Test: Enthalpies for Different Types of Reactions - Chemistry MCQ

Enthalpy of combustion is the enthalpy change that takes place when one mole of a compound undergoes complete combustion in the presence of oxygen. It is represented by ΔHc, it is always negative because the process of combustion is exothermic.

The enthalpy change that occurs when one mole of a molecule breaks into its atoms is known as enthalpy of atomization. The energy released during the conversion of a hydrogen molecule into two hydrogen atoms is enthalpy of atomization.

In case of the decomposition of hydrogen molecule the enthalpy is the same as enthalpy of atomization as well as bond dissociation enthalpy because the bond association enthalpy refers to the breakage of H-H Bond. So that two hydrogen atoms are formed whereas enthalpy of atomization is the breakage of hydrogen molecules in order to form two atoms.

Lattice enthalpy is the enthalpy change when one molecule of an ionic compound dissociates into its ions and gaseous state. Here sodium chloride is an ionic compound and it decomposes into sodium and chloride ions, so the energy released is lattice energy.

Enthalpy change when 1 mole of a substance is dissolved in a large excess of solvent so that on further delusion no appreciable heat change occurs, is known as enthalpy of solution. It can either be positive or negative depending on the reaction, whether it is exothermic or endothermic.

Given that C₂H_6(g) + 3 1/2 O_2(g) → 2CO_2(g) + 3H₂O(l) As we know that, ΔHr = Σ ΔH_f[products] – Σ ΔH_f[reactants]; ΔH_c(C₂H₆) = 2 X (-393.5 kJ mol^-1) + 3 X (-285.8 kJ mol^-1) – (- 84.7 kJ mol^-1) – 2 X (0 kJ mol^-1) = ΔH_c(C₂H₆) = – 1559.7 kJ mol^-1.

The enthalpy of atomization is an enthalpy change that occurs when one molecule of a molecule breaks into its atoms, but here the of a water molecule formation does not satisfy the definition of the enthalpy of atomization.

By following the rule ΔH_r = Σ ΔH_f[products] – ∑ ΔH_f[reactants], we get -(x + 4(412) + 432) + (348 + 6(412)) = -138 KJ/mol; x = 598 KJ/mol. So the carbon carbon double bond energy in Ethane is given as 598 KJ per Mol.

An indirect method, in which we can construct an enthalpy diagram is called a Born-Haber cycle as it is nearly impossible to determine lattice enthalpies directly by experiment. It is an approach to analyze reaction energies.

Enthalpy of dilution is the enthalpy change when 1 mole of a substance is diluted from one concentration to another. So it is dependent on your original concentration of the solution and the amount of solvent added.