1 Rate of reaction

Question 1

If a pupil would like to investigate how the type of catalyst affects the rate of reaction. Name four quantities that must be kept constant to make this a fair test.

Answer 1

concentration

temperature

surface area of reactant/s

surface area of catalyst

Question 2

How does decreasing the temperature affect the rate of reaction?

Answer 2

• reactant particles have less kinetic energy
• there are less particles with the required activation energy
• there are fewer successful collisions
• PLUS- less frequent collisions
• decreased rate of reaction

Question 3

In the reaction between a metal carbonate and acid, why does a graph of amount of CO₂ gas produced against time eventually level off?

Answer 3

Rate of reaction decreases over time.

Number of reactant particles decreases over time of reaction because they are being used up.

Less collisions per unit time results in a slower reaction. (gradient of graph decreases- less gas produced per time)

Eventually the limiting reactant runs out and no more CO₂ will be produced. (graph has levelled off)

Question 4

If a pupil would like to investigate how temperature affect the rate of reaction. Name four quantities that must be kept constant to make this a fair test.

Answer 4

concentration

surface area of reactant/s

presence of a catalyst

surface area of catalyst

Question 5

How does increasing the surface area of a solid reactant affect the rate of reaction?

Answer 5

• higher surface area
• more particles exposed to the liquid reactant
• more frequent collisions
• increased rate of reaction

Question 6

Why does increasing the temperature of reactants result in a greater number of particles with energy greater than the activation energy?

Why does this have such a large effect on the rate of reaction?

Answer 6

• At larger temperatures, particles have more (kinetic) energy
• More particles have energy greater than the acitivation energy
• More reactions are successful AND since the particles are moving faster, there are more collisions per unit time
• This means that temperature has a huge impact of reaction rate.

_{Remember a 10ºC increase doubles the rate of reaction!!!}

Question 7

How does the time for a reaction to finish relate to the rate of reaction?

Answer 7

As the time for a reaction increases, the rate of reaction decreases

Question 8

How does increasing the concentration of a reactant affect the rate of reaction?

Answer 8

• more particles of reactant in a given volume
• more frequent collisions

increased rate of reaction

Question 9

If a pupil would like to investigate how surface area affect the rate of reaction. Name four quantities that must be kept constant to make this a fair test.

Answer 9

concentration

temperature

presence of a catalyst

surface area of catalyst

Question 10

What is the general rule for how the rate of reaction is affected if the temperature increases by 10 degrees?

Answer 10

A 10 degree increase roughly doubles the rate of reaction

Question 11

How can you find the initial rate of reaction from a reaction curve?

Answer 11

Draw a tangent to the initial reaction curve at t = 0 and find its gradient

e.g. gradient = rise / run = 10.4 g / 120 seconds = 0.09 g/second

Question 12

If a pupil would like to investigate how the surface area of catalyst affects the rate of reaction. Name four quantities that must be kept constant to make this a fair test.

Answer 12

concentration

temperature

surface area of reactant/s

type of catalyst

Question 13

In the reaction of Magnesium metal and hydrochloric acid, hydrogen gas is released. If the concentration of the acid is doubled, the rate of reaction should double. Why isn’t this seen when this is done experimentally?

How can the method be improved?

Answer 13

The reaction is exothermic. It is difficult to keep the other control variable constant. Increased temperature speeds up the reaction so that the reaction rate is more than doubled.

You would get better results if the reaction was carried out in a water bath- and temperature controlled.

Question 14

What is the name of this method which is used to study rate of reaction?

Answer 14

Gas collection by downward displacement

Rate = volume of gas collected / time taken

cm³/sec

Question 15

What would happen to the reaction curve if double the mass of calcium carbonate is added to excess acid?

Answer 15

The initial rate of reaction (steepness of the graph) would be greater as there are double the surface area and double the number of reactant particles exposed to the acid.

It would level off at double the volume of gas produced. There is double the amount of calcium carbonate to react with the excess acid.

Question 16

What is meant by rate of reaction?

Answer 16

Rate of reaction is how fast a reaction occurs.

Question 17

Name 4 methods use to study the rate of reaction

Answer 17

1. Collection of a gas in a measuring cylinder by downward displacement of water
2. Collection of a gas in a gas syringe
3. Mass loss using a balance
4. Disappearing cross by formation of a precipitate

Question 18

Draw an energy profile for a reaction with and without a catalyst

Answer 18

A catalyst provides an alternative route with a lower activation energy.

More particles now have the required activation energy!

More successful collisions per unit time

Question 19

How does a catalyst work?

Answer 19

A catalyst provides an alternate route with a lower activation energy than the original

• more of the collisions which take place will have the necesssary activiation energy
• more successful collisions per second
• increased rate of reaction

Question 20

What is the name of this method which is used to study rate of reaction?

Answer 20

Mass loss of a gas using a balance

Rate = mass lost / time taken

g/sec

The conical flask must have a cotton wool bung. The cotton wool allows the gases out but not any splashes of reactant or products.

Question 21

How would the reaction curve vary for a reaction with and without a catalyst?

Answer 21

• A catalsyt provides an alternative route with a lower activation energy
• More particles have the required activation energy
• More successful collisions per unit time

Greater rate of reaction- gas produced more rapidly

Question 22

Use ideas of reaction rate to help explain why a spark in a flour mill with fine flour dust can be very dangerous.

Answer 22

The fine flour dust has a very large surface area.

A very large amount of flour is exposed to oxygen

There is a very large number of collisions per unit time

Increased rate of reaction means a spark can cause an explosion.

Question 23

What are the possible units for the rate of reaction?

Answer 23

1/s - per second

g/s - grams lost per second

cm³/ second - volume of gas produced per second

Question 24

Calcium carbonate reacts with acid to form carbon dioxide.

Below is a reaction curve for three different calcium carbonate chips (powdered, crystalline and large chips)

Which curve represents the reaction curve for powdered calcium carbonate? How do you know?

Answer 24

Curve A represents the reaction curve for powdered calcium carbonate reacting with acid.

• Powdered calcium carbonate has a large surface area
• More particles are exposed to the acid
• More collisions per unit time
• Greater rate of reaction

Question 25

If a pupil would like to investigate how concentration affect the rate of reaction. Name four quantities that must be kept constant to make this a fair test.

Answer 25

temperature

surface area of reactant/s

presence of a catalyst

surface area of catalyst

Question 26

How does increasing the concentration of a reactant affect the rate of reaction?

Answer 26

• more particles of reactant in a given volume
• more frequent collisions

increased rate of reaction

Question 27

How does doubling the surface area of a solid reactant affect the rate of reaction?

Answer 27

• double the surface area
• double the particles exposed to the liquid reactant
• double the number of collisions per unit time
• double the rate of reaction

Question 28

If the reaction between calcium carbonate and hydrochloric acid was being measured. What would the rate be if 50 cm³ of carbon dioxide was collected in 100 seconds.

Answer 28

rate = volume of gas collected / time

rate = 50 cm³ / 100s

rate = 0.5 cm³/s

Question 29

How does acid concentration relate to time for reaction?

Answer 29

As the acid concentration increases, the time for the reaction decreases (it reacts in less time!)

Question 30

What happens to the rate of reaction if the surface area of a catalyst is increased?

Answer 30

The rate of reaction would increase

• surface area of catalyst in contact with reactant has increased
• more collsions per unit time

Question 31

How does decreasing the concentration of a reactant affect the rate of reaction?

Answer 31

• less particles of reactant in a given volume
• less frequent collisions
• decreased rate of reaction

Question 32

What would a graph of concentration against rate of reaction look like?

Answer 32

Rate of reaction is proportional to concentration.

If the concentration is doubled the rate is doubled

Therefore it is a straight line through the origin like shown

Question 33

At which part of the graph is the rate the greatest?

How did you arrive at your answer?

Answer 33

Part A

The gradient of this part of the curve is greatest- it is producing more gas per time.

Greater rate of reaction.

At B the reaction has stopped- gradient is zero.

Question 34

What is the name of this method which is used to study rate of reaction?

Answer 34

Precipitation reaction- time it takes for cross to disappear

Rate = 1 / time taken for cross to disappear

1/sec

Question 35

What assumptions does kinetic theory make about chemical reactions?

Answer 35

1. For a reaction to take place, the particles must collide with one another.
2. The colliding particles must have sufficient energy to react. This energy is known as the activation energy.
3. To increase the rate of reaction it is necessary to increase the frequency of successful collisions.

Question 36

How does increasing the temperature affect the rate of reaction?

Answer 36

• reactant particles have more kinetic energy
• there are more particles with the required activation energy
• there are more successful collisions
• PLUS- more frequent collisions
• increased rate of reaction

Question 37

What is the name of this method which is used to study rate of reaction?

Answer 37

-gas collection using a syringe

Rate = volume collected / time taken

cm³/sec

Question 38

How does decreasing the surface area of a solid reactant affect the rate of reaction?

Answer 38

• lower surface area
• fewer particles exposed to the liquid reactant
• less frequent collisions
• decreased rate of reaction

Question 39

Hydrogen peroxide decomposes when a catalyst is added to it.

Name the catalyst of this reaction

Answer 39

Manganese (IV) oxide

OR

manganese dioxide

Question 40

How does doubling the concentration of a reactant affect the rate of reaction?

(be quantitative!)

Answer 40

• double the number of particles of reactant in a given volume
• double the frequency of collisions
• double the rate of reaction

Question 41

Below is a reaction curve for magnesium reacting with hydrochloric acid.

Explain the shape of the reaction curve.

Answer 41

Question 42

How does increasing the temperature affect the rate of reaction?

Answer 42

• reactant particles have more kinetic energy
• there are more particles with the required activation energy
• there are more successful collisions
• PLUS- more frequent collisions
• increased rate of reaction

Question 43

What is a catalyst?

Answer 43

A catalyst is a substance that increases the rate of a chemical reaction but is chemically unchanged at the end of the reaction

Question 44

How does decreasing the concentration of a reactant affect the rate of reaction?

Answer 44

• less particles of reactant in a given volume
• less frequent collisions
• decreased rate of reaction

Question 45

In a reaction where silver iodide precipitate was formed, the cross at the bottom of the flask disappeared in 50 seconds. Calculate the rate of reaction.

Answer 45

rate = 1 / time

rate = 1/50s = 0.02 s^-1