Kinetics

Question 1

What is kinetics?

Answer 1

Study of factors that affect rates of chemical reaction - how quickly they take place.

Question 2

What must happen for a reaction to occur between molecules between two different gases?

Answer 2

• Need to have collision - Need to have the correct orientation - Must have a minimum amount of energy (Activation Energy)

Question 3

What are the 5 factors affecting the rates of chemical reactions?

Answer 3

• Increasing the temperature - Increasing the concentration of a solution - Increasing the pressure of a gas reaction - Increasing the surface area of solid reactants - Using a catalyst

Question 4

How does increasing the temperature affect chemical reactions?

Answer 4

• Increases the speed of the molecules therefore increasing both their energy and the number of collisions

Question 5

How does increasing the concentration of a solution affect chemical reactions?

Answer 5

• More particles present in a given volume so collisions more likely and the reaction gets faster. As reactions goes on the reactants are used up and their concentration falls. So in most, the rate of reaction drop as the reaction goes on.

Question 6

How does increasing the pressure of a gas reaction affect chemical reactions?

Answer 6

Same effect as increasing the concentration of the solution - more molecules/atoms in a given volume so collisions more likely

Question 7

How does increasing the surface area of solid reactants affect chemical reactions?

Answer 7

• Greater the total SA of a solid, the more of its particles available to collide with molecules in a gas/liquid. Breaking solid lump into smaller pieces increases the rate of reaction as there’s more sites for reaction.

Question 8

How does using the catalyst affect chemical reactions?

Answer 8

• A catalyst is a substance that can change the rate of a chemical reaction without being chemically changes itself. They provide alternate pathway with a lower activation energy.

Question 9

What is activation energy?

Answer 9

• For a collision to take place the molecules must have certain minimum energy, enough to start breaking bond. Ea is the minimum amount of energy to start a reaction.

Question 10

What is a rate of reaction?

Answer 10

The change in concentration of a substance in unit time.

Question 11

What does an exothermic and endothermic reaction look like?

Answer 11

Question 12

What is Maxwell - Boltzmann distribution?

Answer 12

• The energy of a particle in a gas is linked to its speed
• No particles have zero energy
• A few have very high energies
• Average energy is not the same as the most probable
• A plot of energy against the number of particles produces a distinctive shape
• This shape is called a Maxwell - Boltzmann distribution curve

Question 13

For a reaction to take place, a collision between particles must have enough energy to…….

Answer 13

… start breaking bonds (called the activation energy)

Question 14

What does the area under the graph to the right of the Ea line represent?

Answer 14

The number of particles with enough energy to react.

Question 15

Why don’t all reactions that are exothermic occur spontaneously at room temperarure? Give an example.

Answer 15

The need for the Ea to be presnet before a reaction explains this

For example - fuels are mostly safe a room temperature but a small spark may provide enough energy to start the combustion reaction. The heat given out by the initial reaction is enough to supply the Ea for further reactions. Similarly, the chemicals in a match head are quite stable until the Ea is provided by friction

Question 16

In terms of the Maxwell-Boltzmann graph, what is the effect of temperature on reaction rate?

Answer 16

• Higher the temp - peak is lower and moves to the right

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Question 17

What does the total area under the curve represent?

Answer 17

• Total number of particles - same for changes in the temperature

Question 18

What do higher temperatures show about more of the molecules?

Answer 18

• They have energy greater than Ea, so higher percentage of successful collisions will result in reaction
• Small increase in temp produces large increase in the RoR & the number of particles with energy greater than Ea

Question 19

Draw a Maxwell-Boltzmann distribution of the energies of the same number of particles at two temperatures.

Answer 19

Question 20

What is a catalyst?

Answer 20

• Substances that affect the rate that are chemically unchanged at the end

Question 21

Explain the shape of the Maxwell-Boltzmann distribution curve

Answer 21

• Asymmetrical
• Begins at the origin
• Doesn’t tough the x-axis as there’s no upper limit
• Most probable is at the peak
• Area under the curve is the number of particles

Question 22

How does a catalyst work?

Answer 22

They provide a different pathway for the reaction with a lower activation energy.

Question 23

Draw a diagram that shows the effect of a catalyst on a reaction

Answer 23