Topic 9 - Reaction Kinetics

Question 1

Define rate of reaction:

Answer 1

Increasing concentration of a product per time, or decreasing concentration of a reactant per unit time

Question 2

How to calculate rate of reaction

Answer 2

Rate of reaction = change in concentration of a reactant or product/time

Question 3

What is initial rate and how do you calculate it?

Answer 3

Initial rate is the reaction rate at time zero. Change in (P)/t.

Question 4

What is instantaneous rate and how do you calculate it?

Answer 4

The rate at a given time is obtained by measuring the gradient of the tangent drawn to the curve at that time. Change in P/ t

Question 5

What is average rate and how do you calculate it?

Answer 5

Average rate is a measure if the change in the amount or concentration of a reactant or product in a given period of time. Change in P/ t

Question 7

How can you measure the rate of reaction in an experiment that produces H2 (g) in a gas syringe?

Answer 7

It can be investigated by measuring the volume of H2 (g) produced at regular time intervals. Rate = volume of gas produced/ time taken

Question 8

What is collision theory?

Answer 8

The reactant particles must collide with each other pin order for a chemical reaction to occur.

Question 9

Describe the two possible cases of ineffective collision (no product is formed)

Answer 9

Case 1 (High speed, wrong orientation) - If reactant particles do not collide in the right orientation, they may just move apart without forming a product.
Case 2 (Low speed, right orientation) - If reactant particles collide in the right orientation but at a low speed, they may also just move apart without forming a product.

Question 10

Describe the process of effective collision - products are formed

Answer 10

• If reactant particles collide in the right orientation and at a high speed (I.e. with sufficient kinetic energy), they will give the product.
• The minimum energy required for a chemical reaction to occur is the activation energy

Question 11

What is activation energy?

Answer 11

• It is the minimum energy required to start a reaction
• It is an energy barrier that the reactant particles have to overcome before a reaction can occur.
• Any chemical reaction results in the breaking of some bonds (needing energy) and making of new ones (releasing energy). Obviously,some bonds have to be broken before new ones can be made. Activation energy is involved in breaking some of the original bonds.
• Where collisions are relative,y gentle, there’s not enough energy available ale to start the bond-breaking process, and so the particles do not react.

Question 12

What is steric hindrance?

Answer 12

When the shapes of molecules influence reactions, we say that there is a ‘steric factor’ involved in the reaction. In some cases, the atoms in a molecule can hinder the course of a reaction. If an atom or group of atoms is particularly large, then it can get in the way of an attacking species. Meaning, the reaction suffers from steric hindrance.

Question 13

How can increasing the concentration of solution increase the rate of reaction?

Answer 13

Provided a higher concentration of solution, more reacting particles in unit volume, distance between particles is shorter which increases chance of collision between particles. Therefore, a faster rate can be found.

Question 14

How can increasing the pressure of gases increase the rate of reaction?

Answer 14

Providing a higher pressure, volume decreases and so there’s more particles that can react per unit volume. This decreases the distance between particles and increases the chances of collision between particles, therefore a faster rate can be found.

Question 15

How can the surface are of a solid affect the rate of reaction?

Answer 15

The rate is higher if the solid is in a powder form rather than lump form (considering they have the same mass). Powder has a larger surface area and so a higher chance of collision between particles. Therefore, a faster rate can be found.

Question 16

What does the Maxwell-Boltzmann distribution curve show?

Answer 16

It shows the range of kinetic energies that the particles in a gaseous system possess at a particular temperature.

Question 17

What are the characteristic features of the Maxwell-Boltzmann distribution curve

Answer 17

• The area under the curve represents the total number of particles in the system
• There’s no maximum energy for a particle
• The kinetic energy corresponding to the maximum point on the curve represent the kinetic energy possessed by the largest number of particles. This is known as the most probable kinetic energy
• The curve is asymmetric. The average kinetic energy is not at the maximum point of the curve. It lies on the right of the maximum point.
• The shaded area under the curve shows the number of particles having energy equal to or greater than the activation energy. Only a small fraction of particles have sufficient energy to react at that particular temperature.

Question 18

Explain the relationship between temperature and reaction rate using Maxwell-Boltzmann distribution curve

Answer 18

• The area under the curve remains the same
• The number of particles in the system does not change when temperature increases
• At a higher temperature p, the average kinetic energy is higher and the maximum point of the curve shifts to the right and gets lower.
  This shows that:
• The distribution of kinetic energies has widened
• The average kinetic energy of the particles has increased
• The number of particles with very high kinetic energies has increased
• The shaded area under the curve T2 is larger than T1. Meaning the total number of particles with sufficient energy to react is greater at a higher temperature (T2).

Question 20

What are the two ways to make more particles have a higher energy than activation energy, thus increasing the reaction rate?

Answer 20

• Increasing temperature of the particles and so more particles possess an energy greater than activation energy
• Lower the activation energy - more particles at the original temperature possess an energy greater than the new activation energy. This can be done by using a catalyst.

Question 21

Define catalyst

Answer 21

They work by providing an alternative pathway for the reaction to follow that has a lower activation energy.

Question 22

What are two major economic advantages of using catalysts in industry?

Answer 22

• To increase the rate of a chemical reaction meaning that more of the desired product can be made in a given time
• To proceed a reaction at a lower temperature, therefore decrease in energy costs to the manufacturer

Question 23

What is a heterogenous catalyst?

Answer 23

• Is a catalyst that is in different phase from the reacts i.e. in Hager process, gasses are passed over solid iron catalyst
• The reaction happens on the surface of the catalyst
• Increasing the surface area of the catalyst increases the number of molecules that can react the same, increasing the rate of the reaction.
• The catalyst lowers activation energy by providing an alternative pathway

Question 24

What are the economic benefits of the use of catalysts in industrial reactions?

Answer 24

• Lower productions costs
• Form more products in a shorter time
• Use milder conditions so less energy and money are used
• Speed up reactions

Question 25

What is a homogenous catalyst?

Answer 25

- The catalyst is in the same physical state as the reactants - Usually an aqueous catalyst for a reaction between two aqueous solutions - Reactants combine with the catalyst to make an intermediate species, which the reactants form the products and reform the catalyst