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Chemistry - Energetics (Bond Enthalpies (When we calculate enthalpy…
Chemistry - Energetics
Any chemical reaction involves breaking and making bonds. Energy is needed to break bonds, and is given off when making bonds. These two numbers are very rarely the same, so energy is released or absorbed during chemical reactions.
With exothermic reactions, like C + O2 --> CO2, the enthalpy change is -384kJmol-1. The '-' sign shows that it is an exothermic reaction.
In the reaction, the reactants have lost energy, as when burning it, heat is released.
An endothermic reaction is CaCO3 --> CaO + CO2. The enthalpy change here is +178kJmol-1, and the '+' sign shows that the reaction is endothermic, and energy is absorbed.
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Because you require 1 mole of substance at the end of the reaction, you might need to make the left hand side include fractions
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Enthalpy calculations
To work out enthalpy change you use Q=mcAT to work out the energy transferred. You then work out the moles on the ethanol (For example) that are burnt, by doing the mass of the ethanol burnt divided by the Mr. Finally you divide the value for 'Q' by 1000 to get kJ, and divide the two numbers to calculate the enthalpy change.
Accuracy
One reason as to why the chemical reactions for this experiment aren't always 100% accurate is because the heat also heats surroundings.
Hess' Law
Many reactions are difficult to carry out in labs in normal conditions, so their enthalpy changes can't be measured here.
This is where Hess' Law comes into place, which uses the law of conservation of energy. This states that if a reaction can take more than one route, as long as the initial and final conditions are the same, the enthalpy change is the same.
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Bond Enthalpies
When we calculate enthalpy changes, it is the energy taken in or given out that is measured. This is then the difference in energy between breaking and making new bonds.
A strong bond is a bond that requires more energy to break. The same amount of energy will be given out when the same bond is formed.
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For example in methane (CH4), the molecules contains 4 C-H molecules. When the first is broken, the second C-H molecule is then breaking from CH3, and so on. So the enthalpy change for breaking of C-H bonds in methane will be a mean of the four figures.
