Quick questions on Exothermic and endothermic reactions, enthalpy and calorimetry (QCE Chemistry Unit 1)

A reaction profile plots potential energy (y-axis) against reaction progress (x-axis). Five features to label:

A QCAA-grade IA evaluation expects you to identify systematic and random errors:

A standard enthalpy change is measured at 25 degrees C and 100 kPa with reactants and products in their standard states.

Unit 2 builds collision theory and activation energy onto this framework. Unit 3 introduces Hess's law and enthalpy of formation for indirect $\Delta H$ calculations (in some QCAA schools), and reuses enthalpy in the context of equilibrium. Unit 4 connects enthalpy with the energetics of combustion fuels and biofuels.

A catalyst provides an alternative path with a lower transition state, so a lower E_a. $\Delta H$ is unchanged.

Both forward and reverse activation energies can be drawn. Forward E_a minus reverse E_a equals $\Delta H$ (with sign).

Define $\Delta H_c$ and state the sign convention for exothermic reactions. [2 marks]

$25.0 \, \text{mL}$ of $1.00 \, \text{mol L}^{-1}$ $\text{HCl}$ is mixed with $25.0 \, \text{mL}$ of $1.00 \, \text{mol L}^{-1}$ $\text{NaOH}$; temperature rises by $6.50^{\circ}\text{C}$. Calculate $\Delta H$ per mole of water formed. Assume $c = 4.18 \, \text{J g}^{-1} \, ^{\circ}\text{C}^{-1}$ and density $= 1.00 \, \text{g mL}^{-1}$.

A student measures $\Delta H_c$ of methanol. Their value is $25\%$ less negative than the data-book figure. (a) State two heat-loss sources.