Chapter 18 Rates of reaction

Question 1

What is rate of reaction?

Answer 1

The change in the amount of reactants or products per unit time

Question 2

What are four methods for following the rate of reaction?

Answer 2

• Measure the volume of gas evolved
• Measure the loss in mass as a gas is evolved
• Use colorimetry to measure the colour change of a reaction
• Measure the pH change of a reaction

Question 3

How do colorimeters work?

Answer 3

Colorimeters measure the absorbance of a particular wavelength of light by a solution

Question 4

Describe the method for using a colorimeter

Answer 4

1. Set the colorimeter to measure the wavelength of light that you are interested in
2. Calibrate the colorimter by placing distilled water in a sample tube known as a cuvette and place it in the colorimeter and set the absorbance to zero
3. Carry out the reaction, take samples from the reaction mixture at regular intervals and measure its absorbance using the colorimeter

Question 5

What is a calibration curve used for?

Answer 5

Used to convert from absorbance to concentration

Question 6

How do you create a calibration curve?

Answer 6

Measure the absorbance of a set of standard solutions of the reactant or product you’re interested in then plot a graph of absorbance against concentration and drawing a line of best fit

Question 7

How do you work out reaction rate from a concentration time graph?

Answer 7

1. Repeatedly take measurements during a reaction and plot a concentration-time graph
2. The rate at any point in the reaction is given by the gradient at that point on the graph
3. If the graph is a curve, you’ll have to draw a tangent to the curve and find the gradient of that

Question 8

Describe how you would carry out a clock reaction

Answer 8

Measure how the time taken for a set amount of product to form changes as you vary the conc of one of the reactants

Question 9

What two assumptions will you need to make for the clock reaction results?

Answer 9

• The concentration of each reactant doesn’t change significantly over the period of your clock reaction
• The temperature stays constant

Question 10

What does the order of reaction tell you?

Answer 10

Orders tell you how a reactant’s concentration affects the Rate

Question 11

When is a reaction order 0?

Answer 11

When you double the reactant’s concentration and the rate stays the same, the order with respect to that reactant is 0

Question 12

When is a reaction order 1?

Answer 12

If you double the reactant’s concentration and the rate also doubles, the order with respect to that reactant is 1

Question 13

When is a reaction order 2?

Answer 13

If you double the reactant’s concentration and the rate quadruples, the order with respect to that reactant is 2

Question 14

How do you work out an overall order?

Answer 14

An overall order is the sum of all orders of all the different reactants

Question 15

What is the shape of a zero order rate-concentration graph?

Answer 15

A horizontal line in the Y-axis as rate doesn’t change

Question 16

What is the shape of a first order rate-concentration graph?

Answer 16

A straight line through the origin, the rate is proportional to the concentration

Question 17

What is the shape of a second order rate-concentration graph?

Answer 17

A curve as the rate is proportional to the concentration squared

Question 18

How do you draw a rate-concentration graph?

Answer 18

Find the gradient at various points along the concentration-time graph and plot them against concentration and draw a line of best fit

Question 19

What is the Rate equation

Answer 19

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

where m and n are the orders of the reaction

K is the rate constant

Question 20

How can you easily work out the rate constant if the overall reaction is first order?

Answer 20

If the overall reaction is first order, then the rate constant is equal to the gradient of the rate-concentration graph of that reactant

Question 21

What is a half-life?

Answer 21

The time for half the reactant to disappear

Question 22

If you know the half-life of a first order reaction, What equation can you use to work out the rate constant?

Answer 22

k = ln2 / t1/2 (half Life)

Question 23

What is the rate-Determining step?

Answer 23

The slowest step in a multi-step reaction and determines the overall rate

Question 24

Explain why temperature change affects the rate constant

Answer 24

Increasing the temperature gives the reactant particles more kinetic energy. This means the particles speed up, so they collide more often

Increasing the temperature also means more reactant particles will have the required activation energy for the reaction, so a greater proportion of the collisions will result in the reaction actually happening

Therefore increasing temperature increases the reaction rate

Question 25

What is the Arrhenius equation>

Answer 25

K = Ae^(-Ea/RT)

Where:
k = rate constant
Ea = activation energy (J mol^-1)
T = Temperature (K)
R = Gas constant (8..314 K^-1 mol^-1)
A = The pre-exponential factor

Question 26

How do you create an Arrhenius plot?

Answer 26

After using the Arrhenius equation plotting lnK against 1/T

Question 27

What can be determined from an Arrhenius plot?

Answer 27

The gradient will be -Ea/R

The y-intercept will be lnA