Explain in terms of ΔG, why a reaction for which both ΔH and ΔS are positive is sometimes spontaneous and sometimes not.

For a reaction to be spontaneous delta G has to be negative according to the equation delta G = delta H - TdeltaS. Therefore if the temperature of a reaction is very high then -TdeltaS will be greater than delta H, and the value of delta G will be negative meaning the reaction will be spontaneous. Hence if the temperature is low then - TdeltaS will be less than delta H, and delta G will be positive and the reaction will not be spontaneous.