TOPIC 8: ENERGETICS

Question 1

Enthalpy Change of Formation (∆fH o )

Answer 1

The enthalpy change for the formation of 1 mole of a compound from its elements in their standard states

Question 2

Enthalpy Change of Combustion (∆cH o )

Answer 2

The enthalpy change for the complete combustion per mole of a substance in their standard states

Question 3

Enthalpy Change of Neutralisation (∆nH)

Answer 3

The enthalpy change on the formation of one mole of water when an acid is neutralised

Question 4

Enthalpy Change of Reaction (∆Ho)

Answer 4

The enthalpy change when quantities of substances in standard states react completely under standard conditions.

Question 5

What is Hess’s Law?

Answer 5

The enthalpy change is independent of the route taken

Question 6

Define Enthalpy Change

Answer 6

Enthalpy change is the heat energy change in a reaction at constant pressure. The Units are Kj mol^-1

Question 7

Describe what happens in an Exothermic Reaction

Answer 7

Reactions give out heat energy. In Exothermic reactions the temperature often goes up .The enthalpy change is negative.

Question 8

Describe what happens in an Endothermic Reaction

Answer 8

Reactions absorb heat energy and the temperature often falls.The enthalpy change is positive.

Question 9

Describe enthalpy of products and reactants in Exothermic Reactions

Answer 9

The products have less enthalpy than the reactants

Question 10

Describe enthalpy of products and reactants in Endothermic Reactions

Answer 10

The products have more enthalpy than the reactants

Question 11

What is the position of enthalpy change arrows for exothermic reactions?

Answer 11

Pointing Down

Question 12

What is the position of enthalpy change arrows for endothermic reactions?

Answer 12

Pointing Up

Question 13

Describe the Conditions for Enthalpy Changes

Answer 13

Standard Conditions

1. 100 Kpa
2. 298 k

Question 14

Equation to calculate Enthalpy Changes ?

Answer 14

q=mc deltaT
q= heat lost or gained in joules. This is the same as the enthalpy change if the pressure is constant.

m= mass of water in the calorimeter or solution in the insulated container (in grams)

c= Specific heat capacity of water (4.18 J g^-1K^-1)

delta T= The change in temperature of the water or solution (in K)

Question 15

What type of reaction is Bond Breaking ?

Answer 15

Endothermic

Question 16

What type of reaction is Bond forming ?

Answer 16

Exothermic

Question 17

Define Bond Enthalpy

Answer 17

Bond Enthalpy is the amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase

Question 18

Define mean Bond Enthalpy

Answer 18

The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds.

Question 19

How to calculate Enthalpy Change of Reaction

Answer 19

Sum of bond enthalpies of reactants - Sum of bond enthalpies of products

Question 20

Define activation energy

Answer 20

The minimum energy required for a chemical reaction to take place

Question 21

What is the relationship between stability and enthalpy

Answer 21

The less enthalpy a substance has the more stable it is

Question 22

Describe an experiment that could be used to find the enthalpy change of a reaction

Answer 22

1. Add set volume of a substance and measure the temperature of the solution until its stabilised
2. Add a set mass of a substance and stir
3. Measure the temperature of the reaction mixture every 30 seconds until a maximum or minimum temp was reached and draw a graph of temperature against time
4. extrapolate the line where the temperature is returning to initial temperature and find the temperature change
5. if endothermic temperature will fall and temperature change will be negative
6. if exothermic temperature will rise and temperature change will be positive

Question 23

What can we use to measure the enthalpy change of combustion

Answer 23

a calorimeter

Question 24

how to calculate enthalpy change

Answer 24

heat energy/ moles (kj mol-1)