3.2.1 Enthalpy changes Flashcards
(30 cards)
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
