Q.In an exothermic reaction, heat is evolved, and system loses heat to the surrounding. For such system (Note: more than one of the given options may be correct.)

In an exothermic reaction, the system releases heat to the surroundings. This fundamental concept dictates the signs of heat exchange and enthalpy change.

1. By convention, heat released by the system is assigned a negative sign. Therefore, for an exothermic reaction, the heat exchanged at constant pressure, $q_p$, will be negative.
2. The enthalpy change of a reaction, $\Delta_rH$, is defined as the heat exchanged at constant pressure. That is, $\Delta_rH = q_p$. …

When a chemical reaction occurs, energy is either absorbed from or released to the surroundings. This energy exchange is fundamental to understanding the nature of the reaction. An "exothermic reaction" is defined by the release of heat from the reacting system to its surroundings.

To correctly determine the signs of $q_p$ and $\Delta_r H$, we need to recall the standard thermodynamic sign conventions.

1. Understanding Exothermic Reactions and Heat Exchange

   An exothermic reaction is one where the chemical system releases heat energy into its surroundings. This means the internal energy of the system decreases as it converts chemical potential energy into thermal energy that flows out.

2. Sign Convention for Heat ($q$)

   In thermodynamics, the sign of heat ($q$) is determined from the perspective of the system:

   • If the system absorbs heat from the surroundings, $q$ is positive ($q > 0$).
   • If the system releases heat to the surroundings, $q$ is negative ($q < 0$).

   Since an exothermic reaction involves the system losing or releasing heat to the surroundings, the heat exchanged, $q$, must be negative. When this heat exchange occurs at constant pressure, it is denoted as $q_p$. Therefore, for an exothermic reaction, $q_p$ will be negative.

3. Relationship between Heat at Constant Pressure ($q_p$) and Enthalpy Change ($\Delta H$)

   One of the fundamental definitions in thermochemistry is that the heat exchanged by a system at constant pressure ($q_p$) is equal to the change in enthalpy ($\Delta H$) of the system.

   $\Delta H = q_p$

   This relationship holds true for any process occurring at constant pressure, which is a common condition for many chemical reactions carried out in open containers. …