Question

A chemist measures the energy change

Answer 1

Answer:

For (1): The correct option is (a)

For (2): The correct option is (a) and 333.6 kJ of heat will be absorbed when 70.9 g of ammonia reacts.

Explanation:

There are 2 types of reactions that are classified based on enthalpy change:

Endothermic reactions: These are the reactions where heat is absorbed by the reaction. The change in enthalpy of the reaction, $\Delta H_{rxn}$ is positive for these reactions.

Exothermic reactions: These are the reactions where heat is released by the reaction. The change in enthalpy of the reaction, $\Delta H_{rxn}$ is negative for these reactions.

• For (1):

For the given chemical reaction:

$2NH_3(g)\rightarrow N_2(g)+3H_2(g);Delta H=160kJ$

As the change in enthalpy or heat of the reaction is positive. Thus, the reaction is an endothermic reaction because heat is absorbed by the reaction.

• For (2):

When ammonia reacts, some amount of heat will be absorbed by the reaction. Thus, we can say the heat will be absorbed.

The number of moles is calculated by using the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$ ......(1)

Given mass of ammonia = 70.9 g

Molar mass of ammonia = 17 g/mol

Using equation 1:

$\text{Moles of ammonia}=\frac{70.9g}{17g/mol}=4.17mol$

By stoichiometry of the reaction:

If 2 mole of ammonia reacts, the heat absorbed is 160 kJ

So, if 4.17 moles of ammonia reacts, the heat absorbed will be = $\frac{160kJ}{2mol}\times 4.17mol=333.6kJ$

Hence, 333.6 kJ of heat will be absorbed when 70.9 g of ammonia reacts