Calculating the energy released when fuels burn
Some fuels are better than others at releasing energy during combustion reactions. The energy released (sometimes called enthalpy) is calculated using the following equation:
\({E_h} = cm\Delta T\)
\({E_h}\) is the heat energy released (in kJ or kJ mol-1)
\(c\) is the specific heat capacity of water. It is a constant, 4.18 kJ kg-1 藲颁-1
\(m\) is the mass of water (in kg) (100cm3 = 0.1kg)
\(\Delta T\) is the change in temperature (藲颁)
This formula is very easy to recall if you remember that "Every house can't make triangle toast!"
Example
Question
A spirit burner was filled with ethanol (C2H5OH) and used to heat a copper can containing 200 cm3 of water. The temperature of the water rose from 22藲颁 to 27藲颁.
Calculate the heat energy released from this combustion of ethanol.
All of the values for this equation are either known constants \(c\) or available in the question (\(m\) and \(\Delta T\))
\(c\) = the specific heat capacity of water = 4.18 kJ kg-1 藲颁-1
\(m\) = the mass of water in kg so 200 cm3 = 0.2 kg
\(\Delta T\) = the change in temperature = 27 - 22 = 5藲颁
\(\begin{array}{l} {E_h} = cm\Delta T\\ = 4.18 \times 0.2 \times 5\\ = 4.18kJ \end{array}\)