the corresponding concentration.

You can then plot a concentration-graph and find the order of the reaction.

Question 6

Equation for calculating rate constant from half-life.

Answer 6

k = ln2/half-life

Question 7

How do you determine orders from a rate-concentration graph?

Answer 7

• Zero order: horizontal straight line with zero gradient.
• First order: straight line through the origin.
• Second order: upward curve with increasing gradient.

Question 8

Initial rates method

Answer 8

• Use a clock reaction to find initial rate of raction with a single measurement.
• Observe time taken for a visual change to be observed.
• Assume average rate of reaction over this time will be the same as the initial rate.
• Initial rate ∝ 1/t.
• Repeat for different concentrations.

Question 9

Give an example of a clock reaction.

Answer 9

Iodine clock reaction.

Question 10

What is the rate-determining step?

Answer 10

The slowest step in a reaction mechanism.

Question 11

What is the link between the RDS and the rate equation?

Answer 11

Rate equation only includes reactants in RDS, so intermediate molecules are NOT in the rate equation.

Orders in the rate equation match the number of species in RDS.

Question 12

What is the effect of temperature on k (the rate constant)?

Answer 12

When the temperature increases, the rate of reaction increases so k increases.

Question 13

Why does the value of the rate constant increase when the temperature increases?

Answer 13

• Higher proportion of particles have energy that is equal to or exceed the activation energy (main factor).
• Particles have more KE so they more faster and collide more frequently.

Question 14

How would you plot the logarithmic form of the Arrhenius equation, and what is the gradient?

Answer 14

Y-axis is ln k.
X-axis is 1/T.
Gradient is -Ea/R.