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Free Energy

Free energy (G) is energy that is available, or free, to do work! Spontaneous reactions release free energy as they proceed. The formula to find the free energy change of a reaction involves the change in enthalpy (∆H) and entropy (∆S):

\Delta G^\circ=\Delta H^\circ - T \Delta S^\circ

Note: the T in this equation is the temperature in Kelvin, which is always positive.

What are the units for the calculation of ∆G?

Keeping in mind that a spontaneous reaction is one that releases free energy, making the sign of ∆G negative, fill out the following chart to help you remember the different outcomes of ∆G:

\Delta H \Delta S \Delta G
− value (exothermic) + value (disordering) always negative - never positive
+ value (endothermic) + value (disordering) negative at higher temperatures
− value (exothermic) − value (ordering) negative at lower temperatures
+ value (endothermic) − value (ordering) never negative - always positive

Why does the temperature matter for the second and third combinations above?

You can check your answers and find more information on free energy here.

Free Energy Change

Based on the following equation:

\text{CH}_4(g)+\text{H}_2\text{O}(g) \rightarrow \text{CO}(g)+3\text{H}_2(g)

If the ∆H˚ is +206.1 kJ/mol and the ∆S˚ is +215 J/K • mol, calculate the ∆G˚ at 25˚C. Is the reaction spontaneous?

Remember: the ˚ refers to the use of standard conditions when calculating the free energy.

You can check your answers and find more info on calculating free energy change here!

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