Question

How can I calculate “q” thermochemistry?

Answer 1

Thermochemistry deals with the heat energy associated with the chemical reactions and/or physical transformations. Heat can be absorbed or released during a thermochemical reaction.

Complete step by step answer: In thermochemistry “q” stands for heat absorbed or released during a process. Heat is defined as the energy transferred between two substances because of the temperature difference between them. Flow of heat occurs until the temperature of two substances become equal. It is usually expressed in the unit of Joules (J). q can be calculated using the formula,
$q = mc\Delta T$
where m is the mass of the substance, c is the specific heat capacity of the substance and $\Delta T$ is the change in temperature during the process. $\Delta T$ can be calculated using the formula,
$\Delta T = {T_{final}} - {T_{initial}}$
Heat is also expressed using another unit called calorie (cal).
During phase change, temperature remains constant. Hence the equation $q = mc\Delta T$ cannot be used for calculating the heat involved during a phase transition. The difference between any two phases is the energy difference. Hence q during a phase transition can be calculated using the formula,
$q = m\Delta H$
where $\Delta H$ is the heat required for one gram of the substance to undergo phase transition.
Heat is a path function. Its value depends on the path through which the process is taking place.

Note:
The heat absorbed by a system is taken as positive and the heat released by a system is taken as negative. The process in which the system absorbs the heat from the surroundings is called the endothermic process and the process in which heat is released to the surroundings by the system is called the exothermic process.