R1.1.4 Standard enthalpy change Notes

Understanding Standard Enthalpy Change and Heat Calculations

What is Standard Enthalpy Change ($ΔH^\ominus$)?

1. Chemical reactions are accompanied by energy transfers between the system (reactants and products) and the surroundings.
2. When this measurement is conducted under standard conditions, defined as 298 K (25°C), 100 kPa, and reactants/products in their standard states, it is referred to as the standard enthalpy change($ΔH^\ominus$).
   1. Exothermic reactions: Release heat into the surroundings, resulting in a negative $ΔH$ (e.g., combustion of fuels).
   2. Endothermic reactions: Absorb heat from the surroundings, resulting in a positive $ΔH$ (e.g., melting of ice).

Units of Enthalpy Change

1. The standard unit for $\Delta H$ is kilojoules per mole (kJ mol⁻¹).
2. This unit reflects the energy change associated with one mole of reactant or product under standard conditions.

Calculating Heat Transfer $Q$

The heat transferred during a reaction can be calculated using the formula: $$Q = mc\Delta T$$ where:

1. $Q$: Heat transferred (in joules, J)
2. $m$: Mass of the substance being heated (in grams, g)
3. $c$: Specific heat capacity of the substance (in J g⁻¹ K⁻¹)
4. $\Delta T$: Temperature change (in kelvin, K, or degrees Celsius, °C)

Breaking Down the Formula

1. Mass $m$: This is the mass of the substance (often water in calorimetry experiments) that absorbs or loses heat.
2. Specific Heat Capacity $c$: A property of the substance that indicates how much heat is required to raise the temperature of 1 g of the substance by 1 K. For water, $c = 4.18 \, \mathrm{J \ g^{-1} \ K^{-1}}$.
3. Temperature Change $\Delta T $: Calculated as $\Delta T = T_{\text{final}} - T_{\text{initial}}$.