Chapter 18 - Rates of reactions

Question 1

For consistency, what do chemists measure rates of reaction as in a chemical equation?

Answer 1

Rate= (change in concentration)/ (change in time)

Question 2

How can orders of reactions be found?

Answer 2

Orders of reaction must be determined experimentally by monitoring how a physical quantity changes over time. Orders cannot be found directly from the chemical equation.

Question 3

What is the initial rate?

Answer 3

The initial rate is the instantaneous rate at the beginning of an experiment when t=0.

Question 4

What is continuous monitoring, use concentration-time graphs in your example?

Answer 4

Concentration-time graphs can be plotted from continuous measurements taken during the course of the reaction. This is called continuous monitoring.

Question 5

What are the two methods for continuous monitoring of reactions that produce gas as one of the products?

Answer 5

a. Monitoring by gas collection.

b. Monitoring by mass loss.

Question 6

How can you monitor the rate with a colorimeter?

Answer 6

1. In a colorimeter, the wavelength of the light passing through a coloured solution is controlled using a filter. The amount of light absorbed by a solution is measured.
2. A colorimeter measures the intensity of light passing through a sample. The filter is chosen so that is the complementary colour to the colour being absorbed in the reaction. Absorbance is recorded, which is directly linked to the concentration of the solution.

Question 7

How would you analyse by colorimetry for the reaction between propanone, iodine and an acid catalyst, note as the reaction proceeds iodine is used up and it’s orange/brown colour fades and the absorbance of the colour is measured precisely by the colorimeter?

Answer 7

a. Prepare standard solutions of known concentration of the coloured chemical, iodine, in this reaction.
b. Select a filter with the complementary colour of the coloured chemical. For iodine, this would be a green/blue filter, but the colorimeter will usually tell you which setting to use.
c. Zero the colorimeter with water.
d. Measure the absorbance readings of the standard solutions of iodine.
e. Plot a calibration curve of absorbance against iodine concentration. You know have a way of converting an absorbance reading into a concentration of iodine.
f. Carry out the reaction between propanone and iodine. Take absorbance readings of the reaction mixture at measured time intervals.
g. Use the calibration curve to measure the concentration of iodine at each absorbance reading.
h. Finally, plot a concentration-time graph of concentration of iodine against time. From the graph, you can determine the order of reaction with respect to the coloured chemical (in this case iodine).

Question 8

What does the gradient of a concentration-time graph tell you?

Answer 8

The rate of the reaction.

Question 9

What restriction must you have to obtain the order with respect to a reactant in a reaction?

Answer 9

The order with respect to a reactant can only be obtained if all other reactant concentrations remain effectively constant.

Question 10

What does a zero-order reaction look like on a concentration-time graph and how can you find the value of the rate constant K?

Answer 10

A zero-order reaction produces a straight line with a negative gradient. The reaction rate does not change at all during the course of the reaction. The value of the gradient is equal to the rate constant K.

Question 11

What does a first order reaction look like on a concentration-time graph and how can the rate constant K be determined and what is a property all first order concentration-time graphs have?

Answer 11

A first order reaction produces a downward curve with a decreasing gradient over time. As the gradient decreases with time, the reaction gradually slows down. The time for the concentration of the reactant to halve is constant. This time is called the half-life and the rate constant can be determined using the half-life. K=(ln2)/t1/2. Also, a tangent to the curve can be drawn at a particular concentration, where the gradient is the rate of reaction and so K can be deduced using the rate equation.

Question 12

What does a second order reaction look like on a concentration-time graph?

Answer 12

The graph is also a downward curve but steeper at the start and tailing off more slowly when compared to a first order.

Question 13

What is a half-life?

Answer 13

Half-life is the time taken for the concentration of a reactant to decrease to half its original value.

Question 14

What is exponential decay?

Answer 14

First order reactions have a constant half-life with the concentration halving each half-life. This pattern is called exponential decay.

Question 15

How can rate-concentration graphs be plotted?

Answer 15

Rate-concentration graphs can be plotted from measurements of the rate of reaction at different concentrations. They are very important as they offer a route into the direct link between rate and concentration in the rate equation.

Question 16

How does a zero-order reaction with respect to a reactant look on a rate-concentration graph and how can the rate constant k be determined?

Answer 16

A zero-order reactant produces a horizontal straight line with zero gradient. The intercept on the y-axis gives the rate constant k, the reaction rate does not change with increasing concentration.

Question 17

How does a first order reaction with respect to a reactant look like on a rate-concentration graph and how can the rate constant k be determined?

Answer 17

A first order reactant produces a straight-line graph through the origin as rate is directly proportional to the concentration of the reactant. The rate constant k can be determined by measuring the gradient of the straight line of the graph.

Question 18

How does a second order reaction with respect to a reactant look like on a rate-concentration graph and how can the rate constant k be determined?

Answer 18

A second order reactant produces an upward curve with increasing gradient. As this rate concentration graph is a curve, the rate constant cannot be obtained directly from this graph. By plotting a second graph of the rate against the concentration squared, the result is a straight line through the origin. The gradient of this straight-line graph is equal to the rate constant K.

Question 19

How can a log(rate) against log(concentration) graph be used to determine both the order and the rate constant k?

Answer 19

a. Let’s say the rate equation for a reactant A is rate=k[A]^n.
i. Log(rate) = n log[A] + log(k).
ii. Log(rate) = y
iii. n = m
iv. log[A] = x
v. log(k) = c; this relationship follows a y=mx+c equation where the gradient gives the order n and the y-intercept gives log(k), from which k can be calculated.

Question 20

What is a more convenient way of obtaining the initial rate of a reaction by taking a single measurement as opposed to drawing a tangent at t=0 on a concentration-time graph and working out the initial rate?

Answer 20

The clock reaction, the time t from the start of an experiment is measured for a visual change to be observed often a colour or a precipitate. Provided that there is no significant change in rate during this time, it can be assumed that the average rate of reaction over this time will be the same as the initial rate. The clock reaction is repeated several times with different concentrations and values of 1/t are calculated for each experimental run.

Question 21

In the clock reaction, what is the initial rate proportional to?

Answer 21

The initial rate is proportional to 1/t.

Question 22

Describe how the iodine clock reaction works?

Answer 22

a. Firstly, it relies on the formation of iodine, as aqueous iodine is coloured orange-brown, the time from the start of the reaction and the appearance of the iodine colour can be measured. Starch is usually added since it forms a complex with iodine which is an intense dark blue-black colour.
b. Separate experiments are carried out using different concentrations of one of the reactants and all other concentrations are kept constant. The colour change is delayed by including a small amount of another chemical (e.g., aqueous sodium thiosulfate) which actually removes of iodine as it forms.
c. In each experiment, the solution is colourless at the start and the time t is measured for the blue-black colour of the starch-iodine to appear.
d. The initial rate is proportional to 1/t.
e. A graph of 1/t (which is proportional to rate) against concentration is then plotted.
f. Further series of experiments are then carried out in which the concentration of one of the other reactants is changed keeping the other reactant constant. The order with respect to each reactant can then be determined from the shape of a rate-concentration graph and the rate equation can then be written.

Question 23

What restriction on clock reactions is there to make sure the initial rate approximation is a reasonably accurate initial rate?

Answer 23

Provided that less than 15% of the reaction has taken place, the initial rate measured during a clock reaction is still reasonably accurate. Therefore, the accuracy of a clock reaction is dependent in the time measured.

Question 24

Why is it unlikely that a chemical reaction involving more than two reactants takes place in a single step?

Answer 24

A reaction can only take place when particles collide, it is unlikely that more than two particles will collide together at the same time. Also, the stoichiometry in the rate equation does not match the overall equation.

Question 25

What is a reaction mechanism?

Answer 25

The series of steps that make up an overall reaction is called the reaction mechanism.

Question 26

What is a rate-determining step?

Answer 26

The steps in a multi-step reaction will take place at different rates. The slowest step in the sequence is called the rate-determining step.

Question 27

How can you ensure whether a reaction mechanism is likely to be correct?

Answer 27

a. The rate equation only includes reacting species involved in the rate-determining step.
b. The orders in the rate equation match the number of species involved in the rate determining step.

Question 28

When temperature increases, what factors affect the rate and the rate constant and why?

Answer 28

a. Increasing the temperature shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy.
b. As the temperature increases, particles move faster and collide more frequently. These factors increase both the rate and the rate constant of a reaction.
c. To react particles must also collide with the correct orientation.
d. With increasing temperature, the increased frequency of collisions is comparatively small compared with the increase in the proportion of molecules that exceed activation energy from the shift in the Boltzmann distribution. So, change in rate is mainly determined by Ea.

Question 29

How can an experiments rate constant be calculated?

Answer 29

By carrying out the same experiment at different temperatures, rate constants can be calculated at different temperatures.

Question 30

What is the Arrhenius equation?

Answer 30

a. K=Ae^(-Ea/RT)

b. R= gas constant = 8.314

Question 31

What does the exponential factor and the pre-exponential term represent in the Arrhenius equation?

Answer 31

a. The exponential factor [e^(-Ea/RT)] represents the proportion of molecules that exceed Ea and that have sufficient energy for a reaction to take place.
b. The pre-exponential term takes into account the frequency of collisions with the correct orientation. This term does increase slightly with temperature as the frequency of collisions increases but it is essentially constant over a small temperature range. The frequency factor essentially gives the rate if there were no activation energy.

Question 32

What does the logarithmic form of the Arrhenius equation look like?

Answer 32

a. ln(k)=-Ea/RT + ln(A)
b. The gradient is equal to -Ea/R.
c. The y intercept is equal to ln(A).