Heat of Combustion Revision Notes for HSC SSCE Chemistry

Heat of Combustion

What is molar heat of combustion?

Molar heat of combustion is a term commonly used when discussing fuels and foods. It refers to the amount of heat energy released when one mole of a substance burns completely in oxygen under specific conditions.

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The formal definition states that molar heat of combustion is the heat liberated when $1$ mole of a substance undergoes complete combustion with oxygen at a constant pressure of $100.0$ kPa. The final products must be carbon dioxide gas and liquid water.

Combustion (also called burning) always releases heat energy to the surroundings, making it an exothermic process. This is why we feel warmth when something burns.

Understanding the sign convention

One important aspect that often confuses students is the difference between "heat of combustion" and "enthalpy change for combustion". These terms use opposite signs, which you need to remember carefully.

Molar heat of combustion measures heat released, so it is expressed as a positive value. Think of it as the energy output that we can use from the fuel.

Enthalpy change (represented as $\Delta H$ ) measures heat absorbed by the system. Since combustion releases heat (rather than absorbing it), the enthalpy change for combustion is always negative.

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Key Rule for Sign Convention:

Molar heat of combustion (heat released) = positive value
Enthalpy change for combustion (heat absorbed) = negative value

These are actually the same quantity with opposite signs. The molar heat of combustion equals the negative of the enthalpy change.

Example: propane combustion

Propane ( $\text{C}_3\text{H}_8$ ) is a major component of liquefied petroleum gas (LPG), commonly used in portable gas bottles. The molar heat of combustion of propane is $2220$ kJ mol $^{-1}$ .

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Worked Example: Propane Combustion

When propane burns according to the equation:

$\text{C}_3\text{H}_8(\text{g}) + 5\text{O}_2(\text{g}) \rightarrow 3\text{CO}_2(\text{g}) + 4\text{H}_2\text{O}(\text{l})$

The molar heat of combustion is $2220$ kJ mol $^{-1}$ (positive value)

The enthalpy change is $\Delta H = -2220$ kJ mol $^{-1}$ (note the negative sign)

Heats of combustion for common substances

Different fuels and compounds release different amounts of energy when they burn. The table below shows molar heats of combustion for some common substances:

Substance	Molar heat of combustion (kJ mol $^{-1}$ )
Hydrogen, $\text{H}_2(\text{g})$	$285$
Carbon, $\text{C}(\text{s})$	$393$
Carbon monoxide, $\text{CO}(\text{g})$	$280$
Methane, $\text{CH}_4(\text{g})$	$890$
Ethane, $\text{C}_2\text{H}_6(\text{g})$	$1560$
Propane, $\text{C}_3\text{H}_8(\text{g})$	$2220$
Butane, $\text{C}_4\text{H}_{10}(\text{g})$	$2877$
Octane, $\text{C}_8\text{H}_{18}(\text{l})$	$5460$
Ethanol, $\text{C}_2\text{H}_5\text{OH}(\text{l})$	$1360$
Glucose, $\text{C}_6\text{H}_{12}\text{O}_6(\text{s})$	$2803$

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Notice that larger hydrocarbon molecules generally have higher heats of combustion because they contain more carbon and hydrogen atoms to react with oxygen. Compare methane ( $890$ kJ mol $^{-1}$ ) with octane ( $5460$ kJ mol $^{-1}$ ) to see this pattern clearly.

How we measure heat of combustion

Heat of combustion cannot be measured directly. Instead, we use an indirect method based on calorimetry. The burning substance heats a known mass of water, and we measure the temperature increase.

The principle relies on the law of conservation of energy, which states that energy cannot be created or destroyed, only changed from one form to another.

In practical terms:

$\text{Heat released by combustion reaction} = \text{Heat absorbed by water}$

This relationship works because the chemical energy stored in the fuel is converted to heat energy, which then transfers to the water. To get accurate results, we must minimise heat losses to the surroundings by using insulated containers.

The heat absorbed by water can be calculated using the formula:

$q = mc\Delta T$

where:

$q$ = heat energy (in joules)
$m$ = mass of water (in kilograms)
$c$ = specific heat capacity of water ( $4.18 \times 10^3$ J K $^{-1}$ kg $^{-1}$ )
$\Delta T$ = temperature change (in kelvin or degrees Celsius)

Investigation 14.3: measuring heats of combustion

In this practical investigation, you will determine the heat of combustion for three commonly available alcohols: ethanol, 1-propanol, and 1-butanol. These alcohols are contained in spirit burners, which allow controlled combustion.

The method builds on previous investigations by using quantitative measurements. You will burn a measured mass of each alcohol and collect the released heat in a known quantity of water.

Aim: To determine the heat of combustion for several compounds.

Key measurements needed:

For the alcohols:

Mass of alcohol before burning
Mass of alcohol after burning
Identity of the alcohol

For the water:

Mass (or volume) of water
Initial temperature
Final temperature

Independent variable: The type of alcohol used

Dependent variable: The heat released (calculated from temperature change)

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Risk Assessment

Before beginning any practical work, you must identify potential hazards and plan how to manage them safely. Complete a risk assessment table listing:

What are the risks in doing this investigation?
How can you manage these risks to stay safe?

For this investigation, consider risks such as flammable liquids, open flames, hot equipment, and hot liquids. Your teacher should check your risk assessment before you proceed.

Analysing your results

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Steps for Calculating Molar Heat of Combustion:

Calculate how much heat was absorbed by the water using $q = mc\Delta T$
Recognise that the heat absorbed by water equals the heat released by the burning alcohol (from conservation of energy)
Calculate the number of moles of alcohol burnt using: $n = \frac{\text{mass}}{\text{molar mass}}$
Calculate the molar heat of combustion: $\text{Molar heat of combustion} = \frac{\text{heat released}}{\text{moles of alcohol burnt}}$
Compare your experimental value with literature values from reliable sources

Your experimental values will likely be lower than literature values due to heat losses to the surroundings, incomplete combustion, and other sources of error.

Exam tips

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Exam Success Tips:

Always remember: combustion is exothermic, so it releases heat
Check whether a question asks for heat of combustion (positive) or enthalpy change (negative)
When using the conservation of energy equation, ensure all units are consistent
For calculations, work in joules first, then convert to kilojoules at the end
Show all working clearly, including units at every step

Remember!

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Key Points to Remember:

Molar heat of combustion is the heat liberated when $1$ mole of substance undergoes complete combustion with oxygen at $100.0$ kPa, producing $\text{CO}_2(\text{g})$ and $\text{H}_2\text{O}(\text{l})$
Molar heat of combustion (heat released) is positive, while enthalpy change for combustion (heat absorbed) is negative
All combustion reactions are exothermic (they release heat), so $\Delta H$ for combustion is always negative
Heat of combustion is measured indirectly using the principle: heat released by combustion = heat absorbed by water
The law of conservation of energy states that energy cannot be created or destroyed, only changed from one form to another

Heat of Combustion (HSC SSCE Chemistry): Revision Notes