Topic 5 - Energetics

Which statements concerning bond enthalpies are correct?

I. Bond enthalpies refer to both bond breaking and bond formation.

II. The formation of covalent bonds is an endothermic process.

III. Average covalent bond enthalpies refer only to the gaseous state.

• I and II only
• II and III only
• I and III only
• I, II, III only

Which process is endothermic?

A. NaOH(aq) + HCl(aq) → NaOH(aq) + H2O(l)

B. Cl(g) + e− → Cl−(g)

C. 2Na(s) + Cl2(g) → 2NaCl(s)

D. Cl2(g) → 2Cl(g)

• A
• B
• C
• D

Which bond enthalpy has an absolute value rather than an average value?

A. N−N

B. N=N

C. N≡N

D. N−H

• A
• B
• C
• D

Which enthalpy change can be calculated using bond enthalpies and/or average bond enthalpies only?

A. 2C3H6(g) + 9O2(g) → 6CO2(g) + 6H2O(l)

B. C5H10(l) + Cl2(g) → C5H10Cl2(l)

C. C5H10(l) + H2O(l) → C5H11OH(l)

D. C3H6(g) + H2(g) → C3H8(g)

• A
• B
• C
• D

Hydrazine gas, N2H4(g), reacts exothermically with chlorine N2H4(g) + 2Cl2(g) → N2(g) + 4HCl(g) ΔH⦵ = − 463 kJ Some relevant bond energies in kJ mol−1 are: N−N 158; N≡N 945; N−H 391 and Cl−Cl 242. What is the bond enthalpy of the H−Cl bond?

A. 371 kJ mol−1

B. 235 kJ mol−1

C. 255 kJ mol−1

D. 431 kJ mol−1

• A
• B
• C
• D

Which is a correct statement about heat and temperature?

A. Heat and temperature are both forms of energy.

B. 100 g of water at 20 oC contains the same amount of energy as 200 g of water at 20 oC.

C. Heat and temperature are both a measure of the kinetic energy of the particles.

D. When 1.0 kg of water cools from 27 oC to 26 oC it gives out 4.2 kJ of heat.

• A
• B
• C
• D

Which statements are correct about temperature?

I. A rise in temperature of 1.0 oC is the same as a rise in temperature of 1.0 K.

II. Both the degrees Celsius scale and the Kelvin scale are based on the temperature at which pure water melts and boils.

III. Pure water boils at 100 oC and 373 K at 101 kPa (1 atmosphere) pressure.

A. II and III

B. I and III only

C. I, II and III

D. I and II only

• A
• B
• C
• D

Which are correct statements about exothermic and endothermic reactions?

I. The bonds in the products are stronger than the bonds in the reactants in endothermic reactions.

II. Both exothermic and endothermic reactions involve the transfer of heat between the system and its surroundings.

III. ΔH has a positive value for endothermic reactions.

A. II and III only

B. I and III only

C. I, II and III

D. I and II only

• A
• B
• C
• D

Which equation represents the standard enthalpy change of combustion of ethane, ΔHc⦵?

A. 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g)

B. C2H6(g) + 3.5O2(g) → 2CO2(g) + 3H2O(g)

C. 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l)

D. C2H6(g) + 3.5O2(g) → 2CO2(g) + 3H2O(l)

• A
• B
• C
• D

Which equation represents the standard enthalpy change of formation of sodium chloride, ΔHf⦵?

A. Na(s) + 0.5Cl2(g) → NaCl(s)

B. 2Na(s) + Cl2(g) → 2NaCl(s)

C. 2Na(l) + Cl2(g) → 2NaCl(s)

D. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

• A
• B
• C
• D