Chapter 9 enthalpy

Question 1

Define the term enthalpy of combustion

Answer 1

• is the enthalpy change when one mole of a substance
• is burned completley in oxygen
• under standard conditions (298K and 100 Kpa)

Question 2

Write the equation for the standard enthalpy change of combustion of propan-1-ol C3H7OH

Answer 2

C3H7OH (l)+4.5O2 (g) ->3CO2 (g) +4H2O (l)

Question 3

Suggest two reasons for the difference between this value and the one he obtained experimentally

Answer 3

• conditions are not standard
• heat loss
• incomplete combustion

Question 4

Define the term enthalpy of formation

Answer 4

• is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard stated under standard conditions

Question 5

Write and equation including state symbols representing the standard enthalpy change of PbO

Answer 5

Pb (s) + O2 (g) -> PbO (s)

Question 6

Write an equation including tate symbols for the bond enthalpy of I-I

Answer 6

I-I (g) -> 2I (g)

Question 7

Write an ionic equation for magnesium and hydrochloric acid

Answer 7

2H+ (aq) + Mg (s) -> Mg2+ (aq) + H2 (g)

Question 8

Explain the difference between strong and weak acids

Answer 8

• strong acid completley dissociate in solution
• weak acid partially dissociates in a solution

Question 9

Give two propertied of butane that makes it suitable as a fuel

Answer 9

• are exothermic
• low toxicity
• highly flammable

Question 10

What does standard mean in standard conditions

Answer 10

• 298K
• 100 Kpa
• occurs under standard conditions

Question 11

Write an equation for the standard enthalpy change of formation of butane C4H10

Answer 11

4C (s) + 5H2 (g) -> C4H10 (g)

Question 12

Describe how ammonia reacts in a neutralisation reaction

Answer 12

• it is a base
• as it acts as a proton acceptor

Question 13

4NH3 (g) + 5O2 (g) -> 4NO (g) +6H2O (g)
enthalpy change of reations = -908KJmol-1
NH3(g) - -46KJmol-1
Os (g) - 0KJmol-1
H2O (g) - -242KJmol-1
calcu.ate the standard enthalpy change of formation for NO (G)

Answer 13

(4x-46)= 186
(6x-242) = 1452
-184+1452-908 = 360
360/4 = +90 KJmol-1

Question 14

What is meant by the term enthalpy change of reaction (2)

Answer 14

• number of moles of reactants
• as specifies in the balanced equation react together

Question 15

it is very difficult to determine the standard enthalpy change of formation of hexane directly. Suggest a reason why (1)

Answer 15

• reactants don’t react
• many different hydrocarbons would form
• reaction to slow
• activation energy to high