9: Kinetics I

Question 1

What factors do the rate of a chemical reaction depend upon

Answer 1

• Collision frequency
• Collision energy
• Activation energy

Question 2

What does it mean by successful collisions

Answer 2

• Particles collide with the correct orientation
• Particles collide with sufficient energy for the reaction to occur

Question 3

What is collision frequency

Answer 3

The number of collisions between particles per unit of time in a system in a system

Question 4

How can collision frequency be altered

Answer 4

• Changing concentration of reactants
• Changing total pressure
• Changing temperature
• Changing size of reacting particles

Question 5

How does changing the concentration of the reactants affect collision frequency

Answer 5

Increasing the concentration means more particles available to react in the same volume/space leading to more frequent, successful collisions

Question 6

How does changing the total pressure affect collision frequency

Answer 6

Increasing the pressure means that there will be the same number of particles but in a smaller volume leading to more frequent, successful collisions

Question 7

How does changing the temperature affect collision frequency

Answer 7

Increased temperature increases the kinetic energy of the reacting particles resulting in more frequent, successful collisions

Question 8

How does changing the size of the reacting particles affect collision frequency

Answer 8

Decrease in particle size increases the surface area of the reactants meaning there are more particles available to react in the same volume/space leading to more frequent, successful collisions

Question 9

What are unsuccessful collisions

Answer 9

Collisions that don’t result in a reaction (instead the particles bounce off each other)

Question 10

What causes unsuccessful collisions

Answer 10

• Colliding species don’t have enough energy to break the necessary bonds
• Not sufficient energy to result in a chemical reaction

Question 11

Define activation energy

Answer 11

The minimum amount of energy needed to be overcome in order for a reaction to take place

Question 12

What is the activation energy profile for exothermic reactions

Answer 12

Question 13

What is the activation energy profile of endothermic reactions

Answer 13

Question 14

How can the success of a reaction be measured

Answer 14

By measuring the rate of reaction via:
- The amount of reactant lost
- The amount of product formed
- Time taken for precipitate to form/colour change

Question 15

What are common techniques to measure the rate of reaction

Answer 15

• Mass loss
• Gas production
• Concentration changes

Question 16

Describe how to measure rate of reaction by mass loss

Answer 16

• If gas is produced, the mass of the reaction will decrease
• Mass is measured every few seconds, and mass over time is plotted
• Mass can also be measured by difference
• Mass loss provides a measure of the amount of reactant
• Limitation is mass must be sufficiently dense, otherwise change in mass is too small

Question 17

Describe how to measure rate of reaction by volume of gas produced

Answer 17

• Gas produced can be trapped and volume produced over time can measure the rate of reaction
• Gas collection can be done over water or with a gas syringe
• Volume measured every few seconds
• Limitations are gas may be soluble in water, or the gas isn’t dense enough to move the syringe

Question 18

Describe how to measure the rate of reaction by concentration changes

Answer 18

• Titrations (when quenching reaction) can be used
• More efficient is stopping timer when a visible point is reached (e.g. colour change)
• Sodium thiosulphate and HCl form a yellow precipitate when reacted
• Limitation is the human error of observing when the change has happened

Question 19

What is a Maxwell-Boltzmann distribution curve

Answer 19

A graph that shows the distribution of energies at a certain temperature

Question 20

What does this distribution curve show

Answer 20

• A sample of gas showing that some particles have low energy, some have high energy, and most have an energy in between
• The activation energy is also drawn
• Only the particles that meet the activation energy are able to have successful collisions and react

Question 21

How is the Maxwells-Boltzmann curve changed in an increase of temperature

Answer 21

• Increased temperature means particles gain more energy, and have more frequent successful collisions
• As the proportion of successful collisions increases, it results in a higher proportion of particles with the activation energy to cause the reaction
• This causes the curve to flatten and the peak shifts to the right

Question 22

Define a catalyst

Answer 22

A catalyst is a substance that increases the rate of reaction by providing the reactants with an alternative reaction pathway which has a lower activation energy than the usual reaction pathway, and the catalyst isn’t included/used up in the reaction

Question 23

Define a homogenous catalyst

Answer 23

A catalyst in the same phase as the reactants (e.g. reactants and catalysts all in solution)

Question 24

Define a heterogenous catalyst

Answer 24

A catalyst that is in a different phase to the reactants (e.g. reactants are gases but the catalyst is solid)

Question 25

Describe the process of production of ammonia by the Haber Process (and equation)

Answer 25

• N2(g) + 3H2(g) -> 2NH3(g)
• Hydrogen and nitrogen obtained from natural gas and air pumped into the compressor
• The gases are compressed to 200 atm
• The pressurised gases reach 450 degrees C, and are pumped into containers filled with solid iron (heterogenous catalyst) where some hydrogen and nitrogen react forming ammonia
• Unreacted hydrogen, nitrogen and ammonia product pass into the cooling tank where ammonia is liquified and removed
• The unreacted H and N are recycled and used again

Question 26

Why is the temperature 450 degrees C in the production of ammonia

Answer 26

• Higher temperature favours the reverse reaction as its endothermic, so a higher yield of reactants made
• Lower temperature favours the forward reaction as its exothermic, so a higher yield of products made
• BUT this is very slow, so the compromise is 459 degrees C

Question 27

Why is the pressure 200 atm in the production of ammonia

Answer 27

• Lower pressure favours the reverse reaction, so a higher yield of reactants made
• Higher pressure favours the the forward reaction, so a higher yield of products made
• Higher pressure is dangerous and expensive, so 200 atm is a compromise

Question 28

Why is a solid iron catalyst used in the production of ammonia

Answer 28

Without a catalyst the required temperature and pressure for the Haber Process would be much higher, which is more dangerous, expensive, and less environmentally friendly

Question 29

How does the solid iron catalyst work in the production of ammonia

Answer 29

• Diffusion of nitrogen and and hydrogen gas to the iron surface
• Adsorption of the reactant molecules onto iron surface by forming bonds between iron and reactant atoms
• The reaction takes place between the adsorbed nitrogen and hydrogen atoms reacting on the iron surface to form NH3
• Desorption occurs when bonds in NH3 break and then diffuse from the iron surface

Question 30

What are the benefits of catalysts

Answer 30

• Speed up rate of reaction, so lower temperatures and pressures can be used to
• Saves energy costs due to reduced energy demand
• Less electrical pumping costs due to reduced pressure demands
• Eco friendly as less CO2 emissions
• Increase atom economy by increasing product yield and decreasing waste yield

Question 31

How do impurities affect the effectiveness of heterogenous catalysts

Answer 31

• Absorb onto catalyst surface and occupy active sites
• Prevent bond weakening in reactants
• Take up surface area on catalyst by forming strong bonds