A 'QwikCure' pack, used to treat sporting injuries, contains a bag of water inside a larger bag of freshly powdered ammonium nitrate, NH4NO3 - VCE - SSCE Chemistry - Question 2 - 2009 - Paper 1

In the energy profile diagram, the y-axis should represent chemical energy in kJ mol⁻¹, while the x-axis shows the progress of the reaction. The diagram should start at the energy level of NH4NO3(s), rise to the peak representing the activation energy (35 kJ mol⁻¹ for the reverse reaction), and then decrease to the energy level of NH4NO3(aq), with a difference indicating the enthalpy change (+25 kJ mol⁻¹).

The expression for the acidity constant, K_a, for the reaction is:

$K_a = \frac{[H_3O^+][NH_3]}{[NH_4^+]}$

where [H3O⁺] is the concentration of hydronium ions, [NH3] is the concentration of ammonia, and [NH4⁺] is the concentration of ammonium ions.

To find the concentration of H3O⁺ ions, we can use the formula for pH:

$[H_3O^+] = 10^{-pH}$

For a pH of 5.04:

$[H_3O^+] = 10^{-5.04} \approx 9.12 \times 10^{-6} \text{ mol L}^{-1}$

To calculate the mass, we first need to determine the concentration of NH4NO3. Using the relationship:

$ext{mass} = ext{concentration} \times ext{volume} \times M$

where M is the molar mass of NH4NO3 (approx. 80.04 g/mol). Thus, if we find the concentration from previous calculations, we then multiply that by 0.3 L and by M to find the mass in grams.