enthalpy and entropy and the effect on the direction of a reaction

enthalpy

total heat content in a system

which way does a reaction move for enthalpy?

• systems move towards a state of minimum potential energy (enthalpy) to create more stable products

• enthalpy changes favour the exothermic direction of a reaction to produce lower potential energy products

what is entropy

entropy: a way to measure the dispersal of energy among particles in a system

which way do systems move for entropy?

systems tend to move spontaneously towards a state of maximum entropy to disperse energy as much as possible and in more ways; high entropy = more stable

what processes produce an increase in entropy?

• solid to liquid

• liquid to vapour

• solute and solvent → solution

• reaction that causes an increase in the number of gas molecules

entropy and enthalpy compromise

• equilibrium is a state that is a compromise between enthalpy (min potential energy) and maximum entropy

• reactions are considered spontaneous if both factors favour products

• reactions are considered non-spontaneous if both factors favour reactants

• if factors oppose, a mixture of products and reactants can exist at equilibrium

factors that determine equilibrium state

enthalpy and entropy

entropy and enthalpy: if both favour products?

• the reaction is spontaneous

• enthalpy → exothermic → more stable

• entropy → more disorder → spreading energy more → more stable

• more stable products → spontaneous, energetically favourable and likely to occur

entropy and enthalpy: if both favour reactants ?

• the reaction is not spontaneous

• enthalpy → endothermic → less stable

• entropy → less dispersal of energy → less stable

• not enough energy to produce the products

entropy and enthalpy: if entropy and enthalpy oppose?

• a mixture of reactants and products can exist at equilibrium

• reaction becomes temperature dependent

what is favoured if enthalpy<0 and >0?

• less than 0: forwards reaction exothermic and forwards rxn is favoured

• more than 0: forward reaction is endothermic and reverse rxn is favoured

t

what is favoured if entropy<0 and >0?

• less than 0: products have higher entropy and forwards rxn is favoured

• more than 0: products have lower entropy and reverse rxn is favoured

influence of temperature

• at equilibrium, temperature is constant

• low temps: enthalpy has greatest influence, exothermic rxns are generally spontaneous

• high temps: entropy is increased, entropy has greatest influence on equilibrium

what is gibbs free energy?

• energy available to do work

• difference of total energy in system (enthalpy) - energy dispersal (entropy)

• energy left over after energy is dispersed as much as possible

gibbs free energy formula

ΔG = ΔH - TΔS

if ΔG is negative?

• forwards reaction favoured

• spontaneous

• system lost all energy, used all up

if ΔG is positive?

• reverse reaction is favoured

• energy is needed

• system is storing energy and reaching an unstable state

ΔG = zero?

reaction will remain at equilibrium