Chapter 6: Chemical Kinetics

Question 1

What is the rate of a chemical reaction?

Answer 1

The speed at which reactants are used up or products formed

The change in concentration of reactants or products per unit time

How fast or slow the reaction is.

This can be thought of in terms of how quickly the reactants are used up or how quickly the products are formed

Question 2

What are two ways in which the rate of reaction can be measured?

Answer 2

1. measurement of the rate at which CO₂ is produced
2. measurement of the rate at which the mass decreases

Question 3

What are possible problems with experiments using a bung on a flask?

Answer 3

The gas is likely to escape and the pieces (e.g. calcium carbonate pieces) variating in size

Question 4

How can the average rate or the gradient be measured?

Answer 4

Question 5

If a reaction is strongly exothermic what will happen to the temperature of the mixture and the rate?

Answer 5

Reaction mixture becomes hotter during the experiment

Cause rates to be higher

Question 6

What does the cotton wool do in this experiment?

Answer 6

It allows the gas to escape but stops the mass being lost as a result of splashes

Question 7

What is the rate of reaction?

Answer 7

the change in concentration of reactants or products per unit time

Question 8

What colour is iodine?

Answer 8

Brown

Question 9

How does the colour change occur?

Answer 9

Iodine is brown and all the other species are colourless
the reaction mixture fades from brown to colourless as iodine is used up in the reaction and the decreased in the absorption of light can be meausred using the colorimeter

Question 10

What can a colorimeter be used to measure?

Answer 10

the amount of light absorbed at a particular wavelength - the darker the colour of the sample, the more light is absorbed

Question 11

What is stated in the collision theory? What are the two conditions that must be fulfilled for a reaction?

Answer 11

For a reaction to occur particles must collide

• A collision must involve more than a certain minimum amount of energy
• Molecules must collide with the correct orientations

Question 12

Do all collisions result in a reaction?

Answer 12

No because they must also have a minimum amount of energy and collide in the correct orientations

Question 13

What is activation energy?

Answer 13

The minimum amount of energy that colliding particles must possess to result in a reaction

Question 14

What is called when a collision result in a reaction?

Answer 14

Successful or effective collision

Question 15

What is the highest point along the curve called?

Answer 15

Transition state

Question 16

What is this graph?

Answer 16

a potential energy profile, showing the activation energy for an exothermic reaction

Question 17

What is happening here in the reaction between two particles, A and BC?

Answer 17

• as the two particles approach, repulsion between the atoms (internuclear and beween electons) causes an increase in the potential energy
• The B-C bond begins to break and the A-B bond begins to form
• At the transition state (activated complex) along the curve, all three atoms are joined together by partial bonds (A- - B - - C)
• As the A-B bond continues to form, the potnetial energy falls (bond-making releases energy)

Question 18

What are the factors affecting reaction rate?

Answer 18

• Concentration of reactants
• Pressure for (reactions involving gases)
• Surface area of solid reactants
• Temperature
• Catalysis

Question 19

What is the effect of concentration on the reaction rate?

Answer 19

With more particles in a certain volume, the particles collide more often (the collision frequency is higher) and therefore there is a greater chance of a successful collision (i.e. one that results in a reaction) occurring in a certain time (need to have an idea of time in answer)

Question 20

What is the effect of pressure on the reaction rate?

Answer 20

The effect of increasing the pressure is essentially the same as that of increasing the concentration of gaseous reactants. As the pressure is increased, the collision frequency increases

Only reactions involving gases are significantly affected by changing the pressure

Question 21

What is the effect of surface area of solid reactants on reaction rate?

Answer 21

Reactions generally only occur at the surface of a solid.

Making a solid more finely divided increases the surface area and therefore the number of particles exposed at the surface → greater chance of a particle of the other reactant colliding with a particle on the surface and reaction occuring

Question 22

What is the relationship between temperature and the energy of particles in a gas?

Answer 22

For an ideal gas: The average kinetic energy of the particles in a gas is proportional to its temperature in Kelvin

Question 23

What is the effect of temperature on rate of reaction?

Answer 23

As the temperature increases, the rate of reaction increases exponentially

As temperature increases, the molecules have more energy and therefore move faster. This means that the collision frequency increases.

Not only do they collide more often, but they also collide harder, that is, with more energy, so that there is a greater chance of a successful collision

Question 24

At the same temperature, do lighter or heavier particles travel faster?

Answer 24

light particles travel faster

Question 25

What does the Maxwell-Boltzmann distribution look like?

Answer 25

Question 26

What are 4 features of the Maxwell-Boltzmann distribution?

Answer 26

• it is not symmetrical
• no molecules have 0 kinetic energy
• at higher energy the line does not reach the energy axis
• the area under the curve represents the total number of particles and will not change as the temperature changes

Question 27

What is the main reason that the rate of reaction increases with temperature?

Answer 27

there is an increase in the number of particles with energy greater than or equal to the activation energy

Question 28

How does the temperature affect the Maxwell-Boltzmann distrbution?

Answer 28

At higher temperatures
* the curve is flatter
* the maximum has moved to the right
* there are fewer particles with lower energy and more particles with higher energy

with more particles having energy greater or equal to the activation energy at the higher temperature, a greater proportion of collisions will be successfull, and therefore the rate of reaction will increase

Question 29

Why is the area underneath the curve still the same?

Answer 29

because the number of particles (amount of substance) does not change if the temperature is increased

Question 30

What is a catalyst?

Answer 30

A catalyst is a substance that increases the rate of a chemical reaction without itself being used up in the reaction

It acts by allowing the reaction to process by an alternative pathway of lower activation energy

Question 31

Why is the catalyst often written abov e the reaction arrow?

Answer 31

because it does not appear in the chemical equation because it does not change in the reaction

Question 32

Why are catalysts important in many industrial processes?

Answer 32

Without the catalyst, the reaction occurs very slowly, but it is very rapid once the catalyst has been added

i.e. manganese (IV) oxide in the decomposition of hydrogen peroxide solution

Question 33

What is the effect of a catalyst on the Maxwell-Boltzmann distrubtion?

Answer 33