Calculating Enthalpy Change | Full Worked Example For Combustion Reactions

The video introduces calorimetry as the experimental method to determine combustion enthalpy changes, specifically using a copper calorimeter. Copper is chosen for its excellent heat conductivity. An alcohol fuel is burned below a copper can containing a set volume of water. The heat from the burning fuel transfers to the water, and temperature changes are measured to calculate heat energy transfer.

The method involves measuring a set volume of cold water (which equates to its mass due to water's density of 1g/cm³) into the copper calorimeter, noting its starting temperature. The spirit burner is weighed before and after the experiment to calculate the mass of fuel combusted. The fuel is ignited, heating the water, and after a set time, the final temperature is recorded to determine the temperature change.

Several inherent errors affect the accuracy of the experiment compared to data book values. These include heat loss through conduction and radiation to the surroundings, evaporation of water, incomplete combustion of fuel (releasing less energy), air currents causing inefficient heat transfer, formation of soot (uncombusted carbon) insulating the can, and the copper calorimeter itself absorbing some heat energy.

The first step in calculation is to use the equation Q = mcΔT (or mcat). Q represents the heat energy transferred (in joules), m is the mass of water (in grams), c is the specific heat capacity of water (a constant 4.2 J/g°C), and ΔT is the change in temperature (in degrees Celsius). This step calculates the raw heat energy transferred by the burning fuel to the water.

A detailed example is provided: 1.5g of ethanol burned heated 100 cm³ of water from 25°C to 65°C. Step 1: Calculate Q = 100g * 4.2 J/g°C * 40°C = 16,800 Joules. Step 2: Convert Joules to Kilojoules by dividing by 1000, resulting in 16.8 kJ. Step 3: Calculate the moles of fuel (ethanol) burned using moles = mass / relative mass. For ethanol (C2H5OH), the relative formula mass is 46, so 1.5g / 46 = 0.0326 moles.

The final step involves scaling up the calculated heat energy to one mole of fuel. This is done by dividing the kilojoules value by the number of moles of fuel burned. For combustion reactions, the Q value is converted to a negative number to indicate an exothermic reaction (heat released). So, -16.8 kJ / 0.0326 moles = -515 kJ/mol. The video concludes by summarizing the four key steps: Q=mcΔT, convert to kJ, calculate moles of fuel, and finally, -Q / moles of fuel.