সমচাপ বিক্রিয়া বনাম সম আয়তন বিক্রিয়া

$$\Large\color{#D73D75}{ Energy\ And\ Heat\ In\ Calorimeter\ Reactions}$$

When a reaction inside a calorimeter takes place energy or heat is created and transferred to the surroundings. When the energy changes in a reaction it is referred to as the energy change of the system ($\Delta E$) and is represented by the equation:

$$\Delta E = q\ +\ w$$

Where $q$ is the transferred heat and $w$ is work. Work can also be represented as:

$$w=\ -P \Delta V$$

Together, the equations can be rewritten to be expressed in terms of heat transferred or $q$:

$$q=\ \Delta E\ +\ P \Delta V$$

Constant Pressure In Calorimeter Reactions

Most chemical reactions are carried out in an open container. The volume is often changed when one solution is combined with another. This means the pressure of the system is constant and usually atmospheric pressure. The heat transferred at constant pressure is denoted as $q_p$. When this happens the reaction can be monitored for the heat of reactionor enthalpy change, represented by $\Delta H$.

$$q_p=\ \Delta E\ +\ P \Delta V\ =\ \Delta H$$

Constant Volume in Calorimeter Reactions

The equations above can be used when a reaction occurs under constant pressure. Usually most chemical reactions occur under a constant pressure, atmospheric pressure. However, when a reaction is done in a bomb calorimeter, the reaction takes place in a closed container. The volume in the closed system remains constant and the heat transferred is represented by $q_v$. At a constant volume, $\Delta V\ = 0$, which means no work is done. So the equation to represent the heat or energy of the reaction is:

$$q_p=\ \Delta E$$

In this case, the change in temperature is monitored ($\Delta T$). The heat transferred in this type of reaction is represented by:

$$q=\ C\ \times\ (moles) \times\ \Delta T$$

$C$ is the heat capacity which is a extensive property. This means that $C$ depends on the material, size, and composition. These values are often studied and recorded. Since, the heat capacity is dependent on the material, the moles of substance must be taken into account. The change in temperature is found from the thermometer.

(Video) Calorimetry

By David Vanden Bout

Thermodynamics measures the movement of energy. Calorimetry is the technique used to capture that energy moved and quantify it. Calorimetry basics include using a common substance (water) to measure the heat going into or out of the water, and finally measuring this change.

There are two types of calorimetry: constant pressure and constant volume.

Constant pressure is the simplest calorimetry. It can be done easily in a Styrofoam cup. The cup acts as an insulator and keeps our temperature inside the cup constant. The cup is filled with water and one reactant (acid) and the temperature is measured with a thermometer. The second reactant is added (base) and the temperature is measured again. There should be a change if a reaction has occurred. Once the change in temperature is determined experimentally, calculations can be done to determine the amount of heat.

Constant volume or bomb calorimetry is used to measure combustion reactions. This calorimeter is a closed system and the volume doesn't change so no work is done. All the energy is lost as heat into the water. The set-up of a bomb calorimeter is a much more complicated set-up than the cup calorimeter, but the concepts of measurements are the same.