3.1.5 - Kinetics

Question 1

What is collision theory?

Answer 1

Reactions can only occur when collisions take place between particles that have sufficient energy.

Question 2

Define activation energy.

Answer 2

The minimum energy which particles need to collide to start a reaction.

Question 3

What does the Maxwell-Boltzmann distribution show?

Answer 3

The spread of energies that molecules of a gas have at a particular temperature.

Question 4

How can a reaction go to completion if few particles have energy greater than E_a?

Answer 4

Particles can gain energy through collisions.

Question 5

Describe how the MB distribution curve will change when you increase temperature.

Answer 5

Total area should remain constant. Molecules have a wider range of energies than at lower temperatures. Both E_mp and mean energy have higher energies.

Question 6

Define rate of reaction.

Answer 6

The change in concentration of a substance in unit time.

Question 7

Explain how increasing concentration affects the rate of reaction.

Answer 7

More particles per unit volume, more frequent collisions, R.o.r increases.

Question 8

How does the MB curve change if you double concentration/pressure?

Answer 8

Peak is at the same energy. Area will be doubled as there are 2x the number of particles. More particles have energy > E_a (not a greater proportion).

Question 9

What is the effect of increasing temperature on the rate of reaction?

Answer 9

Particles have increased kinetic energy, many more particles have E >= E_a, many more successful collisions, significant increase in rate of reaction.

Question 10

How does increasing surface area affect the rate of reaction?

Answer 10

More particles exposed, more frequent collisions, R.o.r increases.

Question 11

What is the definition of a catalyst?

Answer 11

Substances that increase reaction rates without getting used up.

Question 12

How do catalysts work?

Answer 12

They provide an alternative route/mechanism with a lower activation energy.

Question 13

Why does lowering activation energy increase the rate of reaction?

Answer 13

More particles have energy above E_a, higher frequency of effective collisions.

Question 14

Define catalyst.

Answer 14

A substance that increases the rate of reaction by finding an alternative pathway with a lower activation energy and is chemically unchanged at the end of a reaction.

Question 15

State and explain what must be done to each sample before it’s titrated with alkali.

Answer 15

Stop the reaction, by cooling.

Question 16

Give 3 ways in measuring the rates of reaction.

Answer 16

1. Timing how long a precipitate takes to form. 2. Measuring a decrease in mass. 3. Measuring the volume of gas given off.

Question 17

Formula for the Mr.

Answer 17

Mr = mass/moles.

Question 18

Explain how you heat in a crucible.

Answer 18

Refer to the image for the procedure.

Question 19

Explain the use of gas syringes.

Answer 19

Refer to the image for the procedure.

Question 20

What is activation energy in the context of collision theory?

Answer 20

Activation energy is the minimum energy that particles need to collide with each other to start a chemical reaction.

Question 21

Describe the Maxwell-Boltzmann distribution curve.

Answer 21

The curve shows the spread of energies among molecules at a specific temperature, never touching the x-axis as there’s no maximum energy.

Question 22

How does increasing temperature affect the Maxwell-Boltzmann distribution?

Answer 22

Increasing temperature shifts the distribution curve rightward, indicating more molecules with higher energies.

Question 23

How does increasing the concentration affect reaction rates according to collision theory?

Answer 23

Higher concentration increases the frequency of particle collisions, leading to more effective collisions and a higher reaction rate.

Question 24

How is the rate of a reaction measured experimentally?

Answer 24

Reaction rate can be calculated from the gradient of a concentration vs. time graph, typically at the beginning of the reaction for the initial rate.

Question 25

What role do catalysts play in chemical reactions?

Answer 25

Catalysts provide an alternative pathway for the reaction with a lower activation energy, increasing the frequency of effective collisions without being consumed in the reaction.

Question 26

How does temperature affect the rate of a chemical reaction?

Answer 26

Higher temperatures increase the kinetic energy of particles, leading to more frequent and energetic collisions that surpass the activation energy.

Question 27

Describe how the rate of reaction is measured in the sodium thiosulfate and hydrochloric acid experiment.

Answer 27

The rate is approximated by the time it takes for a cross beneath the reaction mixture to disappear due to cloudiness from sulfur production.

Question 28

What factors commonly affect the rate of a chemical reaction?

Answer 28

The common factors include concentration or pressure, surface area, temperature, and the presence of a catalyst.

Question 29

How do catalysts lower activation energy according to collision theory?

Answer 29

Catalysts provide an alternative reaction pathway with a lower activation energy.

Question 30

What changes occur in the Maxwell-Boltzmann distribution curve at higher temperatures?

Answer 30

At higher temperatures, the peak of the curve flattens and shifts to the right, indicating that a higher proportion of molecules have energies exceeding the activation energy.

Question 31

Explain why most molecules do not react despite having sufficient energy as per the Maxwell-Boltzmann distribution.

Answer 31

Successful reactions require proper orientation and collision timing, which are not always achieved in every collision.

Question 32

How does kinetic theory explain the effect of temperature on reaction mechanisms?

Answer 32

Higher temperatures increase the average kinetic energy of molecules, thus increasing their velocity and the frequency of collisions.

Question 33

Why is understanding reaction kinetics important for industrial chemical processes?

Answer 33

Knowledge of reaction kinetics helps in optimizing conditions for maximum yield, efficiency in chemical manufacturing, ensuring safety by controlling reaction rates, and scaling reactions from lab to industrial scale.

Question 34

Define the rate of reaction.

Answer 34

The change in concentration of a substance in unit time.

Question 35

Describe the initial rates method for determining reaction kinetics.

Answer 35

The initial rates method involves measuring the rate of reaction at the very beginning (initial rate) under different concentrations of reactants to determine how rate varies with concentration.

Question 36

Compare the activation energy with and without a catalyst for the same reaction.

Answer 36

A catalyst lowers the activation energy needed for a reaction to occur, represented by a lower peak on an energy profile diagram, leading to a higher rate of reaction by allowing more molecules to possess sufficient energy to react.

Question 37

How does an increase in temperature modify the reaction mechanism at the molecular level?

Answer 37

Increased temperature boosts the kinetic energy of molecules, resulting in more frequent and energetic collisions. This can lead to new reaction pathways becoming accessible, affecting the reaction mechanism.

Question 38

What role does activation energy play in determining the rate of a chemical reaction?

Answer 38

Activation energy is the energy barrier that must be overcome for reactants to transform into products. A lower activation energy increases the rate of reaction by allowing more reactant molecules to have sufficient energy to react.

Question 39

List factors that can alter the activation energy of a chemical reaction.

Answer 39

Factors include the presence of a catalyst, which lowers activation energy, and changes in molecular structure or bonding within reactants that might either increase or decrease the energy barrier.

Question 40

Provide examples of real-world applications of chemical kinetics.

Answer 40

Chemical kinetics are crucial in pharmaceuticals for optimising drug reactions, in environmental science to understand pollutant degradation, and in materials science for developing faster curing processes.

Question 41

State why the Maxwell-Boltzmann curve starts at the origin.

Answer 41

The curve starts at the origin because there are no molecules with zero energy.

Question 42

Describe the new distribution of molecular energies when half of the gas molecules are removed but temperature remains constant.

Answer 42

The peak of the curve remains at the same energy, but the overall area under the curve is halved, reflecting the reduced number of molecules.

Question 43

Suggest why small amounts of reactants are used in experiments involving toxic gases.

Answer 43

To minimise the production and hence the risk associated with toxic gases such as sulfur dioxide, which could be hazardous to health.

Question 44

General explanation of how catalysts increase the rate of a reaction.

Answer 44

Catalysts provide an alternative pathway with a lower activation energy for the reaction, thereby increasing the rate by allowing more molecules to have sufficient energy to react.

Question 45

Explain the effect of increasing the concentration of hydrogen peroxide on the rate of its decomposition reaction.

Answer 45

Increasing the concentration increases the frequency of effective collisions among reactant molecules, thereby increasing the rate of reaction.

Question 46

Describe the effect of lowering the temperature on the rate of a chemical reaction.

Answer 46

Lowering the temperature decreases the average kinetic energy of molecules, resulting in fewer molecules having energy equal to or greater than the activation energy. This reduces the frequency of successful collisions and, consequently, slows down the reaction rate.

Question 47

Deduce changes to reaction conditions from a Maxwell-Boltzmann distribution curve and explain their impact on the rate of reaction.

Answer 47

A shift in the curve towards lower energies indicates a decrease in temperature or concentration. This change reduces the number of molecules with sufficient energy to overcome the activation energy barrier, leading to fewer effective collisions and a slower reaction rate.

Question 48

How can a graph of concentration vs. time indicate the order of reaction with respect to a reactant?

Answer 48

A graph showing a constant rate of decrease in concentration over time suggests a zero-order reaction with respect to that reactant, as the rate is independent of its concentration.

Question 49

Explain how changes in reaction conditions, such as temperature or catalyst presence, affect the molecular distribution and reaction rate.

Answer 49

Higher temperatures shift the Maxwell-Boltzmann distribution to higher energies, increasing the number of molecules above the activation energy and speeding up the reaction. Catalysts lower the activation energy required, allowing more molecules to participate in the reaction even at lower energies.

Question 50

Describe an experimental procedure to determine the reaction order with respect to a reactant using colorimetry.

Answer 50

Measure concentrations of the reactant at regular intervals using a colorimeter. Start the reaction by mixing known concentrations of reactants, record colorimetric data over time, and analyze the initial rates at different concentrations to determine the reaction order by plotting and interpreting the data appropriately.