Question

How much heat is released when 75 g of octane is burned completely if the enthalpy of combustion is -5,500 kJ/mol C8H18

Answer 1

Answer : The heat released during the reaction is $-8.4\times 10^3kJ$

Explanation :

First we have to calculate the number of moles of octane $(C_8H_{18})$.

$\text{Moles of }C_8H_{18}=\frac{\text{Mass of }C_8H_{18}}{\text{Molar mass of }C_8H_{18}}$

Molar mass of $C_8H_{18}$ = 114 g/mole

$\text{Moles of }C_8H_{18}=\frac{75g}{114g/mole}=0.658mole$

Now we have to calculate the heat released during the reaction.

$\Delta H=\frac{q}{n}$

or,

$q=\Delta H\times n$

where,

$\Delta H$ = enthalpy change = -5500 kJ/mol

q = heat released = ?

n = number of moles of $C_8H_{18}$ = 0.658 mol

Now put all the given values in the above formula, we get:

$q=(-5500kJ/mol)\times (0.658mol)=-8358.66kJ=-8.4\times 10^3kJ$

Therefore, the heat released during the reaction is $-8.4\times 10^3kJ$

Answer 2

Enthalpy change is the total heat that is absorbed or evolved in a certain reaction that is being carried in a constant pressure system. It can be calculated by the product of the specific heat of the substance and the difference of the final temperature and the initial temperature. For this case, the enthalpy of combustion is given, so we simply multiply the amount of octane present to the enthalpy given to determine the total heat that is released. 

Heat = 75 g ( 1 mol / 114.23 g ) (- 5500 kJ / mol ) = - 3611.14 kJ

The negative sign in the calculated value represents that this heat is being released in the process.