The adiabatic temperature rise is larger when the reaction is more exothermic and the system has a lower heat capacity. True or False?

The adiabatic temperature rise is larger when the reaction is more exothermic and the system has a lower heat capacity. True or False?

• A. True
• B. False
• C. Only for fast reactions
• D. Only at high starting temperatures

Answer: (A)

In an adiabatic system, all the heat released by the reaction stays in the mixture and goes into raising its temperature. How big that temperature rise is depends on two things: how much heat the reaction releases (exothermicity) and how much energy the system must absorb to raise its temperature (the heat capacity). If the reaction is more exothermic, more heat is produced for the same amount of reactant, so there is more energy to heat the system. If the system has a lower heat capacity, less energy is needed to raise the temperature by a given amount, so the temperature climbs higher for the same heat release. Put together, a more exothermic reaction in a smaller heat-capacity system yields a larger adiabatic temperature rise. The statement is true. The rate of the reaction or the starting temperature doesn’t set the basic size of the adiabatic rise in this simple energy-balance sense (though heat capacity can vary with temperature, which can tweak the exact value).

In an adiabatic system, all the heat released by the reaction stays in the mixture and goes into raising its temperature. How big that temperature rise is depends on two things: how much heat the reaction releases (exothermicity) and how much energy the system must absorb to raise its temperature (the heat capacity). If the reaction is more exothermic, more heat is produced for the same amount of reactant, so there is more energy to heat the system. If the system has a lower heat capacity, less energy is needed to raise the temperature by a given amount, so the temperature climbs higher for the same heat release. Put together, a more exothermic reaction in a smaller heat-capacity system yields a larger adiabatic temperature rise. The statement is true. The rate of the reaction or the starting temperature doesn’t set the basic size of the adiabatic rise in this simple energy-balance sense (though heat capacity can vary with temperature, which can tweak the exact value).