Question

The addition of 3.15g of Ba(OH)₂.8H₂O to a solution of 1.52g of NH₄SCN in 100g of water in a calorimeter caused the temperature to fall by 3.1˚C. Assuming the specific heat of the solution and products is 4.20J/g˚C, calculate the approximate amount of heat absorbed by the reaction, which can be represented by the following equation:
Ba(OH)₂.8H₂O(s) + 2NH₄SCN (aq) -------> Ba(SCN)₂(aq) + 2NH₃(aq) + 10H₂O(l)

Short answer

The reaction absorbs approximately 1.36 kJ of heat.

Step-by-step solution

Given data

To visualize what is going on, imagine that you combined the two solutions so quickly that no reaction takes place when they are combined, then after mixing, the reaction takes place.The moment the two solutions are mixed, you have a 3.15g of Ba(OH)₂.8H₂O and a 1.52g of NH₄SCN in 100 g of water.

Heat absorbed

The heat absorbed by the reaction is equal to that exuded by the solution.Therefore,q_reaction = -q_solution.

Step 3:

Next, we know that the heat absorbed by the solution depends on its specific heat, mass, and temperature change:

q_solution= (c × m × ΔT)_solution…………………………………(1)

It is given that the specific heat of the solution and the product is 4.20 J/g˚C

Substitute the given values into equation (1).

q_solution =(4.20 J/g˚C)(3.15 g+1.52 g+100 g)(-3.1˚C)

=-1.36 kJ

Finally, q_reaction= -q_solution

= 1.36kJ

The +ve sign indicates that the energy is absorbed in the reaction. Hence, it is an endothermic reaction.