Reaction Rates

Question 1

What is the equation for rate of reaction?

Answer 1

Rate of reaction = amount of reactant used or product formed / time

Question 2

Explain collision theory

Answer 2

Particles in liquids and gases are always moving and colliding with each other. They don’t react every time, only when conditions are right. A reaction won’t take place between two particles unless:

1. They collide in the right direction
2. They collide with at least the minimum amount of kinetic energy

Question 3

Define activation energy

Answer 3

Activation energy is the minimum amount of kinetic energy that particles need to react

Question 4

Particles need what to start a reaction?

Answer 4

Particles must have at least the activation energy to break their bonds and start a reaction. Reactions with low activation energies often happen easily. Reactions with high activation energies don’t, you need to give the particles extra energy by heating them

Question 5

Do molecules of gas all have the same amount of energy?

Answer 5

No, some don’t have much kinetic energy and move slowly. Some have a lot of kinetic energy and move quickly. Most molecules are somewhere in between

Question 6

What do you get if you plot a graph of the number of molecules in a gas with different kinetic energies

Answer 6

A Maxwell-Boltzmann distribution

Question 7

What effect to increasing the temperature have on reaction rates?

Answer 7

If you increase the temperature of a reaction, on average the particles will have more kinetic energy and move faster. This means a greater proportion of molecules will have the activation energy and be able to react. Also, because the molecules are moving faster, they’ll collide more often, making reaction rate faster. If both of these effects happen together, quite small increases in temperature can lead to large increases in reaction rate

Question 8

How does increasing concentration of reactants affect rate of reaction?

Answer 8

If you increase the concentration of reactants in a solution, the particles with on average be closer together, making them more likely to collide. Collisions taking place more frequently means rate of reaction increases

Question 9

How does increasing pressure affect rate of reaction?

Answer 9

If you increase the pressure all the gas particles are closer together, making them more likely to collide. Collisions taking place more frequently means rate of reaction increases

Question 10

What is a catalyst?

Answer 10

A catalyst is a substance that increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is unchanged at the end of the reaction

Question 11

How many reactions do catalysts work on?

Answer 11

Catalysts often only work on a single reaction

Question 12

How much catalyst do you need?

Answer 12

Catalysts don’t get used up in a reaction so you only need a tiny bit of catalyst to catalyse a huge amount of stuff

Question 13

How are catalysts beneficial in the industrial process?

Answer 13

Catalysts allow you to make the same amount of product faster, and often at lower temperatures, this saves money

Question 14

What do catalysts do to activation energy?

Answer 14

Catalysts lower the activation energy, meaning there’s more particles with enough energy to react when they collide. It does this providing an alternative reaction pathway. So, in a certain amount of time, more particles react

Question 15

Name three ways to measure reaction rate

Answer 15

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

Question 16

How do you time how long it takes a precipitate to form? Is this accurate?

Answer 16

You watch a mark through the solution and time how long it takes to be obscured. If the same observer uses the same mark each time, you can compare the rates of reaction, because roughly the same amount of precipitate will have been formed when the mark becomes obscured. This method is subjective - different people may not agree on the exact moment the mark becomes obscured

Question 17

How do you measure a decrease in mass? Is it accurate?

Answer 17

When one or more products is a gas, you can measure the rate of formation using a mass balance. As gas is given off, the mass of the reaction mixture decreases. This method is accurate.

Question 18

How do you measure the volume of gas given off? Is is accurate?

Answer 18

This involves using a gas syringe to measure the volume of gas given off. This method is accurate - gas syringes usually give volumes to the nearest 0.1cm3