Chapter 9: ThermochemistryContent check — Chemistry: Atoms First, Chapter 9: Thermochemistry

(a) C₂H₅OC₂H₅(l)

(b) Na₂CO₃(s)

from the following information:

(i) $2\text{Al}(s)+$ $3{\text{Cl}}_2(g)⟶$ $2{\text{AlCl}}_3(s)$ $\text{Δ}H\text{°}=$ $?$

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

(ii) $\text{HCl}(g)⟶$ $\text{HCl}(aq)$ $\text{Δ}{H}_{(ii)}^{°}=$ $\mathrm{-74.8}\text{kJ}$

(iii) ${\text{H}}_2(g)+$ ${\text{Cl}}_2(g)⟶$ $2\text{HCl}(g)$ $\text{Δ}{H}_{(iii)}^{°}=$ $\mathrm{-185}\text{kJ}$

(iv) ${\text{AlCl}}_3(aq)⟶$ ${\text{AlCl}}_3(s)$ $\text{Δ}{H}_{(iv)}^{°}=$ $\mathrm{+323}\text{kJ/mol}$

(v) $\text{2Al}(s)+$ $6\text{HCl}(aq)⟶$ $2{\text{AlCl}}_3(aq)+$ $3{\text{H}}_2(g)$ $\text{Δ}{H}_{(v)}^{°}=$ $\mathrm{-1049}\text{kJ}$

Using the bond energies in Table 9.4, calculate an approximate enthalpy change, ΔH, for this reaction.

Calculate the heat capacity, in joules and in calories per degree, of the following:

(a) 45.8 g of nitrogen gas

(b) 1.00 pound of aluminum metal

A piece of unknown solid substance weighs 437.2 g, and requires 8460 J to increase its temperature from 19.3 °C to 68.9 °C.

(a) What is the specific heat of the substance?

(b) If it is one of the substances found in Table 9.1, what is its likely identity?

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 approximate amount of heat absorbed by the reaction, which can be represented by the following equation:

Ba(OH)₂·8H₂O(s) + 2NH₄SCN(aq) ⟶ Ba(SCN)₂(aq) + 2NH₃(aq) + 10H₂O(l)

The following sequence of reactions occurs in the commercial production of aqueous nitric acid:
$4{\text{NH}}_3(g)+$ $5{\text{O}}_2(g)⟶$ $4\text{NO}\left(g\right)+$ $6{\text{H}}_2\text{O}\left(l\right)$ $\text{Δ}\text{H}=$ $\mathrm{-907}\text{kJ}$
$2\text{NO}\left(g\right)+$ ${\text{O}}_2(g)⟶$ $2{\text{NO}}_2(g)$ $\text{Δ}\text{H}=$ $\mathrm{-113}\text{kJ}$
$3{\text{NO}}_2+$ ${\text{H}}_2\text{O}\left(l\right)⟶$ $2{\text{HNO}}_3(aq)+$ $\text{NO}(g)$ $\text{Δ}\text{H}=$ $\mathrm{-139}\text{kJ}$

Determine the total enthalpy change for the production of one mole of aqueous nitric acid by this process. Coproducts of the net reaction include water and nitrogen monoxide.

Calculate $\text{Δ}H\text{°}$ for the process
$\text{Sb}\left(s\right)+$ $\frac{5}{2}{\text{Cl}}_2(g)⟶$ ${\text{SbCl}}_5(s)$

from the following information:
$$\begin{array}{l}\\ \text{Sb}\left(s\right)+\frac{3}{2}{\text{Cl}}_2(g)⟶{\text{SbCl}}_3(s)\text{Δ}H\text{°}=\mathrm{-314}\text{kJ}\\ {\text{SbCl}}_3(s)+{\text{Cl}}_2(g)⟶{\text{SbCl}}_5(s)\text{Δ}H\text{°}=\mathrm{-80}\text{kJ}\end{array}$$

Calculate ΔH for the process ${\text{Hg}}_2{\text{Cl}}_2(s)⟶$ $2\text{Hg}\left(l\right)+$ ${\text{Cl}}_2(g)$

from the following information:
$$\begin{array}{l}\text{Hg}(l)+{\text{Cl}}_2(g)⟶{\text{HgCl}}_2(s)\text{Δ}H=\mathrm{-224}\text{kJ}\\ \text{Hg}(l)+{\text{HgCl}}_2(s)⟶{\text{Hg}}_2{\text{Cl}}_2(s)\text{Δ}H=\mathrm{-41.2}\text{kJ}\end{array}$$

Using the data in Appendix G, calculate the standard enthalpy change for each of the following reactions:

(a) $\text{Si}\left(s\right)+$ $2{\text{F}}_2(g)⟶$ ${\text{SiF}}_4(g)$

(b) $2\text{C}\left(s\right)+$ $2{\text{H}}_2(g)+$ ${\text{O}}_2(g)⟶$ ${\text{CH}}_3{\text{CO}}_2\text{H}\left(l\right)$

(c) ${\text{CH}}_4(g)+$ ${\text{N}}_2(g)⟶$ $\text{HCN}\left(g\right)+$ ${\text{NH}}_3(g)\text{;}$

(d) ${\text{CS}}_2(g)+$ $3{\text{Cl}}_2(g)⟶$ ${\text{CCl}}_4(g)+$ ${\text{S}}_2{\text{Cl}}_2(g)$

The white pigment TiO₂ is prepared by the reaction of titanium tetrachloride, TiCl₄, with water vapor in the gas phase: ${\text{TiCl}}_4(g)+$ $2{\text{H}}_2\text{O}\left(g\right)⟶$ ${\text{TiO}}_2(s)+$ $4\text{HCl}\left(g\right).$

How much heat is evolved in the production of exactly 1 mole of TiO₂(s) under standard state conditions?

Ethylene, C₂H₄, a byproduct from the fractional distillation of petroleum, is fourth among the 50 chemical compounds produced commercially in the largest quantities. About 80% of synthetic ethanol is manufactured from ethylene by its reaction with water in the presence of a suitable catalyst. ${\text{C}}_2{\text{H}}_4(g)+$ ${\text{H}}_2\text{O}\left(g\right)⟶$ ${\text{C}}_2{\text{H}}_5\text{OH}\left(l\right)$

Using the data in the table in Appendix G, calculate ΔH° for the reaction.

Propane, C₃H₈, is a hydrocarbon that is commonly used as a fuel.

(a) Write a balanced equation for the complete combustion of propane gas.

(b) Calculate the volume of air at 25 °C and 1.00 atmosphere that is needed to completely combust 25.0 grams of propane. Assume that air is 21.0 percent O₂ by volume. (Hint: We will see how to do this calculation in a later chapter on gases—for now use the information that 1.00 L of air at 25 °C and 1.00 atm contains 0.275 g of O₂.)

(c) The heat of combustion of propane is −2,219.2 kJ/mol. Calculate the heat of formation, $\text{Δ}{H}_{\text{f}}^{°}$ of propane given that $\text{Δ}{H}_{\text{f}}^{°}$ of H₂O(l) = −285.8 kJ/mol and $\text{Δ}{H}_{\text{f}}^{°}$ of CO₂(g) = −393.5 kJ/mol.

(d) Assuming that all of the heat released in burning 25.0 grams of propane is transferred to 4.00 kilograms of water, calculate the increase in temperature of the water.

Using the bond energies in Table 9.3, determine the approximate enthalpy change for each of the following reactions:

(a) ${\text{H}}_2(g)+$ ${\text{Br}}_2(g)⟶$ $2\text{HBr}\left(g\right)$

(b) ${\text{CH}}_4(g)+$ ${\text{I}}_2(g)⟶$ ${\text{CH}}_3\text{I}\left(g\right)+$ $\text{HI}\left(g\right)$

(c) ${\text{C}}_2{\text{H}}_4(g)+$ $3{\text{O}}_2(g)⟶$ $2{\text{CO}}_2(g)+$ $2{\text{H}}_2\text{O}\left(g\right)$

When a molecule can form two different structures, the structure with the stronger bonds is usually the more stable form. Use bond energies to predict the correct structure of the hydroxylamine molecule:

Complete the following Lewis structure by adding bonds (not atoms), and then indicate the longest bond:

Use principles of atomic structure to answer each of the following:⁴

(a) The radius of the Ca atom is 197 pm; the radius of the Ca²⁺ ion is 99 pm. Account for the difference.

(b) The lattice energy of CaO(s) is –3460 kJ/mol; the lattice energy of K₂O is –2240 kJ/mol. Account for the difference.

(c) Given these ionization values, explain the difference between Ca and K with regard to their first and second ionization energies.

(d) The first ionization energy of Mg is 738 kJ/mol and that of Al is 578 kJ/mol. Account for this difference.

For which of the following substances is the least energy required to convert one mole of the solid into separate ions?

(a) MgO

(b) SrO

(c) KF

(d) CsF

(e) MgF₂

Which compound in each of the following pairs has the larger lattice energy? Note: Ba²⁺ and

K⁺ have similar radii; S^2– and Cl^– have similar radii. Explain your choices.

(a) K₂O or Na₂O

(b) K₂S or BaS

(c) KCl or BaS

(d) BaS or BaCl₂

Which of the following compounds requires the most energy to convert one mole of the solid into separate ions?

(a) K₂S

(b) K₂O

(c) CaS

(d) Cs₂S

(e) CaO