Energetics

Question 1

What is enthalpy change?

Answer 1

The heat energy change at a constant pressure

Question 2

What are the units for enthalpy change?

Answer 2

KJ mol-1

Question 3

What is the enthalpy of a reaction?

Answer 3

The heat energy stored in a chemical reaction

Question 4

What is an exothermic reaction? Is ΔH +ve or -ve?

Answer 4

Heat energy is given out to the surroundings, causing the temp of surroundings to increase.
The reactions system looses energy so ΔH is -ve?

Question 5

What is an endothermic reaction? Is ΔH +ve or -ve?

Answer 5

Heat energy is taken in from the surroundings, so temp of surroundings decreases.
The reactions system gains energy, so ΔH is +ve

Question 6

Give two important examples of expthermic reactions

Answer 6

Combustion of fuels
Respiration

Question 7

Give two important examples of endothermic reactions

Answer 7

Photosynthesis
Thermal decomposition of calcium carbonate

Question 8

What key features should an enthalpy profile diagram show?

Answer 8

Products
Reactants
Activation energy
Enthalpy change

Question 9

What are enthalpy profile diagrams?

Answer 9

Diagram with heat energy against time, within a system
Shows the enthalpy changes of a reaction
Box surrounding with thermometer to show surroundings

Question 10

What is the activation energy?

Answer 10

The minmum energy required to start a reaction, by the breaking of bonds

Question 11

What is bond enthalpy?

Answer 11

The heat energy required to break one mole of a covalent bond within a molecule in the gaseous state

Question 12

Why are bond enthalpys always positive values?

Answer 12

Becasue heat energy is ALWAYS required to break a bond

Question 13

What is mean bond enthalpy?

Answer 13

The heat energy required to break one mole of a covalent bond, averaged for that type of bond within a range of different compounds in the gaseous state.

Question 14

Why might the mean bond enthalpy used rather the bond enthalpy?

Answer 14

Because the bond enthalpy for the same covalent bond may be different in different compounds

Question 15

Give the equation for enthalpy change?

Answer 15

ΔH= energy requored to break bonds (energy in) - energy released when new bonds form (energy out)

Question 16

What are the standard contitions considered in standard enthalpy changes?

Answer 16

298K, 100KPa

Question 17

What is a standard state?

Answer 17

The physical state of a compound when it is in standard conditions

Question 18

What is the standard enthalpy change of formation?

Answer 18

The enthalpy change when one mole of a compound is formed from its constituent elements, when all reactants and products are in the standard states

Question 19

What is the standard enthalpy change of formation of an element?

Answer 19

0

Question 20

How does the standard enthalpy change of formation of a compound tell you how stable it is?

Answer 20

The more negative the value, the stable the compound is

Question 21

What is the standard enthalpy change of combustion?

Answer 21

The enthalpy change when one mole of a compiund reacts completely in oxugen, with all reactants and products in the standard states

Question 22

State the first law of thermodynamics

Answer 22

Energy cannot be created or destroyed, but it can be changed from one form to another

Question 23

State Hess’s law

Answer 23

The enthalpy change of a chemical reaction is independent of the route taken.

Question 24

What is Hess’s law used for?

Answer 24

Used to measure the enthalpy change of a reaction that cannot be measured directly, using the enthalpy cycle

Question 25

Why might it not be easy to measure the enthalpy change of a reaction directly?

Answer 25

• the rate of reaction may be too slow
• there may be a very high activation energy

Question 26

When using Hess’s law, which way should the arrows piint when using combustion or formation data?

Answer 26

Formation- arrows point upwards
Combustion- arrows point downwards