Rates of reaction

Question 1

State the formulas for calculating the mean rate of reaction

Answer 1

Quantity of reactant used / time taken

Quantity of product formed / time taken

Question 2

State the units for quantity of reactant or product

Answer 2

grams, volume, moles

Question 3

State the units for rate of reaction

Answer 3

g/s, cm³/s, mol/s

Question 4

Why is the rate of reaction fast at the start of the reaction?

Answer 4

There is a large number of reactant molecules, so lots of them are reacting to form the product

Question 5

What could we use to find the rate of reaction for a specific point in a graph?

Answer 5

A tangent

Question 6

How do we calculate the rate of reaction using a tangent?

Answer 6

Form a triangle

Difference in y / difference in x

Question 7

State the factors affecting rate of reaction

Answer 7

Temperature
Surface area
Concentration
Pressure
Presence of catalysts

Question 8

Explain how temperature affects rate of reaction

Answer 8

The higher the temperature, the faster the rate since collisions are more energetic

Question 9

Explain how surface area affects rate of reaction

Answer 9

The higher the SA, the faster the rate since particles in a solution can only react and collide with particles on the surface

Question 10

Explain how concentration affects rate of reaction

Answer 10

The higher the concentration, the faster the rate because collisions occur more often

Question 11

Explain how pressure affects rate of reaction

Answer 11

The higher the pressure, the faster the rate of reaction as there is less space for particles to be spaced out meaning that collisions happen more often

Question 12

Define activation energy

Answer 12

The minimum amount of energy that particles must have to react (for successful collisions to occur)

Question 13

Define catalyst

Answer 13

A substance that increases the rate of reaction without being used up

Question 14

Explain how catalysts work

Answer 14

They increase the rate by providing a different pathway for the reaction that has a lower activation energy
Less energy is needed to cross the activation barrier
Means that more particles can successfully collide per second
increases the rate

Question 15

What does the collision theory suggest?

Answer 15

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy (activation energy)

Question 16

According to the collision theory, the rate of chemical reactions is determined by..

Answer 16

The frequency of successful collisions

Question 17

Define frequency in terms of reaction

Answer 17

Number of successful collisions per second

Question 18

Explain how to investigate how changes in concentration affects rate by monitoring a change in colour or turbidity (using sodium thiosulfate solution and dilute hydrochloric acid)

Answer 18

1. Add sodium thiosulfate solution into conical flask
2. Put the flask on a printed black cross
3. Add hydrochloric acid while swirling the flask gently and start the stopwatch
4. Look down through the top of the flask and stop the clock when you can no longer see the cross and record the time taken
5. Repeat with different concentrations of sodium thiosulfate (3 times for each)
6. Calculate mean time for each of the sodium thiosulfate concentrations

Question 19

Suggest a problem with the dissapearing cross practical

Answer 19

Not reproducable
People have different eyesights
Some people might see cross for longer than others
However size of cross is the same so problem shouldn’t be too big

Question 20

Explain how to investigate how changes in concentration affect rate by measuring the volume of gas produced (using hydrochloric acid and a strip of magnesium ribbon)

Answer 20

1. Use measuring cylinder to place hydrochloric acid into conical flask
2. Attach conical flask to bung and delivery tube
3. Place delivery tube to container filled with water
4. Place upturned measuring cylinder also filled with water over the delivery tube
5. Add a strip of magnesium into hydrochloric acid and start stopwatch
6. Every 10 seconds, measure volume of hydrogen gas in measuring cylinder, continue until there is no more given off
7. Repeat using different concentrations of hydrochloric acid

Question 21

State the safety precautions for the rate of reaction practicals

Answer 21

Wear safety goggles

Take care when using glassware

Question 22

Define reversible reaction

Answer 22

Reaction where the products can react to produce the original reactants again
Symbol: ⇌

Question 23

Define dynamic equilibrium

Answer 23

When the forward and reverse directions take place at exactly the same rate

Question 24

What is Le Chatlier’s Principle?

Answer 24

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change

Question 25

What happens if the concentration of a reactant is increased (when in equilibrium)?

Answer 25

More products will be formed until equilibrium is reached

Question 26

What happens if the concentration of a product is decreased (when in equilibrium)?

Answer 26

More reactants will react until equilibrium is reached again

Question 27

What happens when the temperature increases for the exothermic reaction:

2NO2 ⇌ N2O4

Answer 27

The equilibrium shifts to the left to decrease the temperature

Question 28

What happens when the temperature increases for the exothermic reaction:

2NO2 ⇌ N2O4

Answer 28

The equilibrium shifts to the right to increase the temperature

Question 29

What happens when there is an increase in pressure for gaseous reactions at equilibrium?

Answer 29

The equilibrium position shifts towards the side with the smaller number of molecules

Question 30

What happens when there is a decrease in pressure for gaseous reactions at equilibrium?

Answer 30

The equilibrium position shifts towards the side with the larger number of molecules