Question

Explain how calorimetry works to calculate ΔH or ΔE for a reaction. Does the temperature of the calorimeter increase or decrease for an endothermic reaction? How about for an exothermic reaction? Explain why ΔH is obtained directly from a coffee cup calorimeter, whereas ΔE is obtained directly from a bomb calorimeter.

Short answer

In calorimetry, the pattern of heat flow can be determined. The heat capacity, mass and temperature change of the surrounding must be determined in order to obtain the heat flow. From these quantities both the values of q_surrandq_syscan be calculated.

A decrease in temperature of the surrounding (the calorimeter contents) can be observed for an endothermic reaction whereas for an exothermic reaction, the temperature increases.

It is known that q_p = ΔH; q_v = ΔE. A coffee-cup calorimeter operates under constant pressure. ΔH is obtained directly from a coffee cup calorimeter. A bomb calorimeter operates under constant volume. So, ΔE is obtained directly from a bomb calorimeter

Step-by-step solution

Explanation

In calorimetry, the pattern of heat flow can be determined. As per the first law of thermodynamics, it can be stated that ΔE_univ=0 and ΔE_sys=-ΔE_surr. The heat capacity, mass and temperature change of the surrounding must be determined in order to obtain the heat flow. From these quantities both the values of q_surrandq_syscan be calculated as -q_sur=q_sys.

A decrease in temperature of the surrounding (the calorimeter contents) can be observed for an endothermic reaction as the surroundings donate heat to the system. Whereas for an exothermic reaction, the system releases heat toward the surroundings (the calorimeter). As a result, temperature increases.

It is known that q_p = ΔH; q_v = ΔE. A coffee-cup calorimeter operates under constant pressure. The amount of heat released or gained at constant pressure is ΔH. So, ΔH is obtained directly from a coffee cup calorimeter. A bomb calorimeter operates under constant volume. The heat released or gained at constant volume is ΔE. So, ΔE is obtained directly from a bomb calorimeter.