A LevelChemistryOCRRevision Notes5. Physical Chemistry & Transition ElementsThermodynamicsThe Feasibillity of a Reaction

The Feasibillity of a Reaction (OCR A Level Chemistry A): Revision Note

Exam code: H432

Written by: Philippa Platt

Reviewed by: Caroline Carroll

Updated on 21 October 2026

The Feasibility of a Reaction

The Gibbs equation can be used to calculate whether a reaction is feasible or not

ΔG^ꝋ = ΔH_reaction^ꝋ - TΔS_system^ꝋ

When ΔG^ꝋ is negative, the reaction is feasible and likely to occur
When ΔG^ꝋis positive, the reaction is not feasible and unlikely to occur
Feasible and spontaneous are fairly similar terms to describe reactions
- Feasible tends to be used to describe reactions which are energetically favourable, so reactions that should go
- Spontaneous tends to be used to describe reactions that go of their own accord

Worked Example

Calculate the Gibbs free energy change for the following reaction at 298 K and determine whether the reaction is feasible.

2Ca (s) + O₂(g) → 2CaO (s) ΔH = -1271 kJ mol^-1

S^ꝋ[Ca(s)] = 41.00 J K^-1 mol^-1

S^ꝋ[O₂(g)] = 205.0 J K^-1 mol^-1

S^ꝋ[CaO(s)] = 40.00 J K^-1 mol^-1

Answer

Step 1: Calculate ΔS_system^ꝋ

ΔS_system^ꝋ= ΣS_products^ꝋ - ΣS_reactants^ꝋ

ΔS_system^ꝋ= (2 x ΔS^ꝋ [CaO(s)]) - (2 x ΔS^ꝋ [Ca(s)] + ΔS^ꝋ[O₂(g)])

= (2 x 40.00) - (2 x 41.00 + 205.0)

= -207.0 J K^-1 mol^-1

Step 2: Convert ΔS^ꝋ to kJ K^-1 mol^-1

= -207.0 J K^-1 mol^-1÷ 1000 = -0.207 kJ K^-1 mol^-1

Step 3: Calculate ΔG^ꝋ

ΔG^ꝋ = ΔH_reaction^ꝋ - TΔS_system^ꝋ

= -1271 - (298 x -0.207)

= -1209.3 kJ mol^-1

Step 4: Determine whether the reaction is feasible

Since the ΔG^ꝋ is negative the reaction is feasible

Feasibility and Temperature

The feasibility of a reaction can be affected by the temperature
The Gibbs equation will be used to explain what will affect the feasibility of a reaction for exothermic and endothermic reactions

Gibbs equation: delta G = delta H (reaction) - T delta S (system), with delta H (reaction) labelled the first term and T delta S labelled the second term.

Exothermic reactions

In exothermic reactions, ΔH_reaction^ꝋ is negative
If the ΔS_system^ꝋ is positive:
- Both the first and second term will be negative
- Resulting in a negative ΔG^ꝋ so the reaction is feasible
- Therefore, regardless of the temperature, an exothermic reaction with a positive ΔS_system^ꝋ will always be feasible
If the ΔS_system^ꝋ is negative:
- The first term is negative and the second term is positive
- At very high temperatures, the -TΔS_system^ꝋ will be very large and positive and will overcome ΔH_reaction^ꝋ
- Therefore, at high temperatures ΔG^ꝋ is positive and the reaction is not feasible
Since the relative size of an entropy change is much smaller than an enthalpy change, it is unlikely that TΔS > ΔH as temperature increases
These reactions are therefore usually spontaneous at normal conditions

Flow chart for an exothermic reaction (delta H negative): with negative delta S, delta G is negative at low temperature (feasible) but positive at high temperature (not feasible); with positive delta S, delta G is always negative, so always feasible.

The diagram shows under which conditions exothermic reactions are feasible

Endothermic reactions

In endothermic reactions, ΔH_reaction^ꝋ is positive
If the ΔS_system^ꝋ is negative:
- Both the first and second term will be positive
- Resulting in a positive ΔG^ꝋ so the reaction is not feasible
- Therefore, regardless of the temperature, endothermic with a negative ΔS_system^ꝋ will never be feasible
If the ΔS_system^ꝋ is positive:
- The first term is positive and the second term is negative
- At low temperatures, the -TΔS_system^ꝋ will be small and negative and will not overcome the larger ΔH_reaction^ꝋ
- Therefore, at low temperatures ΔG^ꝋ is positive and the reaction is not feasible
- The reaction is more feasible at high temperatures as the second term will become negative enough to overcome the ΔH_reaction^ꝋ resulting in a negative ΔG^ꝋ
This tells us that for certain reactions which are not feasible at room temperature, they can become feasible at higher temperatures
- An example of this is found in metal extractions, such as the extraction if iron in the blast furnace, which will be unsuccessful at low temperatures but can occur at higher temperatures (~1500 ^oC in the case of iron)

Flow chart for an endothermic reaction (delta H positive): with negative delta S, delta G is always positive, so never feasible; with positive delta S, not feasible at low temperature but feasible at high temperature.

The diagram shows under which conditions endothermic reactions are feasible

A summary table of free energy conditions

Summary table of delta H, delta S, the effect of increasing temperature and whether the reaction is spontaneous: negative and positive, always; negative and negative, usually; positive and positive, yes above a certain temperature; positive and negative, never.

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

I would just like to say a massive thank you for putting together such a brilliant, easy to use website.I really think using this site helped me secure my top gradesin science and maths. You really did save my exams! Thank you.

Beth
IGCSE Student

This website is soooo useful and I can’t ever thank you enough for organising questions by topic like this. Furthermore, the name of the website could not have been more appropriate as it literally did SAVE MY EXAMS!

Fathima
A Level Student

Incredible! SO worth my money, the revision notes have everything I need to know and are so easy to understand. I actually enjoy revising! It makes me feel a lot more confident for my GCSEs in a few months.

Kate
GCSE Student

Absolutely brilliant, both my girls used it for A levels and GCSE. It's saves on paper copies, also beneficial exam questions ranked from easy to hard. It's removed a lot of stress from the exams.

Sameera
Parent

Just to say that your resources are the best I have seen and I have been teaching chemistry at different levels for about 40 years

Mark
Chemistry Teacher

Excellent