3.2.1: Enthalpy changes

Question 1

Explain in terms of bond breaking and forming why a reaction can be exothermic

Answer 1

More energy released forming bonds than energy required to break them

Question 2

What are standard conditions?

Answer 2

1 atm
298K

Question 3

Give a feature of delta H f

Answer 3

Always 0 for elements

Question 4

Define average bond enthalpy

Answer 4

The breaking of one mole of bonds in gaseous molecules

Question 5

Give equation to work out energy of a reaction

Answer 5

q = mc delta T

Question 6

Give the equation to work out delta H

Answer 6

Delt H = q
——
moles in reaction

Question 7

Define enthalpy change of reaction

Answer 7

The number of moles of reactants as specified in the balanced equation which react together

Question 8

Define enthalpy change of formation

Answer 8

Enthalpy change when one mole of a substance is made from its constituent element in their standard states under standard conditions

Question 9

Define enthalpy change of combustion

Answer 9

Enthalpy change when one moles of substance is burned completely in oxygen under standard conditions

Question 10

Define enthalpy change of neutralisation

Answer 10

Enthalpy change when one mole of water is made in a neutralisation reaction under standard conditions

Question 11

Why do Br2 and I2 not exist in gaseous states under standard conditions

Answer 11

Energy is needed to break London forces between molecules

Question 12

Give 3 reasons why a calculated enthalpy changes of combustion may differ from data book value

Answer 12

• Non-standard conditions
• Heat released to surroundings
• Incomplete combustion

Question 13

Is combustion an exothermic or endothermic process ?

Answer 13

Exothermic (so - sign)

Question 14

Define activation energy

Answer 14

Minimum energy required for a reaction to take place

Question 15

Define standard states

Answer 15

The physical states under standard conditions

Question 16

What procedure is used to determine an enthalpy change of solution ?

Answer 16

• Use polystyrene cup to prevent heat loss
• Add acid and measure temp until it is constant for a fixed amount of time
• Add alkali, stir and measure temperature change using thermometer in a set time

Question 17

What procedure is used to measure enthalpy change of combustion ?

Answer 17

• Weigh fuel before and after to determine mass burnt
• Burn fuel to raise water temp by a specific amount
• Energy from fuel is transferred to water (some to surroundings)
• Lid placed on beaker with thermometer to prevent heat loss and wind shields by flame to prevent draught
• Use q = mc deltaT

Question 18

Which way do arrows point for enthalpy of formation ?

Answer 18

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