3.2.1 Enthalpy changes
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what is Enthalpy, H
• Enthalpy H is the measure of heat energy in a chemical system > chemical system is the atoms, molecules or ions making up the chemicals
what is the enthalpy change
• enthalpy change = H products - H reactants
what is the law of conservation of energy
• energy cannot be created or destroyed
when a chemical reaction involving enthalpy takes place, heat energy is transferred between system + surroundings: what are the two types
• exothermic = energy from system to surroundings • endothermic = energy from surroundings to system
for enthalpy, what do we measure the temp of
• the surroundings only
in exothermic reactions, what happens (enthalpy)
• energy transferred to surroundings from system > temp of surroundings inc • products have less energy than reactants • enthalpy change = negative
in endothermic reactions, what happens (enthalpy)
• energy transferred from surroundings to system > temp of surroundings dec • products have more energy than reactants • enthalpy change = positive
what is activation energy
• minimum amount of energy required to start a reaction > this energy is required to break bonds in reactants so that atoms can rearrange to form new products
what is standard enthalpy change
• an enthalpy change under standard conditions > measured in KJmol-1
what are the standard conditions required for standard enthalpy change
• standard pressure, 100 kPa • standard temp, 298K, 25C • standard conc, 1 moldm-3 • standard state
what is the standard enthalpy change of reaction
• the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in standard states
what is the standard enthalpy change of formation
• the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
what is the enthalpy change of combustion
• the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all the reactants and products in their standard states
what is the enthalpy change of neutralisation
• the enthalpy change that accompanies the reaction of an acid with a base to form one mole of H2O(l) under standard conditions, with all reactants and products in their standard states
what is the equation used to calculate energy change
Q = m x c x change in temp - q = J - m = g - c = Jg-1K-1 - temp = K or C
how do you calculate the enthalpy change of combustion (steps)
• calculate Q • convert to KJ • work out enthalpy change = KJ / mol
why is the experimental value of enthalpy change of combustion less than that of the data book
• heat loss to surroundings other than water (air/beaker) • some incomplete combustion occurring instead of complete • evaporation of ethanol (not all reacted) • non-standard conditions
how do you calculate the enthalpy change of neutralisation (steps)
• Calculate Q • Convert to KJ • work out limiting reactant mol • use limiting mol to work out enthalpy change = KJ / mol
what is average bond enthalpy
• energy required to break one mole of a specified type of bond in a gaseous molecule
bond breaking is …. and they always have a …. enthalpy value
• endothermic • positive
bond making is …. and enthalpy value is ….
• exothermic (energy released when bonds form) • negative
why are bond enthalpies always positive
• energy is always required to break bonds > bond enthalpies always endothermic so positive enthalpy value
in exothermic reactions, why is the overall enthalpy change negative
• energy required to break bonds is less than energy released to make bonds
in endothermic reactions, why is the overall enthalpy change positive
• energy required to break bonds is more than energy released to make bonds