Topic 7 - Rates Of Reaction And Energy Changes

Question 1

How do you work out the rate of reaction?

Answer 1

(Amount of reactant used or Amount of product formed) ÷ time

Question 2

Describe an investigation to find out how concentration affects the rate of reaction.

Answer 2

1. Add the first of the concentrations of sodium thiosulphate to the flask. Add 10cm3 of HCL and start the stopwatch
2. Watch the solution as it clouds over. Once the cross has disappeared stop the clock.
3. Record the time in a results table
4. Repeat the above steps for the other concentration of sodium thiosulphate. Repeat the experiment 3 times for each of the concentrations.
5. Record all results in a table and work out the average time for each.
6. Show the results in a graph and you should find that as the concentration gets stronger, the time take is shorter.

Question 3

Describe an investigation to find the effect of surface area on the rate of reaction.

Answer 3

1. Put dilute HCL into a conical flask and place marble chips inside
2. Immediately place a bung connected to a gas syringe onto the conical flask and take the first volume reading whilst starting the stop watch
3. Take readings at regular time intervals until the marble chips disappear and record the results in a table. You can also plot a graph
4. Repeat the experiment with the same volume and concentration of acid and exactly the same mass of the marble chips but the marble must be more crushed.
5. Then repeat with the same mass of marble but as powdered chalk.

Question 4

What does the rate of a chemical reaction depend on?(4)

Answer 4

• Temperature
• Concentration
• Pressure
• Surface area

Question 5

How does temperature increase the rate of reaction?

Answer 5

When the temperature is increased, the particles move faster. If they move faster, they’re going to have more collisions. Also, higher temperatures increase the energy of the collisions, since the particles are moving faster. This means that there will be more successful collisions as more particles will collide with enough energy to react.

Question 6

How does concentration affect the rate of reaction?

Answer 6

If a solution is made more concentrated, there are more particles of reactant in the same volume. This makes collision more likely, so the reaction rate increases.

Question 7

How does pressure affect the rate of reaction?

Answer 7

Increasing pressure means that the particle are more crowded. This means that the frequency between particles will increase therefore increasing the rate of reaction.

Question 8

How does surface area affect the rate of reaction?

Answer 8

Increasing the surface area means that the particles around it have more area to work on, so the frequency of collisions will increase. This means that the rate of reaction is faster for solids with a larger surface area to volume ratio.

Question 9

What is a catalyst?

Answer 9

A substance which increase the rate of reaction without being changed chemically or used up in the reaction. Also, it won’t change the products of the reaction. Catalysts are specific to a chemical reaction.

Question 10

How do catalysts work?

Answer 10

They decrease the activation energy needed for a reaction to occur. They do this by providing an alternative reaction pathway that has a lower activation energy. As a result, more of the particles have at least the minimum amount of energy needed for a reaction to occur when the particles collide.

Question 11

What are enzymes?

Answer 11

Biological catalysts. They speed up the chemical reactions in living cells.

Question 12

What can enzymes be used for?

Answer 12

Enzymes from yeast cells are used in the fermentation process which is used to make alcoholic drinks. They catalyse the reaction that converts sugars into Ethanol and Carbon Dioxide.

Question 13

What is an exothermic reaction?

Answer 13

A reaction which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature.

Question 14

What is an endothermic reaction?

Answer 14

A reaction which takes in energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature of the surroundings.

Question 15

How does a reaction profile show an exothermic reaction?

Answer 15

• The products are at a lower energy than the reactants

- The difference in height represents the energy given out in the reaction

Question 16

How does a reaction profile show if a reaction is endothermic?

Answer 16

• The products are at a higher energy than the reactants

- The difference in height represents energy being taken in during the reaction.

Question 17

What is the activation energy?

Answer 17

The minimum amount of energy needed for bonds to break and a reaction to start. On a reaction profile, it’s the energy difference between the reactants and the highest point on the curve.

Question 18

How do measure the temperature change when dissolving salts in water?

Answer 18

Add the salt to a polystyrene cup of water and measuring the change in temperature when the salt has dissolved.
Dissolving ammonium chloride decreases the temperature of the reaction mixture - its endothermic
Dissolving calcium chloride causes the temperature of the solution to rise- it’s exothermic

Question 19

How do you measure temperature change in a neutralisation reaction?

Answer 19

In a neutralisation reaction an acid and a base react to form a salt and water. Most neutralisation reactions are exothermic, e.g.
HCl + NaOH = NaCl +H2O. However, the neutralisation reaction between ethanoic acid and sodium carbonate is endothernic

Question 20

How do you measure temperature change in a displacement reaction?

Answer 20

In a displacement reaction, a more reactive element displaces less reactive element in a compound. These types of reactions are accompanied by a release of energy- they’re exothermic. Zinc powder and copper sulfate react in a displacement reaction forming zinc sulfate and copper.

Question 21

How do you measure temperature change in a precipitation reaction?

Answer 21

Precipitates are insoluble solids which can sometimes form when two solutions are mixed together. Precipitation reactions are exothermic. For example, the reaction between lead (ii) nitrate solution and potassium iodide forming a lead iodide precipitate would result in an increase in the temperature of the surroundings.

Question 22

What type of process is bond breaking?

Answer 22

Energy must be supplied to break the existing bonds so it is an endothermic process.

Question 23

What type of process is bond making?

Answer 23

Energy is released when new bonds are formed so it is a exothermic process.

Question 24

MEXO BENDO

Answer 24

Making Bonds is
EXOthermic
Breaking bonds is
ENDOthermic

Question 25

How do you calculate an overall energy change?

Answer 25

Energy required to break bonds - Energy released by forming bonds

Question 26

What does a positive energy change mean?

Answer 26

It is an endothermic reaction. Less heat energy is released in forming new bonds in the products than is required in breaking bonds in the reactants.

Question 27

What is a negative energy change?

Answer 27

It is a exothermic reaction. More heat energy is release in forming new bonds in the products than is required in breaking existing bonds in the reactants.

Question 28

How do you do a bond energy calculation?

Answer 28

1) Work out the energy required to break the original bonds in the reactants (LHS of eq)
2) Work out the energy released by forming the new bonds in the products (RHS in eq)
3) Work out overall change