Energy Changes

Question 1

Why does the overall amount of energy not change during a chemical reaction ?

Answer 1

This is because in either exothermic and endothermic reactions energy is conserved. (Conservation of energy)

Question 2

Define an exothermic reaction.

Answer 2

A reaction that releases energy into the surroundings. Usually thermal energy. This is shown by a rise in temperature.

Question 3

Give 3 examples of exothermic reactions.

Answer 3

• combustion
• neutralisation reactions
• aerobic respiration
• oxidation reactions ( e.g sodium + water ; releases energy when it moves about in the surface of the water being oxidised)

Question 4

Give and explain two uses of exothermic reactions.

Answer 4

• hand warmers use exothermic oxidation of iron in the air with a salt solution catalyst to release energy.
• self heating cans of hot chocolate and coffee rely on exothermic reactions between chemicals in their bases.

Question 5

Define an endothermic reaction.

Answer 5

Takes in energy from the surroundings. This is shown by a fall in temperature.

Question 6

Give 3 examples of an endothermic reaction.

Answer 6

• photosynthesis
• thermal decomposition ( e.g heating calcium carbonate causes it to decompose into calcium oxide + carbon dioxide.
• citric acid + sodium hydrogencarbonate

Question 7

Name and explain an everyday use with an endothermic reaction.

Answer 7

Sports injury packs - the reaction allows the pack to become instantly cooler without having to put it on the freezer.

Question 8

Is bond breaking an exothermic or endothermic process and explain why.

Answer 8

Bond breaking is an endothermic process as energy is required in order to break the existing bonds. - the energy used to break bonds is greater than the energy released forming them.

Question 9

Is bond forming an exothermic or endothermic process and explain why.

Answer 9

An exothermic process as energy is releases when new bonds are formed. - the energy releases by forming bonds is greater than the energy used to break them.

Question 10

What do reaction profiles show ?

Answer 10

Show the relative energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction.

Question 11

How can we tell if a reaction profile shows an exothermic reaction ?

Answer 11

This is because the products of the reaction are at a lower energy than the reactants.

Question 12

What does the difference in height between the reactants and products of an exothermic reaction show on a reaction profile ?

Answer 12

Represents the overall energy change in the reaction ( the energy released ) per mole.

Question 13

What does the initial rise in the line of an exothermic reaction profile show

Answer 13

Represents the energy required to break the old bonds and Start the reaction - ( activation energy )

Question 14

Define activation energy.

Answer 14

The minimal amount of energy the reactants need to collide with each other and react .

Question 15

Name one way that activation energy can be supplied to start the reaction.

Answer 15

By heating the reaction mixture .

Question 16

How do we know if a reaction profile is endothermic ?

Answer 16

Because the products are at a higher energy than the reactants.

Question 17

What does the difference in height between the reactants and products of an endothermic reaction profile show ?

Answer 17

The overall energy change during the reaction. ( the energy taken in) per mole.

Question 18

What is the equation used to calculate the overall energy change ?

Answer 18

Total energy required to break bonds in reactants - energy released when new bonds are made products

Question 19

How would you carry out the practical of measuring energy transfer using temperature change ? ( Required practical 4 )

Answer 19

1) measure 30cm³ of NaOH solution using measuring cylinder.

2) transfer solution into a polystyrene cup

3) place the polystyrene cup into a beaker so it doesn’t fall over.

4) using a thermometer measure the temperature of the acid and record in a table.

5) using a measuring cylinder measure 5cm³ of HCL and transfer into the polystyrene cup.

6) place a plastic lid over the cup and place a thermometer through the hole in the lid.

7) stir the mixture using the thermometer ; record the maximum temperature in the table after it stops rising.

8) Repeat steps 5-7 until you’ve added 60ml in total of HCL

9) Repeat the whole experiment again and calculate a mean value for the temperature reached for each volume of HCL acid.

• At the end of the experiment it should be clear that as we increase the volume of HCL the maximum temperature reached also increases.

Question 20

Suggest why the maximum temperature reached starts to decrease at a certain volume of HCL in required practical 4 .

Answer 20

This is because we are adding so much HCL, that there is not enough NaOH solution so some of the HCL is unable to react. Therefore the amount of energy released by the reaction has reached a maximum.

Question 21

Why do we use a polystyrene cup with a lid in required practical 4 ?

Answer 21

This is because we want to reduce heat loss as we are measuring the temperature , since polystyrene is a good thermal insulator it reduces heat loss to the sides and the bottom of the cup.
The lid reduced heat loss to the air.

Question 22

What is the independent, dependent and control variable in required practical 4 ?

Answer 22

Independent - Volume of HCL

Dependent - maximum temperature reached

Control - Volume of NaOH solution
- The concentrations of both the HCL and the
NaOH solution.