A typical chemical reaction will be spontaneous if

1. Analyze the question The question asks what condition makes a reaction spontaneous. This is a thermodynamics question about Gibbs free energy.

2. Explain the concept The spontaneity of a reaction is decided by the Gibbs free energy change, ΔG:

ΔG = ΔH − TΔS

• ΔG < 0 (negative): spontaneous
• ΔG = 0: at equilibrium
• ΔG > 0 (positive): non-spontaneous

A negative ΔH (an exothermic reaction) pushes ΔG towards being negative. This is why most spontaneous reactions at ordinary temperatures are exothermic, which is why the word "typical" matters here.

3. Check each option

• A. Enthalpy change is negative. Exothermic reactions release energy, and this makes ΔG negative in most common cases. This is the best choice for a "typical" reaction.
• B. Free energy change is greater than one. A positive ΔG means non-spontaneous, and "greater than one" is not a valid criterion anyway. Wrong.
• C. Entropy change is zero. Then ΔG = ΔH, so spontaneity would depend entirely on ΔH. A zero entropy change does not guarantee spontaneity. Wrong.
• D. Enthalpy and free energy change are equal. This only happens when TΔS = 0, and it says nothing about the sign. Wrong.

4. Answer Option A is the best answer. Strictly speaking, spontaneity depends on ΔG being negative, but ΔG is not offered in the correct form among the options. Of the choices given, only a negative enthalpy change is a typical indicator of spontaneity.

5. Exam tip When no option says "ΔG is negative," look for the choice that normally leads to it: a negative ΔH. Remember: negative ΔG = spontaneous, and exothermic reactions usually favour that.