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Energy changes and rates of reaction. Combustion is a process in which a self-sustaining chemical reaction occurs at temperatures above those of the surroundings. More simply, combustion is burning. Explosions are also forms of combustion.
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Energy changes and rates of reaction
Combustion is a process in which a self-sustaining chemical reaction occurs at temperatures above those of the surroundings. More simply, combustion is burning. Explosions are also forms of combustion.
Some common combustion reactions are the burning of coke (carbon), petrol (for example, octane) and natural gas (methane). The reactants are oxygen from the air and the greyish-black solid (coke), the colourless liquid (octane) and the colourless gas (methane). Each of these burns with a bluish fl ame to form colourless carbon dioxide gas and colourless water vapour. Chemical reactions have occurred here (rather than just physical changes) because:
The amounts of heat released in the above reactions are 393 kJ/mol for coke, 890 kJ/mol for methane and 5460 kJ/mol for octane. All combustion reactions liberate large amounts of heat. They are what we call exothermic reaction
EXOTHERMIC AND ENDOTHERMIC REACTIONS discuss signs etc
Examples of endothermic chemical reactions are: There are far more exothermic reactions than endothermic ones. When we carry out an exothermic reaction in a test tube, the test tube gets hot. This is because as the reaction occurs there is a decrease in chemical energy and the ‘lost’ chemical energy is released as heat, which warms up the test tube and its contents. When an endothermic reaction occurs, the test tube gets cold. This is because the reaction as it occurs needs to take in heat (to convert to chemical energy). The only place the reaction can get this heat is from the test tube and its contents and so they get cold. Unfortunately the heat released or absorbed during a reaction depends to some extent on the conditions under which the reaction is carried out, in particular upon whether it is performed at constant volume (in a closed vessel) or at constant pressure (in a container open to the atmosphere). This is particularly true for reactions involving gases. When we compare heats from different reactions, we should do so using constant conditions. Hence we introduce a new term called enthalpy.
ENTHALPY Enthalpy is a measure of the total energy possessed by a substance or group of substances. We can think of enthalpy as being mainly the chemical energy stored in a substance. Unfortunately we cannot measure this total energy or enthalpy of a substance; all we can do is measure changes in it. Since most experiments we shall be dealing with occur at constant pressure (open to the atmosphere), the heat absorbed or released will be a direct measure of H. By ‘change in enthalpy’ we mean the increase in enthalpy in going from reactants to products: H = enthalpy of products – enthalpy of reactants
activated an exothermic change Ea Enthalpy H reactants heat released products initial enthalpy Reaction time (s)
activated state Enthalpy H Ea products Heat absorbed reactants initial enthalpy An endothermic change
