C7 - Energy Changes

Question 1

What happens during an exothermic reaction

Answer 1

During exothermic reactions the reacting molecules give off energy to their surroundings

This causes an increase in the temperature of the surroundings

Question 2

Describe an energy profile diagram for an exothermic reaction

Answer 2

In an energy profile diagram for an exothermic reaction the products have less energy that their surroundings. This is because during the reaction they give off energy to the surroundings

Question 3

What does the difference between the energy of the reactants and the energy of the products tell

Answer 3

The difference between the energy of the products and the enrgy of the reactants tells us the amount of energy given off to the surroundings during the reaction

Question 4

What are some examples of products with exothermic reactions

Answer 4

Hand warmers

Self heating cans

Question 5

What happens during an endothermic reaction

Answer 5

During an endothermic reaction energy is taken in from the surroundings

This causes a decrease in the temperature of the surroundinhs

Question 6

Describe an energy profile for a endothermic reaction

Answer 6

In an energy profile for an endothermic reaction the products have more energy than the reactants because energy is being taken in from the surroundings

The amount of energy being taken in can be worked out by the difference between the products and the reactants

Question 7

What is the collision theory

Answer 7

The collision theory states that particles have to collide with sufficient energy in order to react

Question 8

What is the activation energy and how can it be represented on a graph

Answer 8

The activation energy is the minimum amount of energy needed for particles that collide to react

It is represented on a energy profile diagram by the difference in energy between the reactants and the peak in the curve

Question 9

How are exothermic reactions represented in terms of energy change

Answer 9

Exothermic reactions are shown as negative energy changes because energy is being transferred to the surroundings

Question 10

How are endothermic reactions represented in terms of energy changes

Answer 10

Endothermic reactions are represented through a positive energy change.Energy is taken in from the surroundings

Question 11

What does making bonds do to energy

Answer 11

Making bonds releases energy and so is exothermic

Question 12

What does breaking bonds do to energy

Answer 12

Breaking bonds requires energy and so is endothermic

Question 13

What do cells and batteries use to produce electricity and what will eventually happen to the reactants

Answer 13

Cells and batteries use a chemical reaction to produce electricity but eventually the reactants run out

Question 14

What is the equation for the negative electrode of a hydrogen fuel cell

Answer 14

H^2—->2H+ + 2e-

The hydrogen is split into 4 hydrogen ions and 4 electrons

These electrons run through the wire and provide the electricity

Question 15

What is the equation for the positive electrode

Answer 15

02 + 4H+ 4e- —-> 2 H20

The hydrogen ions combine with the oxygen to produce water

Question 16

What is the overall equation for the hydrogen fuel cell

Answer 16

2H+ + O2 —–>H2O

The hydrogen has been oxidised

Question 17

What are the advantages of a hydrogen fuel cell

Answer 17

• The fuel cell will produce as electricity for as long as you supply hydrogen
• The fuel cell does not become less efficient over time

Question 18

What are disadvantages of a hydrogen fuel cell

Answer 18

• the hydrogen fuel cell run on hydrogen which is an explosive gas that is hard to store
• Hydrogen fuel cells produce a relativley low amount of potential energy so lots have to be used together

Question 19

How does a cell produce electricity

Answer 19

When to different metals are placed in an electrolyte they can produced electricity

An electrolyte is a solution which conducts electricity

Question 20

How long can a cell produce electricity for

Answer 20

A cell can only produce electricity for a certain amount of time before the chemicals in the cell run out and the reaction stops

Question 21

How does the difference in reactivity affect the electricity produced by the cell

Answer 21

The greater the difference in reactivity between the two metals in the cell , the greater the amount of energy produced .

Question 22

What does a battery contain

Answer 22

A battery contains two or more cells in a series to produce a greater voltage

Question 23

What are the advantages of rechargeable batteries

Answer 23

• They require no dangerous gases to run

- They produce a higher potential difference than hydrogen cells

Question 24

What are disadvantages of rechargeable batteries

Answer 24

• They can store less energy each time they go through a charging cycle and eventually need to be replaced
• They can run out and have to be recharged

Question 25

What are the disadvantages of non rechargeable batteries

Answer 25

The reactants in non reachargeable batteries get used up and the reaction can no longer take place

The reaction is irrevesable

Question 26

What is a method you can use for a caliometry experiment

Answer 26

• Use a measuring cylinder to measure 25cm3 of sodium hydroxide solution
• Put a polystyrene cup in the 250cm3 beaker
• Use another measuring cylinder to measure 25cm3 of hydrochloric acid
• Add the sodium hydroxide solution to the polystyrene cup. Use a thermometer to measure the temperature of the sodium hydroxide solution every thirty seconds whilst gently stirring it
• After exactly 2 minutes add the hydrochloric acid to the solution, and now measure the temperature of the solution every 30 seconds for 10 minutes
• Measure the temperature change in the solution and decide from this wether the reaction is endo or exo thermic ( Endo if the temperature goes up , exo if it goes down )