Differential thermal analysis:

• On further increasing the temperature, melting occurs in the sample. As a result, the rise in the temperature stops as shown in figure 21.02(a), and hence the temperature difference increases.

• Upon ending the melting, the temperature curve rapidly reverts back to the baseline (see Figure 21.02(a)), resulting in a peak as shown in Figure 21.02(b).

• This provides the sample's transition temperature and the reaction temperature from the temperature difference signal.

• A close look at Figure 21.02 (b) shows that the temperature difference due to samples endothermic (exothermic) change is shown as negative (positive) direction.

• An exothermic reaction occurs when the temperature of a system increases due to the evolution of heat. The heat is released into the surroundings, resulting in an overall negative quantity for the heat of reaction.

For example:

C (solid) + O₂ (gas) → CO₂ (gas) ................................................ Δ H = - 393.5 kJ < 0

On the other hand, an endothermic reaction occurs when the temperature of an isolated system decreases while the surroundings of a non-isolated system gains heat. Therefore, the endothermic reactions result in an overall positive heat of reaction.

For example:

C (solid) + 2S (gas) → CS₂ (liquid)................................................ ΔH = + 92.0 kJ > 0