Question

If the combustion of 1 g of graphite produces 20.7 kJ of heat, what will be molar enthalpy change? Give the significance of sign also.

Answer 1

"$Molar\quad enthalpy\quad change\quad of\quad graphite\quad =\quad enthalpy\quad change\quad for\quad 1\quad g\quad carbon\quad \times \quad molar\quad mass\quad of\quad carbon$

$=\quad -20.7\quad KJ.{ g }^{ -1 }\quad \times \quad 12\quad g.{ mol }^{ -1 }$

$\therefore \quad \Delta H\quad =\quad -2.48\quad \times \quad { 10 }^{ 2 }\quad KJ.{ mol }^{ -1 }$

Negative value of ΔH implies exothermic reaction. So, the above reaction is a exothermic reaction. "

Answer 2

The enthalpy of the reaction is -249.4 kJ/mol

Explanation:

To calculate the number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

Given mass of graphite = 1 g

Molar mass of graphite = 12 g/mol

Putting values in above equation, we get:

$\text{Moles of graphite}=\frac{1g}{12g/mol}=0.083mol$

Sign convention of heat:

When heat is absorbed, the sign of heat is taken to be positive and when heat is released, the sign of heat is taken to be negative.

To calculate the enthalpy change of the reaction, we use the equation:

$\Delta H_{rxn}=\frac{q}{n}$

where,

q = amount of heat released = -20.7 kJ

n = number of moles = 0.083 moles

$\Delta H_{rxn}$ = enthalpy change of the reaction

Putting values in above equation, we get:

$\Delta H_{rxn}=\frac{-20.7kJ}{0.083mol}=-249.4kJ/mol$

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