Question

If the temperature drops when a salt is dissolved in water. Its heat of solution is

Select one:

a. exothermic

b. positive

c. cold

d. negative

Answer 1

Enthalpy or Heat of solution is the amount of heat energy released or absorbed when a solution is formed.

There are two components in a solution

1. Solute
2. Solvent

Three steps are involved in the solvation process

1. Breaking of bond between solute molecules
2. Breaking of intramolecular and intermolecular attraction between solvent
3. New Solute-Solvent bond formation

You might already know making of new bonds releases energy whereas breaking requires energy. In step 3 energy is released as new bonds are formed, In step 1 and 2 energy is absorbed.

∆H_sol is negative when more energy is released in making new bonds, than is used in breaking bonds, and it is positive when more energy is required to break bonds.

In your question the temperature of the solution decreases, it means the system is absorbing heat from the surrounding.

So its like (II) which means a endothermic reaction.

And ∆H_sol is Positive.

Correct Answer : b) Positive