Rates Of Reaction

Question 1

Define rate of reaction.

Answer 1

Change in concentration with unit time.

Question 2

What happens in reactions?

Answer 2

Concentration of substrate decreases and concentration of product increaseses.

Question 3

What is the usual unit for rate?

Answer 3

Seconds

Question 4

Define Kinetics.

Answer 4

Study of factors that effect rate of chemical reactions.

Question 5

Example of a short and long reaction?

Answer 5

Short - 1 sec - popping test for hydrogen

Long - several years - rusting nail

Question 6

Collision theory details.

Answer 6

• Particles must collide for reactions to occur.
• need to collide with enough energy to break bonds.
• orientation is key - collision must take place between parts of molecule that are going to react.

So lots of collisions = lots of particles in small volume.

Particles need to move fast to have enough energy to break bonds.

Fast react rate = lots of fast moving particles in small volume.

Question 7

What are the factors affecting reaction rate?

Answer 7

Temperature
Concentration
Pressure
Surface area
Catalysts

Question 8

What are the effects of temperature on reaction rate?

Answer 8

Higher temperature = higher molecules speed

Increases their energy and therefore number of reactions.

Question 9

Effects of concentration on reaction rate?

Answer 9

Higher concentration = higher amount of particles present in a given volume.

Makes collisions more likely so faster reaction rate.

But as reactions proceed, reactants are used up and their concentration falls.

So eventually reaction rate drops

Question 10

Effects of pressure on temperature?

Answer 10

Higher pressure = forces more molecules into a smaller area, more successful collisions. (Pretty much same as concentration).

Question 11

Effect of surface area on rate of reaction?

Answer 11

Larger SA = more particles available for collision.

As there’s more sites for reaction to take place.

Question 12

Effects of catalyst on reaction rate?

Answer 12

Lowers reaction activation energy.

Catalysts aren’t used up in reactions

Catalysts have no effect on chemical reaction itself.

Question 13

What is activation energy?

Answer 13

Minimum energy required to cause a reaction by breaking bonds.

Question 14

What are the basic details of the Maxwell - Boltzmann distribution?

Answer 14

• Particles with different speeds have different energies.
• No particles have zero energy
• Most particles have intermediate energy - around curve peak.
• No limit to particles energy
• All particles on right of activation energy line are ones with enough energy to break bonds.

Question 15

How does the Maxwell -Boltzmann distribution graph change with temperature?

Answer 15

• Higher temp has peak of curve lower and to the right.
• Number of particles with very high energy increases
• Due to this there’s more successful collisions.

Question 16

Define a catalyst.

Answer 16

Substance that affects chemical reaction rate without being chemically changed themselves.

Question 17

Why are catalysts used in industry?

Answer 17

Cheaper to use to speed up reactions than raising temperature.

As they are not used up in reactions they are re-usable.

Question 18

What do catalysts provide?

Answer 18

Reaction pathway with lower activation energy.

Question 19

What are the types of catalyst?

Answer 19

Heterozygous catalysts - catalyst in a different matter phase to reactants.

Homogeneous catalysts - catalysts and reactants are the same matter phase.

Question 20

What does rate expression tell us?

Answer 20

About contributions of the species that do effect the reaction rate.

Question 21

What are the three different rates in rate expression that depend bon concentration?

Answer 21

If rate isn’t effected by species concentration then the reaction is 0 to that species.

If rate is directly proportional to concentration of the species then reaction is 1st order to that species.

If the rate is proportional to the square of the concentration of the species then the reaction is 2nd order.

Question 22

What’s the experiment process for finding reaction order?

Answer 22

• Experiment carried out at constant temperature
• Each one with different combo of initial reactant concentrations.
• Experiments planned so between any pair of Experiments, only one species concentration changes. The rest remains the same.
• Comparing initial concentrations and rates for pairs of Experiments allows order of each reactant to be found.