Asked by alex
How does a decrease in potential energy during a chemical reaction relate to the temperature of the surroundings?
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When a chemical reaction lowers the chemical (potential) energy of the system, the excess energy is released to the surroundings. Much of that released energy appears as increased kinetic energy of surrounding molecules—i.e., heat—which raises the temperature of the surroundings. (Conversely, if the system’s potential energy increases, the reaction absorbs heat and the surroundings cool.)
Quantitatively:
- At constant pressure the heat exchanged with the surroundings is q = ΔH (for the system). If the system loses energy, ΔH < 0 and the surroundings gain q_surroundings = −ΔH > 0.
- The resulting temperature change of the surroundings is ΔT = q_surroundings / C (or ΔT = q/(m·c) for a mass m with specific heat c), assuming all released energy becomes thermal energy of the surroundings.
Caveats: not all released energy must become heat (some may do work, be emitted as light, or be stored); the observed temperature rise therefore depends on how the energy is distributed and on the heat capacity/mass of the surroundings.
Quantitatively:
- At constant pressure the heat exchanged with the surroundings is q = ΔH (for the system). If the system loses energy, ΔH < 0 and the surroundings gain q_surroundings = −ΔH > 0.
- The resulting temperature change of the surroundings is ΔT = q_surroundings / C (or ΔT = q/(m·c) for a mass m with specific heat c), assuming all released energy becomes thermal energy of the surroundings.
Caveats: not all released energy must become heat (some may do work, be emitted as light, or be stored); the observed temperature rise therefore depends on how the energy is distributed and on the heat capacity/mass of the surroundings.
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