Enthalpy of Combustion

Enthalpy of Combustion

By Angela Toh

​Recall:

Multiple Choice

Match the definition below to the correct term.

The enthalpy change that takes place when one mole of a compound is formed from its elements in their standard states under standard conditions.

• The enthalpy of neutralisation

• The enthalpy of combustion

• The enthalpy of formation

• The enthalpy of reaction

Multiple Choice

Diagram below shows

• Standard lattice enthalpy
• Standard enthalpy of precipitation
• Standard enthalpy of formation
• Standard enthalpy of hydration

Multiple Choice

What do we mean by the term standard conditions

• 1atm, 20C and 1moldm³

• 1atm, 0K and 1moldm³

• 10atm, 273K and 10mol/dm³

• 100kPa, 298K and 1mol/dm³

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​Checkpoint 6 Q2

​Try drawing the energy cycle.

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​Checkpoint 6 Question 3

​Try drawing the energy cycle.

Multiple Choice

Define standard enthalpy of combustion.

• Heat released when one mole of substance is burnt completely in excess oxygen.

• Heat absorbed when one mole of substance is burnt completely in excess oxygen under standard state.

• Heat released when one mole of substance is burnt completely in excess oxygen under standard conditions with all substances being in their standard states.

• Heat change when one mole of substance is burnt partially in excess oxygen under standard state.

Multiple Choice

What does the circle mean in ΔH°?

• Standard Conditions (T=298.15 K P=1 atm)

• Standard Temperature and Pressure (T=273.15 K P=1 atm)

• Degree K

• Degree C

Multiple Choice

What is the name of this enthapyl change

• Enthalpy change of formation

• Enthalpy change of combustion

• Enthalpy change of reaction

• Enthalpy change of atomisation

Multiple Choice

Which of these equations could represent a standard enthalpy change of combustion? (They are all correctly balanced)

• 2 C₄H₁₀ (g) + 13 O₂ (g) → 8 CO₂ (g) + 10 H₂O (l)

• C₄H₁₀ (g) + 6.5 O₂ (g) → 4 CO₂ (g) + 5 H₂O (l)

• C₄H₁₀ (g) + 4.5 O₂ (g) → 4 CO (g) + 5 H₂O (l)

• C₄H₁₀ (g) + 6.5 O₂ (g) → 4 CO₂ (g) + 5 H₂O (g)

Multiple Choice

Diagram below shows

• Standard enthalpy of combustion
• Standard enthalpy of solution
• Standard enthalpy of formation
• Standard enthalpy of reaction

Example 10 in Notes Pg 29

Energy Cycle:

The formation of ethyne (C₂H₂(g) can be represented by the following equation:

2C(graphite) + H₂(g) → C₂H₂(g)​

Draw an enthalpy cycle diagram relating the above equation to carbon dioxide and water.

Heat of combustion

​Example 10 (Pg 29)

Multiple Choice

The following equation shows the combustion of heptane, C₇H₁₆, in excess oxgen.

C₇H₁₆(l) + 11 O₂(g) → 7 CO₂(g) + 8 H₂O ΔH= -5 512 kJ mol^-

The combustion of heptane in excess oxygen releases 1 378 kJ of energy.

What is the mass of heptane used?

[Use the relative atomic mass of H = 1 and C = 12]

• 25.0 g

• 36.0 g

• 77.0 g

• 88.0 g

​Calculations using enthalpies of combustions

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​Answer

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​Answer

Multiple Choice

A simple calorimeter was used to determine the enthalpy of combustion of ethanol. The experimental value obtained was -920 kJ/mol. The Data Booklet value is -1371 kJ/mol. Which of the following best explains the difference between the two values?

• incomplete combustion of the fuel

• heat loss to the surroundings

• poor ventilation in the laboratory

• inaccurate temperature measurements