Exchange of energy between the system and surrounding

Exothermic reactions

When chemical reactions take place, energy is either absorbed or evolved. A reaction is said to be exothermic, if heat is 'evolved'. The heat energy produced during the reactions is indicated by writing + q or more precisely by giving the actual numerical value on the products side. In general, exothermic reactions may be represented as:

In the exothermic reactions, the enthalpy of the products will be less than the enthalpy of the reactants so that the enthalpy change is negative as shown below:

DH = H_p - H_r (since H_p <>_r)

DH = -ve

Some examples of reactions evolving energy are given below:

• Coal is burnt simply for the large amount of energy available during its combustion:

• When water is added to quick lime (CaO) for preparing whitewash, a considerable amount of heat is produced during the reaction. The heat produced warms up the water.

• When we add dilute hydrochloric acid to a test tube containing granulated zinc, hydrogen gas is evolved. The reaction is accompanied by evolution of heat.

• One mole of carbon reacts with one mole of oxygen to form one mole of carbon dioxide and 393.5 kJ of heat is evolved at constant temperature and constant pressure. The reaction may be expressed as:

or,

Similarly,

Endothermic reactions

Reactions that are accompanied by absorption of energy from the surroundings are called endothermic reactions. Since heat is added to the reactants in these chemical reactions, it is indicated by either putting +q or by writing the actual numerical value of heat on the reactants side.

Alternatively, this may be written as,

We know DH = the heat absorbed at constant temperature and constant pressure. Because of the absorption of heat, the enthalpy of the products will be more than the enthalpy of the reactants. Consequently, DH will be positive (+ ve) for the endothermic reactions.

DH = H_p - H_r since (H_p > H_r)

DH = +ve

For example:

• When a small quantity of ammonium chloride (NH₄Cl) is dissolved in water in a test tube, the tube becomes colder than before. During this chemical reaction heat is absorbed from the surroundings (test tube).

• When crystals of sodium thiosulphate (Na₂S₂O₃.5H₂O) commonly called hypo, are dissolved in water a cooling effect takes place.

• One mole of nitrogen reacts with one mole of oxygen to form two moles of nitric oxide. 180.5kJ of heat is absorbed at constant temperature and the reaction may be expressed as:

or,

Similarly,

Fig: 5.2 - Enthalpy changes for exothermic and endothermic reactions

Similarly, if we consider heat change at constant volume and temperature,

DE is -ve for exothermic reactions and DE is +ve for endothermic reactions.

Thus it may be concluded that:

For exothermic reactions, DH or DE = -ve

For endothermic reactions, DH or DE = +ve

Problem

4. Calculate, Q, W, DE and DH for isothermal reversible expansion of one mole of ideal gas from initial pressure of 1 bar to 0.1 bar at a constant temperature of 273 K.

Solution

For isothermal reversible expansion of ideal gas

q = -w = -(- 5227.16) = 5227.16 J

DE = 0

DH = 0.

Other Energy Changes in Chemical Reactions

Energy changes in chemical reactions can also take place in forms other than heat, such as light, electricity, mechanical energy, etc. Some common examples are:

• The burning of petrol or diesel in motor engines of cars, trucks, tractors or buses produces mechanical energy, which is used in running these vehicles.

• Chemical reactions taking place in batteries produce electrical energy to run transistors radios, torches and watches, etc. For example, in a Daniel cell, the chemical reaction between zinc metal and copper sulphate solution is accompanied by electrical energy.

• Photosynthesis

In this process chlorophyll in green plants converts carbon dioxide and water into glucose and oxygen, energy is provided by the energy of sunlight,

This reaction is very important for the life of plants.