Energetics

Question 1

Enthalpy Change

Symbol

Units

Standard Conditions

Answer 1

The heat energy transferred in a reaction at constant pressure.

Delta H

kJ mol -1

100 kPa pressure and a temperature of 298 K (25oC)

Question 2

Exothermic Reactions

Answer 2

Give out energy

Delta H is negative

Question 3

Endothermic Reactions

Answer 3

Absorb energy

Delta H is positive

Question 4

Endothermic reactions and bonds

Answer 4

Energy is needed to break bonds, so bond breaking is endothermic (delta H is positive).

Stronger bonds take more energy to break

Question 5

Exothermic reactions and bonds

Answer 5

Energy is released when bonds are formed, so bond making is exothermic (delta H is negative).

Stronger bonds release more energy when they form

Question 6

Enthalpy Change of a Reaction

Answer 6

The overall effect of bonds broken and bonds being formed.

If you need more energy to break bonds than is released when bonds are made, delta H is positive.

If it’s less, delta H is negative

Question 7

Bond Enthalpy

Answer 7

The energy required to break bonds

Question 8

Mean Bond Enthalpy

Answer 8

The average energy needed to break a certain type of bond, over a range of compounds

Question 9

Formula for calculating enthalpy change of reaction

Answer 9

Total energy absorbed - total energy released

Question 10

Standard Enthalpy of Formation

Answer 10

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 11

Standard Enthalpy of Combustion

Answer 11

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

Question 12

Formula for calculating enthalpy changes from a calorimetry experiment

Describe each part and give the units

Answer 12

q=mc delta T

where:
q= heat lost or gained (in joules). Same as the enthalpy change if pressure constant
m= mass of water (or other solution) in the calorimeter (in grams)
c= specific heat capacity of water (4.18 J g-1 K-1)
Delta T= the change in temperature (K) of the water or solution

Question 13

Hess’s Law

Answer 13

The total enthalpy change of a reaction is independent of the route taken

Question 14

Two assumptions when calculating enthalpy changes

Answer 14

1. You can assume that all solutions have the same density as water.
2. Although the official unit for delta T is K, the value is actually the same in oC

Question 15

Specific Heat Capacity of Water

Answer 15

The amount of energy it takes to raise the temperature of 1 g of water by 1 K