Gibbs free E change

what is the Gibbs free E change and what does it tell us?

• balance between entropy and enthalpy

• determines the feasibility of a reaction

give the eqn for the Gibbs free E change:

ΔG = ΔH - TΔS

for a reaction to be feasible, what must the ΔG value be?

0 or-ve

what are the units of ΔG? what does this mean when we are using the ΔG eqn?

• kJ mol^-1

• must /ΔS by 1000 to convert into kJ K^-1 mol^-1

can you complete this table? (1st 3 columns - +ve/-ve)

what is the value of ΔG during a change of state?

0 kJ mol^-1

for temperatures below the mpt/bpt, is ΔG +ve or -ve?

+ve

for temperatures above the mpt/bpt, is ΔG +ve or -ve?

-ve

how can we plot a graph of the Gibbs free energy?

• eqn ΔG = ΔH - TΔS can be plotted as a straight line graph of ΔS against T (respectively: y = c + m(-x)

• ∴ gradient = -ΔS and the y intercept is

ΔH

what does the x-intercept represent on a Gibbs-free energy graph?

temperature at which ΔG becomes 0 and so the reaction becomes feasible

how can we determine whether a reaction is endo or exo based on its Gibbs-free energy graph?

• when ΔS is +ve (gradient is -ve) = endo

• when ΔS is -ve (gradient is +ve) = exo

give the eqn to calculate T derived from the Gibbs free E change:

T = ΔH / ΔS