Chapter 9 (Enthalpy)

Question 1

What is the definition of enthalpy ?

Answer 1

Enthalpy is defined as the total heat content within a system

Question 2

How to calculate enthalpy change ?

Answer 2

ΔH = H(products) - H(reactants)
- because energy has to be conserved this change in energy has to be transferred, this takes place between system and your surroundings

Question 3

What are exothermic reactions ?

Answer 3

• transfer energy to their surroundings, this can be identified be an increase in temperature of the surroundings
• ΔH = negative

Question 4

What does the enthalpy and reaction profile looks like for an exothermic ?

Answer 4

Question 5

What is an endothermic reaction?

Answer 5

• Energy transfer from surroundings to the system
• ΔH = positive

Question 6

What does the Enthalpy and reaction profile look like for endothermic reactions ?

Answer 6

Question 7

What are the standard conditions ?

Answer 7

• standard pressure: 100 kpa
• standard temperature: 298K
• standard concentration: 1 mol dm-3
• standard state: the physical state of a substance under standard conditions

Question 8

What is the standard enthalpy changes of a reaction ΔHr ?

Answer 8

enthalpy change that accompanies a reaction in the molar quantities stated in an equation, under standard conditions with all reactants and products in their standard state

Question 9

What is the standard enthalpy changes of formation ΔHf ?

Answer 9

Enthalpy change when 1 mole of a compound is formed from its elements, under standard conditions with all reactants and products in their standard state

Question 10

What is the standard enthalpy change of formation of an element ?

Answer 10

• The formation of an element has to be 0 kj mol -1
• because you are forming an element from an element

Question 11

What is standard enthalpy change of combustion ΔHc ?

Answer 11

Enthalpy change when 1 mole of a substance reacts completely with oxygen, under standard conditions with all reactants and products in their standard state

Question 12

What is the standard enthalpy change of neutralisation ΔHneut ?

Answer 12

• enthalpy change that accompanies a reaction of an acid and base to form 1 mole of H2O under standard conditions with all reactants and products in their standard state
• always -57 Kj mol-1

Question 13

How to calculate heat energy in a temperature change ?

Answer 13

• Δq = mc Δt
• where Δq is the change in energy
• m is the mass
• Δt is the change in temperature

Question 14

How to determine enthalpy change of combustion ?

Answer 14

• Record the volume of water (use density to calculate)
• record the mass of fuel before and after heating
• record the temperature change of water
• use q = mct (convert to kj)
• find the moles of fuel burnt

Question 15

What are possible sources of error in the spirit burner ?

Answer 15

• heat transferred to beaker / surrounding air instead of H2O heat loss to surroundings
• soot formed on beaker - incomplete combustion
• draughts (winds) effect flame
• evaporation of fuel - ethanol vaporises very easily - weigh as soon as possible after extinguishing the flame
• non-standard conditions

Question 16

What is the definition of bond enthalpy and average bond enthalpy ?

Answer 16

• Energy required to break 1 mole of a specified type of bond in a gaseous molecule
• this value depends on the bond environment
• average bond enthalpies are used for bonds that occur in different molecules (different environments)

Question 17

What are the limitations of average bond enthalpy ?

Answer 17

• using average bond enthalpies rather than the actual energy involved to make and break
• the energy of individual bonds will be slightly different
• average enthalpy is in gaseous state, so if something is liquid you need to take into account the enthalpy change for condensing from gas to liquid