If the reaction produces heat (increases the temperature of the surroundings) then it’s exothermic. If it decreases the temp (i.e. absorbs heat) then it’s endothermic. Also, the yield of an equilibrium reaction which is exothermic will be increased if it occurs at low temps, and so for endothermic reactions at high temperatures. 6.1.2
Exothermic : A reaction which produces heat.
Endothermic : A reaction which absorbs heat.
Enthalpy of reaction : The change in internal energy (H) through a reaction is △H. 6.1.3
△H will be negative for exothermic reactions (because internal heat is being lost) and positive for endothermic reactions (because internal energy is being gained). 6.1.4
The most stable state is where all energy has been released. Therefore when going to a more stable state, energy will be released, and when going to a less stable state, energy will be gained. On an enthalpy level diagram, higher positions will be less stable (with more internal energy) therefore, if the product is lower, heat is released (more stable, △H is negative) but if it is higher, heat is gained (less stable, △H is positive). 6.1.5
Formation of bonds : Release of energy.
Breaking of bonds : Gain / absorption of energy.
6.2 Calculation of enthalpy changes
Change in energy = mass x specific heat capacity x change in temperature
(E = m x C x ΔT) 6.2.2
Enthalpy changes (ΔH) are related to the number of mols in the reaction. If all the coefficients are doubled, then the value of ΔH will be doubled. Attention must be paid to limiting reagents though, because enthalpy changes depend on the amount of reactants reacted (extensive property of enthalpy). 6.2.3
When a reaction is carried out in water, the water will gain or lose heat from (or to) the reaction, usually with little escaping the water. Therefore, the change in energy, and so the ΔH value, can be calculated with E = m x c x ΔT where E is equal to ΔH, m is the mass of water present, and c = 4.18 kJ Kg-1...
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