Chemistry Multiple Choice Questions on “Thermodynamics – Enthalpies for Different Types of Reactions”.
1. Enthalpy of combustion as always ________
a) positive
b) negative
c) 0
d) infinity
Answer: b
Clarification: 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.
2. Calculate the heat of combustion of ethane, in the reaction C2H6(g) + 3 1/2O2(g) → 2CO2(g) + 3H2O(l) where the heats of formation of ethane gas, carbon dioxide gas and water liquid are –84.7 kJ mol-1, -393.5 kJ mol-1 and –285.8 kJ mol-1 respectively.
a) -1559.7 kJ mol-1
b) -155.7 kJ mol-1
c) -159.7 kJ mol-1
d) -559.7 kJ mol-1
Answer: a
Clarification: Given that C2H6(g) + 3 1/2 O2(g) → 2CO2(g) + 3H2O(l) As we know that, ΔHr = Σ ΔHf[products] – Σ ΔHf[reactants]; ΔHc(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) = ΔHc(C2H6) = – 1559.7 kJ mol-1.
3. The enthalpy of 435 KJ per Mol is obtained from the conversion of hydrogen molecule into two hydrogen atoms.
a) True
b) False
Answer: a
Clarification: 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.
4. In which of the following reactions the energy released is not an enthalpy of atomization?
a) decomposition of a hydrogen molecule into two hydrogen atoms
b) decomposition of an oxygen molecule into 2 oxygen atoms
c) formation of a water molecule
d) decomposition of chlorine molecule
Answer: c
Clarification: 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.
5. In case of the decomposition of hydrogen molecule into two hydrogen atoms, the enthalpy of atomization is same as the _________
a) bond dissociation enthalpy
b) enthalpy of formation
c) enthalpy of combustion
d) enthalpy of sublimation
Answer: a
Clarification: 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.
6. Calculate the carbon carbon double bond energy in ethane from the following reaction, H2C=CH2(g) + H2(g) –> H3C−CH3(g) ΔH = −138 kJ/mol. If Bond enthalpies are: C−C = 348; H−H = 436; C−H = 412 in KJ/mol.
a) 498 KJ/mol
b) 593 KJ/mol
c) 508 KJ/mol
d) 598 KJ/mol
Answer: d
Clarification: By following the rule ΔHr = Σ ΔHf[products] – ∑ ΔHf[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.
7. Decomposition of sodium chloride into a sodium ion and chloride ion releases energy of 788 KJ per Mol, what is the energy called?
a) lattice energy
b) translation energy
c) dilution energy
d) neutralization energy
Answer: a
Clarification: 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.
8. Which of the following cycle allows has to analyze reaction energies?
a) carbon cycle
b) born Haber cycle
c) nitrogen cycle
d) chemical cycle
Answer: b
Clarification: 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.
9. Enthalpy of solution can either be positive or ________
a) negative
b) does not exist
c) 0
d) infinity
Answer: a
Clarification: 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.
10. The enthalpy of dilution of a solution is __________ on the original concentration of the solution and the amount of solvent added.
a) dependent
b) independent
c) may be dependent
d) may be independent
Answer: a
Clarification: 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.