3.1.2 Enthalpy changes

Question 1

Define enthalpy

Answer 1

measure of the heat energy stored in a chemical system

Question 2

Define enthalpy change

Answer 2

difference between enthalpy of reactants and enthalpy of products
- can be positive or negative

AH = H (products) - H ( reactants)

Question 3

What happens to the energy in an exothermic reaction?

Answer 3

heat energy is tranferred** to the surroundings ** from the system - temperature of the surroundings increases
(chemical energy is changed into thermal)

Products have less energy that the reactants so there is a negative enthalpy change

Question 4

What happens to the energy in an endothermic reaction?

Answer 4

heat energy is transferred from the surroundings to the system - temperature of the surroundings decreases
(thermal energy is changed into chemical)

Products have more energy than the reactants so enthalpy change is positive

Question 5

Define activation energy (Ea)

Answer 5

minimum energy required for a reaction to take place

Question 6

What are the standard conditions for enthalpy change?

Answer 6

• 100 kPa
• 298 K
• 1 mol dm -3
• Standard states

Question 7

Define standard enthalpy change of reaction

Answer 7

enthalpy change when molar quantities as states in the chemical equation react under** standard conditions**

Question 8

Define standard enthalpy change of formation

Answer 8

enthalpy change when one mole of a substance is formed from its** consituent elements** in their standard states in their standard conditions

standard enthalpy change of formation in elements = zero

Question 9

Define standard enthalpy change of combustion

Answer 9

enthalpy change when **one mole **of a substance is completely burned in oxygen under standard conditions

• always exothermic

Question 10

Define standard enthalpy change of neutralisation

Answer 10

enthalpy change when an acid and a base in their standard states react to form one mole of water under standard conditions

Question 11

Give the equation used in calculating enthalpy change

Answer 11

q = mcAT

q = heat lost or gained
m = mass of surroundings (water or solution)
c = specific heat capacity
AT = temperature change of water or solution

Question 12

What is the unit for enthalpy change?

Answer 12

KJ mol-1

Question 13

What is the method for calculating enthalpy change?

Answer 13

1. Use q = mcAT to calculate the heat given out
2. calculate moles of reactants
3. divide energy change by the smallest number of moles to get enthalpy change
4. add a sign (positive for endothermic, negative for exothermic)

Question 14

Define bond enthalpy

Answer 14

energy needed to break a particular bond

Question 15

Define bond dissociation energy

Answer 15

energy needed to break one mole of the bond to give separated atoms
(everything must be in the gas state)

Question 16

Define average bond enthalpy

Answer 16

mean amount of energy required** to break one mole** of a specific type of covalent bond in gaseous molecules

Question 17

What happens to the bonds in an exothermic reaction?

Answer 17

more energy is RELEASED in forming new bonds

Question 18

What happens to the bonds in an endothermic reaction?

Answer 18

more energy is NEEDED to break bonds

Question 19

Why do we use the average bond enthalpy?

Answer 19

every bond in a compound has a slightly different bond energy

Question 20

What is the formula for standard enthalpy change of reaction using average bond enthalpy values?

Answer 20

enthalpy change of reaction =
(sum of bonds broken) - (sum of bonds formed)

Question 21

What formula is used if standard enthalpy change of formation values are provided?

Answer 21

enthalpy change of reaction =
(enthalpy change of formation of products) - (enthalpy change of formation of reactants)

Question 22

What formula is used if standard enthalpy change of combustion values are provided?

Answer 22

enthalpy change or reaction =
(enthalpy change of combustion of reactants) - (enthalpy change of formation of products)

Question 23

Define Hess’ law

Answer 23

total enthalpy change for the reaction is independent of the route taken
(energy is conserved

Question 24

How can you use a hess cycle to find the enthalpy change of reaction, when given formation values?

Answer 24

1. construct enthalpy cycle (reactants, products, elements in their standard states)
2. reactants and products are formed from their elements so arrows point up
3. add formation values for reactants and products
4. calculate the unknown enthalpy change using (products - reactants)

with the arrow = same sign
against the arrow = change sign

Question 25

How can you use a hess cycle to find the enthalpy change of reaction, when given combustion values?

Answer 25

1. construct enthalpy cycle (reactants, products and common combustion products)
2. reactants and products react to form combustion products so arrows point down
3. add combustion values for reactants and products
4. calculate unknown enthalpy change (reactants - products)

with the arrow = same sign
against the arrow = change sign

Question 26

Errors in the enthalpy change of reaction practical method

Answer 26

• energy transfer to the surroundings
• approximation of the SHC of the solution
• reaction or dissolving may be incomplete or slow

Question 27

Errors in the enthalpy change of combustion practical method

Answer 27

• energy loss from calorimeter
• incomplete combustion of fuel
• incomplete transfer of energy
• evaporation of fuel after weighing
• measurements not carried out under standard conditions as water is a gas not a liquid in the experiment