Overview
This lecture explains how to calculate the enthalpy change (ΔH) of a reaction using data from calorimeter experiments, including sample problems and step-by-step solutions.
Calorimeter Basics
- A calorimeter measures heat transfer during chemical reactions.
- The system is where the reaction occurs; the surroundings are water and calorimeter walls.
- Exothermic reactions release heat absorbed by the surroundings, following the principle: heat released = heat absorbed.
Heat Calculations in Calorimetry
- ( Q = mc\Delta T ) for water/solution, where:
- ( Q ) = heat (in Joules)
- ( m ) = mass (grams)
- ( c ) = specific heat capacity (J/g·K)
- ( \Delta T ) = change in temperature (K or °C)
- For calorimeter: ( Q = C\Delta T ), where ( C ) is heat capacity (J/K).
- If only specific heat is given: ( \Delta H = -\frac{mc\Delta T}{n_{mol}} ).
- If calorimeter heat capacity is known: ( \Delta H = -\frac{mc\Delta T + C\Delta T}{n_{mol}} ), with ( n_{mol} ) = moles of limiting reactant.
Example 1: HCl + NaOH Reaction
- Volumes: 50 mL HCl (0.4 M) + 50 mL NaOH (0.1 M); density = 1 g/mL.
- Temp rises from 25°C to 28°C; ( c = 4.2 ) J/g·K.
- Total mass = 100 g, ( \Delta T = 3 )°C.
- Moles limiting reactant = 0.02 mol (20 mmol).
- Calculation: ( \Delta H = -\frac{100 \times 4.2 \times 3}{0.02 \times 1000} = -63 ) kJ/mol.
Example 2: HNO₃ + KOH Reaction
- 33 mL HNO₃ (1.4 M) + 42 mL KOH (1 M); density = 1 g/mL.
- Temp rises from 25°C to 32°C; ( c = 4.2 ) J/g·K.
- Total mass = 75 g, ( \Delta T = 7 )°C.
- Limiting reactant moles = 0.042 mol (42 mmol).
- Calculation: ( \Delta H = -\frac{75 \times 4.2 \times 7}{0.042 \times 1000} = -52.5 ) kJ/mol.
Example 3: Methane Combustion in Bomb Calorimeter
- 250 mL water, 200 mg methane, temp rises 25°C to 35°C.
- ( C_{calorimeter} = 75 ) J/°C, ( c_{water} = 4.2 ) J/g·°C.
- Water mass = 250 g, ( \Delta T = 10 )°C.
- Moles methane = 0.0125 mol.
- Calculation: ( \Delta H = -\frac{(250 \times 4.2 \times 10) + (75 \times 10)}{0.0125} = -900 ) kJ/mol.
Key Terms & Definitions
- Calorimeter — Device to measure heat changes in chemical reactions.
- Enthalpy Change (ΔH) — Heat change at constant pressure, usually reported per mole.
- Specific Heat (c) — Amount of heat required to raise 1 g of substance by 1°C.
- Heat Capacity (C) — Amount of heat needed to raise calorimeter temperature by 1°C.
- System — The part of the experiment where reaction occurs.
- Surroundings — Everything outside the system absorbing/releasing heat.
- Exothermic Reaction — Reaction that releases heat.
- Limiting Reactant — Reactant that determines the amount of product formed.
Action Items / Next Steps
- Practice similar calorimeter problems.
- Review concepts of limiting reactant and heat calculations.
- Prepare for questions on enthalpy calculations using provided formulas.