Question

Take about 2g of barium hydroxide in a test tube. Add 1g of ammonium chloride and mix with the help of a glass rod. Touch the bottom of the test tube with your palm. What do you feel, is it an exothermic or endothermic reaction?

Answer 1

Hint: Think about what would happen if this reaction is carried out in a beaker that is surrounded by water. Consider the enthalpies of the reactants and the products for further clarity.

Complete step by step solution:
We will first look at the overall reaction of barium hydroxide and ammonium chloride to check whether heat is absorbed or given out during the reaction. In this reaction, one mole of barium hydroxide octahydrate combines with two moles of ammonium chloride to give one mole of barium chloride dihydrate, two moles of ammonia, and eight moles of water. The reaction is as follows:
\[Ba{{(OH)}_{2}}\cdot 8{{H}_{2}}O+2N{{H}_{4}}Cl\xrightarrow{heat}BaC{{l}_{2}}\cdot 2{{H}_{2}}O+2N{{H}_{3}}+8{{H}_{2}}O\]
Here, we can see that the absorption of heat is required to complete this reaction. While converting to the products, the reactants absorb heat. If this reaction is carried out in a beaker that is placed in water, we will see that the water around the beaker will be frozen after the reaction is completed as the energy required for this reaction is large.
For an endothermic reaction, the enthalpy of the products is greater than that of the reactants. So according to the formula:
\[\Delta H{}^\circ =\sum{\Delta H{{{}^\circ }_{f(products)}}}-{{\sum{\Delta H{}^\circ }}_{f(reactants)}}\]
The total change in enthalpy here has to be positive since the enthalpy of the products is greater than that of the reactants. The value of $\Delta H{}^\circ $ of the reaction is found to be $+63.5kJ$ which is a considerable amount of energy. So, the water around the beaker will freeze and the temperature will be about $-20{}^\circ C$.

Note: Check for the limiting reagent in this reaction to determine how much of the reactants will react to form the product and how much net energy will be required for the reaction to be carried out. According to the calculations, 0.006 moles of barium hydroxide octahydrate is present and 0.018 moles of ammonium chloride is present. So, the limiting reagent is barium hydroxide octahydrate.