9 - Reaction kinetics

Question 1

what is the Rate of Reaction

Answer 1

• how fast a product is being made

- how fast a reactant is being used up.

Question 2

Equation for rate of reaction

Answer 2

change in concentration / time taken

Question 3

When given a graph, how do you find the rate at a specific time?

Answer 3

Draw a tangent to the curve at the specified time, and work out the gradient of the tangent drawn.

Question 4

3 things that collision theory states

Answer 4

• particles must collide
• particles must collide with sufficient energy, Ea.
• particles must collide in the correct orientation.

Question 5

In a reaction between ethene (CH2=CH2) and HCl, why cant a reaction occur if the chlorine end of the H-Cl bond approaches the double bond?

Answer 5

• the double bond has a high concentration of negative charge around it, due to the electrons in the bond.
• Chlorine end of the H-Cl bond is slightly negative due to being more electronegative than hydrogen.
• When they approach each other, the repulsion causes the molecules to bounce off each other (negative-negative means repulsion).
• So, H end of the H-Cl bond must approach the double bond.
• This is an example of correct orientation.

Question 6

What is steric hindrance?

Answer 6

The shape of a molecule can influence reactions. If an atom or group of atoms is particularly large, they can prevent the attacking species from approaching their target species.

Question 7

What is activation energy?

Answer 7

The minimum energy that particles need to have in order to react when they collide.

Question 8

Why do reaction have an activation energy?

Answer 8

Chemical reactions results in breaking of some bonds (needing energy) and making of new bonds (releasing energy). Bonds have to be broken before new ones can be made. Activation energy is involved in breaking some of the original bonds.

Question 9

Why is it good for exothermic reactions to have an activation energy?

Answer 9

If activation energy did not exist, exothermic reactions would take spontaneously. Fuels would spontaneously combust, which would result in an explosion.

Question 10

Effect of surface area on rate of reaction?

Answer 10

• more particles on surface of one reactant available to collide with particles of the other reactant.
• frequency of collisions increases
• frequency of successful collisions increases.
• ROR increases
• decreasing the size of reactant particles increases surface area. Powders have a very large surface area.

Question 11

Effect of concentration/pressure on rate of reaction?

Answer 11

• number of particles per unit volume increases.
• frequency of collisions increases.
• frequency of successful collisions increases.
• rate of reactions increases.

Question 12

difference between pressure and concentration on ROR?

Answer 12

• pressure change effects gas reactants only.

- increasing pressure of a gas is the same as increasing the concentration of the gas.

Question 13

Effect of temperature on rate of reaction?

Answer 13

• higher temp, particles have greater energy, particles move faster, frequency of collisions increases.
• higher proportion of particles will have energy greater or equal to the activation energy.
• frequency of successful collisions increases.
• ROR increases.

Question 14

What are the 5 things that affect rate of reaction?

Answer 14

• pressure
• concentration
• temperature
• surface area
• catalysts

Question 15

What does the origin tell you in the Maxwell boltzmann distribution?

Answer 15

no particles have zero energy so the curve passes through the energy.

Question 16

What is most probable energy?

Answer 16

• the point at which the curve peaks is the most probable energy. Most particles have energy at around this value at any given time.

Question 17

what is average energy?

Answer 17

the area under the curve on either side of the line drawn through the average energy would be the same.

Question 18

What does the area under the curve represent in the distribution?

Answer 18

the area under the curve represents the total number of particles in the sample.

Question 19

What is the activation energy in the distribution?

Answer 19

When the activation energy is marked on the x axis on the graph, the area to the right of this represents the number of particles with energy greater or equal to the activation energy.

Question 20

What does the asymptote show in the distribution?

Answer 20

A few particles have very high energy. There is no limit on how high an energy these particles can have, so the curve is asymptomatic to the x axis.

Question 21

How does increase in temperature affect the distribution?

Answer 21

The curve moves right and downwards.
- area under curve towards the right of Ea is greater, indicating that a higher proportion of particles have energy greater or equal to the Ea.

Question 22

Why doesn’t volume/concentration affect the Boltzmann curve?

Answer 22

• although frequency of successful collisions increases
• velocity of the particles doesn’t change.
• amount of kinetic energy in the particles does not change.
• the boltzmann distribution shows the number of molecules at a certain energy.
• no change in kinetic energy in particles means that the curve is not affected.

Question 23

What are catalysts and how do they increase rate of reaction?

Answer 23

• catalysts are substances which increase the rate of reactions while remaining chemically unchanged at the end of the reaction.
• catalysts provide an alternative reaction pathway with a lower activation energy.

Question 24

what does a double-humped reaction profile show?

Answer 24

• It shows that it is a two-step catalysed reaction.

Question 25

How do catalysts affect the Maxwell Boltzmann distribution?

Answer 25

Ea moves left on the x axis.

• area under curve on the right of the Ea is greater, Higher proportion of particles have energy greater or equal to the Ea.
• frequency of successful collisions increases.

Question 26

What are heterogenous catalysts?

Answer 26

Heterogenous catalysts are catalysts which are in a different phase to the reactants.

• usually solid and the reactants are liquid/gas.
• e.g iron catalysts in ammonia production
• e.g Ziegler-Natta in production of polyethene.

Question 27

What are the advantages of the use of catalysts? (especially in industry)

Answer 27

• lower energy demands
• less CO2 produced
• less environmental impact
• lower production costs.

Question 28

What is an example of a non-industrial catalyst?

Answer 28

enzyme catalysis in biological washing powders.

Question 29

Except temperature and catalysts, what else affects the distribution curve?

Answer 29

• molar mass.
• as molar mass increases, the most probable speed decreases.
• the distribution curve moves up and left .