To determine the enthalpy for the combustion of methanol, you can use the equation:
q = mcΔT
where q is the heat released or absorbed, m is the mass of the substance being burned (methanol in this case), c is the heat capacity of the calorimeter, and ΔT is the change in temperature.
Given:
Mass of methanol (m) = 17.62 g
Heat capacity of the calorimeter (c) = 4.5 kJ/°C
Change in temperature (ΔT) = (99.03 °C - 21 °C) = 78.03 °C
Now, plug in these values into the equation:
q = (17.62 g) * (4.5 kJ/°C) * (78.03 °C)
Notice that the units for mass and heat capacity need to be consistent, so convert grams to moles since the molar mass is given.
To convert grams to moles, use the formula:
moles = mass (g) / molar mass (g/mol)
Given:
Molar mass of methanol (MM) = 32.04 g/mol
moles = 17.62 g / 32.04 g/mol
Calculate moles:
moles = 0.549 moles (rounded to three decimal places)
Now, multiply the moles by the change in enthalpy per mole to find the total heat released (enthalpy) for the combustion of methanol:
q = (0.549 moles) * (−782.86 kJ/mol)
Notice that the enthalpy of combustion of methanol is -782.86 kJ/mol, meaning it releases energy.
Calculate q:
q = -429.51 kJ (rounded to two decimal places)
So, the enthalpy for the combustion of methanol is approximately -429.51 kJ.