Question

In an experiment, a spirit burner is used to heat $250\,c{m^{^3}}$of water by burning methanol ($C{H_3}OH$).
(At. Values C= 12.0, H= 1.0, O=16.0; specific heat capacity of water = $4.18\,J{g^{ - 1o}}{C^{ - 1}}$) Results:
Starting temperature of water = ${20.0^{^0}}C$
Starting mass of burner + fuel = $248.8\,g$
Final temperature of water= ${43.0^{^0}}C$
Final mass of burner + fuel = $245.9\,g$
(A) How many joules of heat energy went into the water?
(B) How many moles of fuel were burnt?
(C) Calculate an experimental value for the enthalpy change of combustion of methanol from these results.
(D) Suggest three reasons why your answer is much smaller than the accepted standard enthalpy of combustion of methanol.

Answer 1

Hint: We will firstly find out the joules of heat energy went into the water and then how many moles of fuels were burnt. We will also state the reason why our answer will be smaller than the accepted enthalpy. Using our knowledge, we will be approaching our answer.

Complete step by step solution:
(A) Change in the temperature $\Rightarrow \,43 - 20\, = {23^{^0}}C$
Change in mass $\Rightarrow \,\,248.8\, - \,245.9\, = \,2.9\,g$
We know,
$q\,\, = \,\,mc\vartriangle t$
$\Rightarrow \,\,2.9\, \times \,4.18\, \times 23 \\\ \Rightarrow \,\,278.8J \\\$
Now,
(B) Moles of fuel burnt = $2.9\,/$molar mass
$\Rightarrow \,\,2.9\,/\,32$
$\Rightarrow 0.09$

(C) The amount of heat which is evolved or absorbed in the reaction which is carried out at constant pressure is known as enthalpy change and its symbol is $\vartriangle H$ and is pronounced or read as delta.
Physical and Chemical states are the standard states for enthalpy change. Elements which have allotropes have the most energetically stable standard state.

(D) My answer is much smaller than the accepted standard enthalpy of combustion of methanol because of the energy loss and incomplete reaction. The water may end up in the gaseous state or reactants may not be in the standard states.

NOTE: The heat of a reaction or the Enthalpy of reaction is defined as the change in the enthalpy of a chemical reaction. It describes the quantity of the heat released or absorbed in the reaction. The term ‘Enthalpy Change’ is only applied to the reactions which are done at the constant pressure because most of the lab reactions are performed in flasks or in test tubes which are open to the atmosphere so that pressure remains constant at atmospheric pressure.