Question

Although dissolution $N{H_4}Cl$ of in water is endothermic yet it dissolves as it takes heat from surrounding so as to overcome lattice energy therefore, it dissolves and yet the process is endothermic but $\Delta S$is +ve.

Answer 1

Hint:We will take Gibbs enthalpy relation for finding out the sign of each quantity. We know for endothermic reaction enthalpy is negative and as reaction is performing in nature so its Gibbs energy is also negative according to sign conventions. So if we try to take signs for each symbolic term we get to know why $\Delta S$ is showing positive value.

 Complete solution:
Let’s try to understand it from the basics that endothermic reaction is a reaction in which heat is transferred from the surrounding to the system. If we take a beaker with water and add ammonium chloride in it, then we see that reaction happens and it will dissolve completely into the water and form a solution. What we also see when we perform the experiment is that the endothermic change is happening. Heat is transferring from the surroundings to the system and we start feeling cold by the handling of the beaker.
This chemical change can be understood using Gibb’s enthalpy relation. $\Delta G = \Delta H - T\Delta S$
For a spontaneous reaction $\Delta G < 0$ and for a endothermic reaction $\Delta H < 0$
When we dissolve ammonium chloride in water the beaker starts becoming cool and conversion of ammonium chloride to ammonium hydroxide takes place. The enthalpy of reaction decreases as heat is transferred from surrounding to system and as reaction is going on it is called to be a spontaneous reaction taking Gibbs energy value also less than one.
On putting the signs in the Gibbs relation, we get a positive value of $\Delta S$ .

Note:While solving theoretical questions of thermodynamics take from the basic what the examiner is trying to ask. If it is given that the reaction is endothermic, it means that the value of any state function like enthalpy is negative. Like this if we take signs of Gibbs energy, it will increase from zero as reaction is spontaneous.