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An endothermic process is one in which a system absorbs heat from its surroundings. This means that energy is required to make the process happen. The absorbed energy usually comes in the form of heat. A common way to figure out if a process is endothermic is to look at the heat exchanged, denoted as \( q \).
- If \( q > 0 \), this indicates that the system takes in heat, marking the process as endothermic.- Such processes often involve melting, boiling, or subliming of substances.
For instance, when ice melts to form water, it requires energy absorbed from the surroundings, making it endothermic. In the given exercise, case (a) with \( q = 0.763 \text{ kJ} \) shows the system absorbs heat, confirming an endothermic process.

Exothermic processes are the opposite of endothermic processes. Here, a system releases heat to its surroundings. This results in a decrease in the energy of the system while increasing the energy of the surroundings. We can determine if a process is exothermic by examining the sign of \( q \).
- If \( q < 0 \), it means the system is giving off heat, marking it as exothermic.- Common examples include combustion, condensation, and freezing.
Exothermic processes usually make the surroundings warmer as they release energy. For instance, when water vapor condenses into liquid, it releases energy into the environment. In the exercise, case (b) releases \(66.1 \text{ kJ}\) of heat to its surroundings. This negative \( q \) value identifies it as exothermic.

Energy change, denoted by \( \Delta E \), represents the total change in a system’s internal energy. It combines the heat exchange \( q \), and the work done \( w \) using:\[\Delta E = q + w\]
- Work typically relates to expansion or contraction of gas, which is expressed as work done by or on the system.- Converting units may be necessary, for instance, if work \( w \) is given in joules, and you need to calculate \( \Delta E \) in kilojoules.
For case (a), converting \( w = -840 \text{ J} \) to \( \text{kJ} \) helps solve \( \Delta E \) as reflectively detailed in the calculation steps. Both case (a) and (b) emphasize how changes in \( q \) and \( w \) affect the energy state of the system. In case (b), a large negative heat exchange leads to an overall negative \( \Delta E \), indicative of net energy loss, a characteristic of exothermic reactions.