P10
Combustion
Fossil fuels are our main source of energy at present and this energy is released
through combustion reactions. This chapter investigates the energy considerations
involved in chemical reactions and the factors that influence reaction rates.
Combustion
Process in which a self-sustaining chemical reaction occurs at
temperatures above those of the surroundings.
Combustion is burning.
Combustion reactions are exothermic.
Combustion of fossil
fuels
Fossil fuels are compounds of carbon and produce carbon
dioxide and water as well as energy.
Heat of reaction
The amount of heat released or absorbed when a chemical
reaction occurs.
Calculate the molar
heat of a reaction



Change in enthalpy
(H)
Calculate the amount of heat released or absorbed
(q=mCT)
Calculate the number of moles that reacted (n = m/M)
Calculate the heat released or absorbed per mole
Total heat released
Number of moles reacted
The heat absorbed per mole of a specified reactant or product,
when the reaction occurs at constant pressure.
H = (enthalpy of products) - (enthalpy of reactants)
H depends on the physical state of the reactants and products
and therefore the physical states are always indicated in
chemical equations.
Exothermic reactions
H is negative as energy is released by these reactions.
Exothermic reactions include
 Combustion reactions
 Reactions involving the combination of elements
 Reactions of metals with water and acids
 Reaction of acids and bases
Endothermic
reactions
H is positive as energy is absorbed by these reactions.
Endothermic reactions include
 Decomposition reactions
 Photosynthesis
 Some precipitation reactions
 Many industrial processes
Heat of combustion
Activation energy
Ignition temperature
The heat liberated when one mole of the substance undergoes
complete combustion with oxygen at a constant pressure of
one atmosphere.
The numerical value for the heat of combustion of any
substance is always written as a positive number because it is
defined as the heat released.
The minimum amount of energy reactant molecules must
possess in order to form products
The minimum temperature to which a fuel-air mixture must be
heated in order for combustion to occur.
The greater the activation energy the higher the ignition
temperature.
A wick
Piece of absorbent string, rope or fabric that dips into a fuel to
help the fuel burn.
A wick is needed if
 The fuel is relatively non-volatile
 The fuel has a high ignition temperature
Pollution from
burning fossil fuels




The incomplete combustion of fuels because of
insufficient oxygen results in the formation of carbon
monoxide and/ or carbon (soot)
The presence of sulfur or sulfur compounds as impurities
in fuels results in the formation of sulfur dioxide.
The reaction of oxygen and nitrogen at high temperatures
produces a number of different oxides of nitrogen
The combustion of coal forms particulates
Greenhouse effect
Global warming caused by increased levels of carbon dioxide
and other gases in the atmosphere trapping heat that
previously was radiated into space.
The burning of fossil fuels is a major contributor to global
warming.
Rates of reaction
The rate of change of concentration with time.
The rate of a reaction at any time in a reaction is equal to the
slope of the reaction’s concentration/ time graph at that time.
The reaction rate decreases as the reaction proceeds.
Catalysts
Substances that increase the rate of a reaction without
undergoing a permanent change in the reaction.
Catalysts increase the reaction rate by providing an alternative
pathway with a lower activation energy
Reaction rate and
temperature
Small increases in temperature can cause large increases in
reaction rates because there may be many more molecules
with energy greater than the activation energy.
Enzymes
Biological catalysts.
Explosions
Very rapid combustion reactions.