Energy changes andrates of reaction

Combustion is a process in which a self-sustaining chemical reaction occursat temperatures above those of the surroundings. More simply, combustion isburning. 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 theair and the greyish-black solid (coke), the colourless liquid (octane) and the colourlessgas (methane). Each of these burns with a bluish fl ame to form colourless carbondioxide 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 callexothermic reaction

EXOTHERMIC AND ENDOTHERMICREACTIONS 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 andthe ‘lost’ chemical energy is released as heat, which warms up the test tube andits contents. When an endothermic reaction occurs, the test tube gets cold. Thisis because the reaction as it occurs needs to take in heat (to convert to chemicalenergy). The only place the reaction can get this heat is from the test tube and itscontents and so they get cold. Unfortunately the heat released or absorbed during a reaction depends tosome 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 theatmosphere). This is particularly true for reactions involving gases. When we compareheats from different reactions, we should do so using constant conditions. Hence weintroduce a new term called enthalpy.

ENTHALPYEnthalpy is a measure of the total energy possessed by a substance or group ofsubstances. We can think of enthalpy as being mainly the chemical energy storedin a substance. Unfortunately we cannot measure this total energy or enthalpy ofa substance; all we can do is measure changes in it.

Since most experiments we shall be dealing with occur at constant pressure (opento 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 fromreactants to products:H = enthalpy of products – enthalpy of reactants

NOT GOOD.....in a small way.......I will explain

an exothermic change

reactants

activated

products

Enth

alp

y H

Reaction time (s)

Ea

heatreleased

initial enthalpy

reactants

activated state

products

Enth

alp

y H

Ea

Heatabsorbed

An endothermic change

initial enthalpy

http://www.chem.ufl.edu/~itl/2045/matter/FG05_017.GIF

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