CHEM1011 Lecture Notes - Lecture 23: Enthalpy, Thermodynamics, Dynamic Equilibrium
9 Jun 2018
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Lecture 23: Fuels and combustion / Equilibrium
Recap: Hess’s Law
- Hesss La – we can use enthalpy data for known reactions to calculate enthalpy
changes for new or unknown reactions, ∆Hrxn
- We looked at two different ways of doing this, using either:
“tadard ethalpies of foratio, ∆H°f (red route)
Aerage od ethalpies, ∆H°bond (blue route)
Fuels and heat engines
The First of Thermodynamics Law tells us that heat can be converted into work. The efficiency
of this conversion – the fratio of the aailale eerg, ∆Hrxn, turned into work – depends on
the specific process or path.
A heat engine is a device that converts heat into work, by exploiting the energy difference
between a hot source and a cold sink. The efficiency of a heat engine is defined simply as the
fraction of the available energy that can be used for work. E.g. laptops generate heat- metal
chip that connects hot source (chip) to fan (sink)
Enthalpy can tell us how much heat can be released by combustion of a particular fuel - the
available energy qin from a process such as a phase change or reaction (combustion of a fuel).
Fuel types and efficiencies
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What are the considerations?
- Energy output per unit (or weight if we are fuelling something mobile), or calorific value.
- Safety: explosive, toxic, or toxic by-products. Storage as a gas, liquid or solid?
- Environmental impact
These depend on application. For car, truck or aircraft fuels, we might add ease of re-fuelling
ad prefer soethig that reats ith oge, hih e dot hae to arr aroud. For
spaceships, re-fuelling is not relevant, and we need to carry both reactants.
Petrol or gasoline fuels
These contain a mixture of saturated (single C-C bonds) and unsaturated (some multiple C-C
bonds) hydrocarbons, with a distribution of carbon numbers between about 3 and 10, obtained
by refining crude oil. The general combustion reaction is
- Do not need to know the equation (just be able to balance it)
The heat of combustion of various fuels may be measured in a bomb calorimeter and depends
on whether the water remains as a gas (as it does in a car) or condenses to a liquid, in which
case it increases the calorific value of the fuel through its heat of vapourisation. (Net calorific
value → gross calorific value) Hydrocarbon fuels all have fairly consistent calorific values
Petrol or gasoline fuels: stoichiometry
The stoichiometry of combustion thus relies on the fuel to oxygen ratio. For n-heptane this is
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