Exam Practice Questions Energetics

Bond Breaking, Bond Making

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Question 1

What is the energy released when 1 mole of ethanol is burned in excess air?

Question 2

What is the energy released for the following reaction:7

$N_2+3H_2\rightleftharpoons 2NH_3$

Question 3

What is the Br-Cl bond enthalpy if the enthalpy of formation of BrCl is 14.4 kJ mol^-1?

$Br_2+Cl_2\rightarrow 2BrCl$

Hess’s Cycle

Question 1

Find the enthalpy change of the following reaction:

$H_2O_{(l)}\rightarrow H_2O_{(g)}$

Question 2

Find the enthalpy change of the following reaction:

$Cl_2+2HBr\rightarrow 2HCl+Br_2$

Question 3

Find the enthalpy change of the following reaction:

$C_2H_5Br\rightarrow C_2H_4+HBr$

Question 4

Find the enthalpy change of the following reactions:

$CH_2=CH_2+H_2\rightarrow CH_3CH_3$

$CH_3CH=CH_2+H_2\rightarrow CH_3CH_2CH_3$

Why are the two values similar but not exactly the same?

Question 5

Which compound releases more energy per gram, burning hydrogen or burning methane?

Born Haber Cycle

Question 1

Use the following data to calculate the enthalpy of formation of strontium chloride. You must write all thermochemical equations for the steps of the cycle.

The enthalpy of sublimation of strontium = +164 kJ/mole
First ionization energy for strontium = +549 kJ/mole
Second ionization energy for strontium = +1064 kJ/mole
The enthalpy of dissociation of chlorine, Cl₂ = +243 kJ/mole
The electron affinity of chlorine, Cl = -349 kJ/mole
Lattice energy of strontium chloride = -2150 kJ/mole

Question 2

Use the following data to calculate the lattice energy of cesium oxide. You must write all thermochemical equations for the steps of the cycle.

Enthalpy of formation of cesium oxide = -233 kJ/mole
Enthalpy of sublimation of Cs = +78 kJ/mole
First ionization energy of Cs = +375 kJ/mole
Enthalpy of dissociation of O₂ (g) = +494 kJ/mole of O₂ molecules
First electron affinity of O = -141 kJ/mole of O atoms
Second electron affinity of O = +845 kJ/mole of O^– ions

Gibbs Free Energy

Question 1

Predict whether the following reaction is still spontaneous at 500^oC:

$N_2+3H_2\rightarrow 2NH_3$

Assume that ΔH = 92.22 kJ mol ^-1 and ΔS = -198.75 J K^-1 mol^-1

Question 2

Consider the following reaction for the decomposition of calcium carbonate:

$CaCO_3 \rightarrow CaO + CO_2$

Above which temperature will this reaction become favorable?

Assume ΔHr°and ΔSr° are temperature independent and use the following values for the enthalpies and entropies of formation of the reactant and products.