Enthalpy Review

Phileas Fogg, the character who went around the world in 80 days, was very fussy about his bathwater temperature. It had to be exactly 38.0^o C. You are his butler, and one morning while checking his bath temperature, you notice that it’s 42.0^oC. You plan to cool the 100.0 dm³ of water to the desired temperature by adding an aluminum-duckie, originally at freezer temperature (-24.0^oC). Of what mass should the Al-duckie be?

Specific heat of Al = 0.900 J/(g^oC); specific heat of water = 4.18 J/(g^oC); density of water = 1.00 g/cm³. Assume that no heat is lost to the air.

• 30.0 g

• 30000 g

• 367000 g

• 367 g

Construct a Born-Haber cycle and calculate the lattice energy of CaC₂(s). Note that this solid contains the diatomic ion C₂^2-. Use the Data Booklet for missing values, but make sure you scroll down to the bottom of the question first...!

Useful Information:

ΔH^o_f (CaC₂ (s))= -60 kJ/mol

ΔH_atom (Ca) = +178 kJ/mol

ΔH_sub (C) = +717 kJ/mol

Bond dissociation enery of C₂ (g) = +614kJ/mol

Second ionization energy of Ca (g) = +1143 kJ/mol

First electron affinity of C₂ (g) = -315kJ/mol

Second electron affinity of C₂ (g) = +410kJ/mol

• 2807 kJ/mol

• 3397 kJ/mol

• 2886 kJ/mol

• 2381 kJ/mol

The addition of 3.15 g of Ba(OH)₂•8H₂O to a solution of 1.52 g of NH₄SCN in 100 g of water in a calorimeter caused the temperature to fall by 3.1 °C.

Assuming the specific heat of the solution and products is 4.20 J/g °C, calculate the ΔDH_rxn, which can be represented by the following equation:

Ba(OH)₂⋅8H2O(s)+2NH₄SCN(aq)→Ba(SCN)₂(aq)+2NH₃(aq)+10H₂O(l)

• 1302 kJ/mol

• 130.2 kJ/mol

• -130.2 kJ/mol

• -1302 kJ/mol

Calculate the heat of combustion of 1 mole of ethanol, C₂H₅OH(l) using Table 12, the enthalpies of formation in your data booklet.

• -1367 kJ/mol

• -1366 kJ/mol

• -1234 kJ/mol

• -1081 kJ/mol

Using Hess’s Law, chlorine monofluoride can react with fluorine to form chlorine trifluoride:

(i) ClF(g)+F₂(g)⟶ClF₃(g). ΔH°=?

Use the reactions here to determine the ΔH° for reaction (i):

(ii) 2OF₂(g)⟶O₂(g)+2F₂(g). ΔH∘_(ii) = −49.4kJ

(iii) 2ClF(g)+O₂(g)⟶Cl₂O(g)+OF₂(g) ΔH∘_(iii) = +205.6kJ

(iv) ClF₃(g)+O₂(g)⟶1/2 Cl₂O(g)+3/2 OF₂(g) ΔH∘_(iv) = +266.7kJ

• 521.7 kJ

• -139.2 kJ

• -114.5 kJ

• -11.7 kJ

Ammonia reacts with oxygen to form nitrogen dioxide and steam, as follows:

4NH₃(g) + 7O₂(g) ---> 4NO₂(g) + 6H₂O(g)

The enthalpy change of this reaction has been measured as -1135 kJ. Use the bond dissociation enthalpies in your data booklet to determine the bond energy of the N−O bond of NO₂

• 470 kJ

• 587 kJ

• 214 kJ

• 283 kJ

Using the lattice enthalpy of magnesium chloride, MgCl₂, in Table 18, estimate the enthalpy change of solution of MgCl₂ using Table 20 in your data booklet.

• -218 kJ/mol

• +141 kJ/mol

• +218 kJ/mol

• -141 kJ/mol

The vaporization of bromine requires 31.0 kJ/mol and has an increase in entropy (ΔS˚ = +93.0 J/(mol•K)). At what temperature will this process be spontaneous?

• above 333 K

• below 333 K

• above 276 K

• above 0.333 K

Find the enthalpy of formation of methane, CH₄(g) using Table 13 – the standard enthalpies of combustion.

• -74.0 kJ/mol

• -75 kJ/mol

• -73 kJ/mol

• +211 kJ/mol

The conversions of ATP to ADP is the source of energy in biology. Under standard conditions, the Gibbs Free Energy change is -30.5 kJ/mol. (Under physiological conditions it is more than 50% higher!). Using Table 11, determine the enthalpy change based on the reaction below. Determine the change in entropy at 298 K.

• DH = -463 kJ/mol, DS = +1.45 J/mol K

• DH = +463 kJ/mol, DS = -1.66 J/mol K

• DH = -463 kJ/mol, DS = +1451 J/mol K

• DH = +463 kJ/mol, DS = +1656 J/mol K