Chapter 8 - Chemical energies (L)

Question 1

What two components make up chemical energy?

Answer 1

Kinetic energy

Potential energy

Question 2

What is kinetic energy a measure of?

Answer 2

The motion of particles in a substance

Question 3

What is potential energy a measure of?

Answer 3

How strongly particles in a substance interact with one another (attract and repel)

Question 4

What is heat energy?

Answer 4

The portion of kinetic and potential energy of a substance that is responsible for the temperature of the substance

Question 5

What is heating of a substance the result of?

Answer 5

The transfer of heat energy from one system to another, which in turn produces a change in temperature

Question 6

What is the enthalpy change of a process?

Answer 6

The heat energy that is transferred between system and surroundings

Question 7

What is an exothermic change?

Answer 7

When heat energy is transferred from the system to the surroundings

Question 8

What is an endothermic change?

Answer 8

When heat energy is transferred from the surroundings to the system

Question 9

In a chemical reaction, what does ‘system’ refer to?

Answer 9

The reaction mixture

Question 10

In a chemical reaction, what does ‘surroundings’ refer to?

Answer 10

Everything outside of the system (which in practice is the air in the room of the reaction)

Question 11

What is the definition of standard enthalpy change of reaction?

Answer 11

The enthalpy change of a reaction according to the molar quantities in the equation, under standard conditions

Question 12

What are standard conditions?

Answer 12

100kPa pressure and a stated temperature (often 298K)

Question 13

At the end of exothermic reactions, which has a greater enthalpy, reactants or products?

Answer 13

Reactants have a greater enthalpy, as during the reaction, heat energy has been transferred from the system to the surroundings

Question 14

At the end of endothermic reactions, which has a greater enthalpy, reactants or products?

Answer 14

The products will have a greater enthalpy than the reactants, as heat energy has been transferred from the surroundings into the system

Question 15

In exothermic reactions, is the enthalpy change positive or negative?

Answer 15

Negative

Question 16

In endothermic reactions, is the enthalpy change positive or negative?

Answer 16

Positive

Question 17

How do you measure the change in enthalpy?

Answer 17

Hproducts - Hreactants

Question 18

What is the definition of standard enthalpy of neutralisation?

Answer 18

The enthalpy change when an acid and an alkali react to form 1 mole of water, under standard conditions

Question 19

What is the definition of standard enthalpy of combustion?

Answer 19

The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions

Question 20

What is the definition of standard enthalpy change of formation?

Answer 20

The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions

Question 21

What type of process is bond breaking?

Answer 21

An endothermic process, as energy needs to be absorbed for the bonds to break

Question 22

What type of process is bond forming?

Answer 22

Exothermic, as energy is released as a result of bonds being formed

Question 23

How do you work out enthalpy change from bond energies?

Answer 23

Energy needed to break the bonds - energy released from bond formation

Question 24

What is bond enthalpy?

Answer 24

The amount of energy required to break 1 mole of a bond type in a molecule (in gaseous state)

Question 25

Why do we take a mean bond enthalpy for the same type of bond (e.g. C-H bonds)?

Answer 25

Because different C-H bonds will have different individual bond enthalpies

Question 26

In calorimetry, what is the equation to work out heat energy change (Q)?

Answer 26

Q= m x c x ∆T

Question 27

What 3 things do you need to know to work out q (heat energy change)?

Answer 27

Mass of solution (m)
Specific heat capacity (c)
Change in temperature (∆T)

Question 28

What is the equation to work out enthalpy change after working out Q?

Answer 28

Enthalpy change = energy change / moles

Question 29

What are possible sources of error in working out the enthalpy of combustion?

Answer 29

• Some heat energy produced would be transferred to the air and the copper can and not the water
• Some incomplete combustion may occur, meaning the substance is not completely burned in oxygen

Question 30

How do you conduct an experiment to work out the enthalpy of combustion for ethanol?

Answer 30

• Add known volume of water to copper can and measure temperature of water before and after
• Weigh the spirit burner containing ethanol before and after
• Work out temperature change and mass of ethanol burned

Question 31

How do you conduct an experiment to work out the enthalpy of neutralisation?

Answer 31

• Add 25cm³ of 1 mole/dm³ acid into a polystyrene cup, and measure the temperature of the acid
• Measure the temperature of the alkali, and then add 25cm³ of the alkali to the acid
• Stir the solution until the maximum temperature is reached

Question 32

How do you work out temperature change in enthalpy of neutralisation experiments?

Answer 32

Take the average of the acid and alkali initial temperatures.
Subtract this value from the final solution temperature.

Question 33

What else must you work out in enthalpy of neutralisation calculations?

Answer 33

The moles, by multiplying the concentration and the volume (in dm)

Question 34

What uncertainties are there in enthalpy of neutralisation experiments?

Answer 34

• Usual uncertainties of measurement involved with the use of the pipette and thermometer
• Some heat energy would be transferred to the thermometer and cup

Question 35

Why are state symbols important in standard enthalpy of formation tests?

Answer 35

Because elements must be in their standard states; the value for enthalpy change of formation for gaseous ethanol is different to that of liquid ethanol

Question 36

Why is Hess’s law often used in standard enthalpy of formation calculations?

Answer 36

As most standard enthalpy of formation cannot be determined experimentally

Question 37

What does Hess’s law state?

Answer 37

The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.
I.e. going from reactants A -> products B and reactants A -> intermediates -> products B will have the same overall enthalpy change (much like vectors in maths)