S-Cool Revision Summary

S-Cool Revision Summary

Chemical reactions can either release energy to the surroundings, exothermic or energy can be transferred from the surroundings, endothermic.

The law of conservation of energy states that energy cannot be destroyed or created.

Enthalpy change is the term used to describe the energy exchange that takes place with the surroundings at constant pressure. H is the symbol.

Copyright S-coolH = Copyright S-coolH products - Copyright S-coolH reactants

The units for enthalpy change are Kilojoules per mole.

An exothermic reaction has a negative enthalpy value whereas an endothermic enthalpy has a positive value.

Standard conditions are used to measure enthalpies of different reactions so that they may be fairly compared.

The standard enthalpy change of reaction: Copyright S-coolHrCopyright S-cool is the enthalpy change when amounts of reactants shown in the equation for the reaction, react under standard conditions to give products in their standard state.

The standard enthalpy of formation: Copyright S-coolHfCopyright S-cool is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.

The standard enthalpy of combustion: Copyright S-coolHcCopyright S-cool is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.

For a chemical reaction to occur bonds must break before new bonds can be made. When bonds break energy is absorbed (endothermic). When bonds form energy is released (exothermic).

Bond energy:

The amount of energy required to break a covalent bond, it indicates the strength of a bond. Value always quoted as bond energy per mole, E. The value is positive due to it being an endothermic reaction.

To measure enthalpy changes we use the following calculation:

Energy transfer = mcCopyright S-coolT (joules)

c = specific heat capacity.

m = mass.

Copyright S-coolT = temperature change.

Hess' law states that the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place.