Kinetics

Question 1

What is kinetics?

Answer 1

The experimental study of reaction rates and factors affecting rates.

Question 2

What are some of the applications of kinetics?

Answer 2

Cooking food and food preservation, farming and growth of crops, construction, archaeological dating, fuels, the chemical industry and reaction mechanism studies

Question 3

What is the definition of rate of reaction?

Answer 3

The rate of a chemical reaction is the change in concentration of reactant or product per unit time (units are usually moldm^-3 s^-1)

Question 4

What is the explanation of the shape of the graph of concentration of reactant against time?

Answer 4

Gradient greatest at the start as this is when the most reactant is available and concentration of reactants are known, then gradient decreases as the concentration of reactants decreases until the gradient becomes 0 when all the reactants have been used up

Question 5

How can the instantaneous rate of reaction be found from a graph of reactant concentration against time or product concentration against time?

Answer 5

Draw tangents to the curve at the point where the rate is being found and calculate the gradient

Question 6

What are the requirements of collision theory?

Answer 6

• The first requirement is for molecule A to collide with molecule B.
• The second requirement is that the molecules need to collide with sufficient energy to cause a reaction. Collisions that cause the reaction to occur have to be high energy in order to break covalent bonds and start the reaction

Question 7

What is the general way to increase rate of reaction?

Answer 7

Increase the number of collisions per second - the collision frequency - of molecules with sufficient energy to react

Question 8

What is a homogeneous reaction?

Answer 8

A reaction that takes place in a single state, e.g. all species present are gases, or all species are in the same solution

Question 9

How does changing the pressure affect the rate of reaction and why?

Answer 9

An increase in pressure at constant temperature increases the reaction rate of gas reactions, as there are more gas particles per unit volume. Kinetic energy of the particles does not change, so proportion of collisions does not change, but there are more collisions per second, so rate increases.
There is no affect on rate of reaction between solids, liquids or solutions

Question 10

How does changing the concentration affect the rate of reaction and why?

Answer 10

For reactions in solution, an increase in concentration means that the frequency of collisions between solute molecules is increased as an increased concentration means more solute particles in a given volume.

Question 11

How does particle size affect rate of reaction?

Answer 11

In heterogeneous systems involving a solid, a larger surface area of the solid gives a faster rate of reaction.
Powder < granules < lumps/chips

Question 12

How does temperature affect rate of reaction?

Answer 12

Increased temperature always increases rate of reaction because the molecules have higher kinetic energy so collide more frequently but it also means that a greater fraction of the molecules have the energy necessary to react on collision/energy greater than the activation energy which is the main reason that rate increases.

Question 13

What does the Maxwell-Boltzmann distribution curve look like?

Answer 13

Asymptotic, bell shaped curve which starts at the origin but does not touch the x axis at the right hand side. The activation energy can be marked on a point on the curve with a vertical line and the area under the graph at the right of this point gives the fraction of molecules with energy greater than the activation energy.
Plotted with “fraction of molecules with a given energy” on y axis, and “energy” on x axis

Question 14

How does increasing temperature change the Maxwell-Boltzmann distribution curve?

Answer 14

It moves the peak to the right hand side and lower. Area under the graph after the activation energy will increase

Question 15

What is activation energy?

Answer 15

Activation energy is defined as the minimum combined energy that molecules must have on collision for a reaction to occur

Question 16

What is a catalyst?

Answer 16

A catalyst is substance that alters the rate of a chemical reaction but remains permanently chemically unchanged at the end of the reaction

Question 17

How does a catalyst work?

Answer 17

A catalyst speeds up the rate of reaction by providing an alternative route of lower activation energy.

Question 18

How does the addition of a catalyst change the Maxwell-Boltzmann distribution curve?

Answer 18

The activation energy moves to the left on the x axis, meaning there is a greater area under the curve, indicating a greater fraction of molecules with energy > activation energy

Question 19

How does an energy reaction profile diagram illustrate the difference between a catalysed and uncatalysed reaction?

Answer 19

The line on an uncatalysed reaction goes straight from reacts to products with a high activation energy, but there is a lower line just above the reactants line with a catalysed reaction which allows for a lower activation energy, and the line goes from reactants to that line to products, showing that, although the route is longer, the activation energy is lower

Question 20

What are the economic advantages of catalysts?

Answer 20

• Increase the rate of a chemical reaction, allowing more of a desired product to be made in a given time
• Allow reactions to be performed effectively at lower temperatures, saving energy and money

Question 21

What is a phase?

Answer 21

A phase is one of the 3 states of matter, solids, liquids and gases. Chemical systems often have more than 1 phase. Each phase is distinct but doesn’t need to be pure; a solid in equilibrium with its saturated solution is a 2 phase system.

Question 22

What is a heterogeneous catalyst?

Answer 22

One that is in a different phase from the reactants. Generally it is a solid, while reactants are gases or in solution

Question 23

What is the benefit of a heterogeneous catalyst?

Answer 23

It can be easily removed from the product when a homogeneous catalyst cannot as easily

Question 24

How can a rate of reaction be estimated experimentally?

Answer 24

• Measuring the time for a certain amount of the mixture to react
• Measuring the time for a certain amount of product to be formed
  The assumption is then made that 1/time is a measure of the rate - although this is not strictly true as the rate will decrease from the moment reactants are mixed

Question 25

How can production of a gas be used to measure rate of reaction?

Answer 25

The time taken to produce a certain volume of gas (collected over water or in a gas syringe) can be measured.
A reduction in mass can also be measured (using a conical flask with cotton wool to stop solution spraying out while allowing gas to be released on a mass balance)

Question 26

What is the issue with measuring loss in mass to calculate rate of reaction if the gas given off is hydrogen?

Answer 26

Hydrogen gas has a very low Mr and so any mass decrease will be very small and difficult to measure

Question 27

How can production of a solid be used to measure rate of reaction?

Answer 27

The time taken to produce enough solid to hide a cross on a piece of paper underneath the apparatus.
An example of this is the reaction between hydrochloric acid and aqueous sodium thiosulphate where a yellow white precipitate of sulfur is formed