# IB DP Chemistry Reactivity 1.3 Energy from fuels IB Style Question Bank HL Paper 1

### Question

The enthalpy of formation of ammonia gas is -46kJ $$mol^{-1}$$

$$N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$$

What is the energy released, in KJ, in the reaction?

A. 23
B. 46
C. 69
D. 92

Markscheme: D

The enthalpy of formation ($$\Delta H_f$$) of a reaction is the heat released or absorbed when one mole of a compound is formed from its elements in their standard states.

In this case, the enthalpy of formation of ammonia ($$NH_3$$) is given as -46 kJ/mol. The balanced chemical equation for the formation of ammonia from nitrogen ($$N_2$$) and hydrogen ($$H_2$$) is:

$N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$

To find the energy released in the reaction, you need to multiply the enthalpy of formation by the stoichiometric coefficient for the product. In this case, the coefficient for $$NH_3$$ is 2. Therefore:

$\text{Energy released} = 2 \times \text{Enthalpy of formation of } NH_3$

$\text{Energy released} = 2 \times (-46 \, \text{kJ/mol})$

$\text{Energy released} = -92 \, \text{kJ}$

### Question

What is the enthalpy change for the following reaction?

A. -74 – 242 + 111
B. +74 + 242 + 111
C. -74 + 242 – 111
D. +74 + 484 – 222