Meaning and mechanism
In exothermic reactions, the energy within the reaction vessel is transferred to the surroundings, which can cause the solution temperature to increase. Conversely, in endothermic reactions, the solution temperature can decrease as the solution receives energy from the surroundings.
The container, in addition to the liquid, stores heat. If we ignore the container’s heat capacity, we can underestimate the amount of energy needed to achieve the same temperature change.
Relationships and conditions
In the experimental approximation of constant pressure, the value is −nΔH=(mc+Ccal)ΔT. If the reaction is exothermic, where ΔH is negative, then under other conditions, the temperature change on the right side is positive.
It is essential to have the property of completely proceeding with the reaction and preventing heat from escaping externally. Factors such as evaporation, enthalpy of solution, sensor delay, and the difference in specific heat of the actual solution also affect the measurement.
What to observe in the simulation
Heat loss reduces the observed temperature rise. Cooling curve extrapolation approximates the loss, but outliers and residuals remain.
It's a hypothetical exothermic reaction. Assume c=4.18 J/(g·K), ambient/initial 20 °C, reaction time 20 s, perfect mixing, constant heat generation rate. The loss is Newtonian cooling with dT/dt=q̇/C−0.020(T−20). The temperature at which the cooling curve is returned to the 10 s in the middle of the reaction is expressed as the highest extrapolation correction temperature. This approximation is not the actual maximum temperature and excludes actual reaction rates, evaporation, and thermometer delays.
Key relationships and symbols
−nΔH = (m·c + Ccal)ΔT
| Symbol | Meaning | Unit |
|---|---|---|
| ΔH | Enthalpy change per mole of reaction | J/mol |
| Ccal | Heat capacity of the calorimeter | J/K |
| ΔT | Observed temperature change | K |
Worked example
What is the combined heat capacity of the solution and calorimeter?
The solution and container receive the heat of reaction together. qSolution=−nΔH=(mc+Ccal)ΔT and the sign of q is opposite to that of the reaction.
Common misconceptions
- It is not correct to write the ΔH value for an exothermic reaction as positive simply because the temperature has increased.