Methanol can be used as a fuel, in a variety of different ways - Scottish Highers Chemistry - Question 7 - 2015

The thermometer was likely placed incorrectly as it may not have been submerged in the water, leading to inaccurate temperature readings. It should be placed in the water and not above the flame or in the air to ensure it measures the water temperature accurately.

The student should have kept the amount of methanol used in each experiment constant to ensure valid comparisons between results.

To calculate the enthalpy of combustion, the following relationship can be used: $q = m \cdot c \cdot \Delta T$ where:

• $m = 100 g$ (mass of water)
• $c = 4.18 \text{ J/g°C}$ (specific heat capacity of water)
• $\Delta T = 23°C$ (temperature rise)

Calculating the energy released: $q = 100 g \cdot 4.18 \text{ J/g°C} \cdot 23°C = 9614 \text{ J} \text{ or } 9.614 \text{ kJ}$

This energy corresponds to the combustion of 1.07 g of methanol, so to find the enthalpy change per mole of methanol:

• Moles of methanol: $n_{methanol} = \frac{1.07 g}{32 g/mol} = 0.0334 \text{ mol}$

The enthalpy of combustion is: $\Delta H_c = \frac{-9.614 \text{ kJ}}{0.0334 \text{ mol}} = -288.08 \text{ kJ/mol}$

Thus, the enthalpy of combustion for methanol is approximately -288 kJ/mol.

To calculate the density of methanol, use the formula: $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

Given that the mass of the 25 cm$^3$ sample is 19.98 g, the density can be calculated as: $\text{Density} = \frac{19.98 g}{25.0 cm^3} = 0.7992 g/cm^3$

Therefore, the density of methanol is approximately 0.799 g/cm$^3$.

It is important for chemists to predict the nature of the reaction because exothermic reactions release heat and can increase temperature, while endothermic reactions absorb heat and can lower temperature. Understanding this is crucial for optimizing reaction conditions and ensuring safety during industrial processes.

Using bond enthalpies to determine the enthalpy change for the reaction of methanol with steam, the total bond energies of the reactants and products will be calculated:

• Bonds broken in reactants (methanol and steam) and bonds formed in products (hydrogen and carbon monoxide) will provide the necessary data to calculate the enthalpy change.
• Following this calculation systematically based on the bond enthalpy values provided in the data booklet will yield the required enthalpy change.