Chem 1 - Handout 13

The following are hypothetical thermochemical equations: 2P → Q ΔH = +90 kJ ½ Q → R ΔH = -30 kJ 2S → T ΔH = -75 kJ Calculate ΔH for the reaction P → R using Hess's law.

Equation 1 ÷ 2 ⇒ P → 1/2 Q --- ΔH = +45 kJ Equation 2 ⇒ 1/2 Q → R --- ΔH = -30 kJ _____________________________________ Add ⇒ P → R --- ΔH = +15 kJ

Given the following thermochemical equations N2 (g) + O2 (g) → 2 NO (g) ΔH = +180.7 kJ 2 NO (g) + O2 (g) → 2 NO2 (g) ΔH = -113.1 kJ 2 N2O (g) → 2 N2 (g) + O2 (g) ΔH = -163.2 kJ Use Hess's law to calculate the ΔH for the reaction N2O (g) + NO2 (g) → 3 NO (g)

Label the given equations 1, 2 and 3. Equation 3÷2 ⇒ N2O (g) → N2 (g) + 1/2 O2 (g) ΔH= -81.6 kJ Equation 2 × (−1/2) ⇒ NO2 (g) → NO (g) + 1/2 O2 (g) ΔH = +56.55 kJ Equation 1 ⇒ N2 (g) + O2 (g) → 2NO (g) ΔH = +180.7 kJ _____________________________________________________________ Add ⇒ N2O (g) + NO2 (g) → 3NO (g) ΔH = +155.65 kJ (or) +155.7 kJ

The conversion of methane to ethylene occurs as shown below: 2 CH4 (g) → C2H4 (g) + 2 H2 (g) Apply Hess's law and calculate the ΔH˚ for the above reaction. Useful thermochemical data are given below. CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l) ΔH˚ = - 890.3 kJ C2H4 (g) + H2 (g) → C2H6 (g) ΔH˚ = - 136.3 kJ 2 H2 (g) + O2 (g) → 2 H2O (l) ΔH˚ = - 571.6 kJ 2 C2H6 (g) + 7 O2 (g) → 4 CO2 (g) + 6 H2O (l) ΔH˚ = - 3120.8 kJ

Label the given equations 1, 2, 3 and 4. Equation 1 × 2 ⇒ 2CH4 (g) + 4O2 (g) → 2CO2 (g) + 4H2O (l) ΔH = -1780.6 kJ Equation 2 × (-1) ⇒ C2H6 (g) → C2H4 (g) + H2 (g) ΔH = +136.3 kJ Equation 3 × (−1/2) ⇒ H2O (l) → H2 (g) + 1/2 O2 (g) ΔH = +285.8 kJ Equation 4 × (−1/2) ⇒ 3H2O (l) + 2CO2 (g) → C2H6 (g) + 7/2 O2 (g) ΔH = +1560.4 kJ __________________________________________________________________________ Add ⇒ 2CH4 (g) → C2H4 (g) + 2H2 (g) ΔH = +201.9 kJ

A certain type of rock is used in a solar-heated home to store heat. The specific heat of the rock is 0.82 J/g-K. (a) Calculate the quantity of heat absorbed by 50.0 kg of the rock if its temperature increases by 12.0 ˚C. (b) What temperature change would the rock undergo if it emitted 450 kJ of heat?

a) q = 492000 J = 492 kJ b) ΔT = 10.98 ˚C = 11 ˚C

Using the 'Enthalpy of Formation' values from Appendix C of the text book, calculate the value of ΔH˚ for each of the following reactions. i. CaO (s) + 2 HCl (g) → CaCl2 (s) + H2O (g) ii. 4 FeO (s) + O2 (g) → 2 Fe2O3 (s) iii. C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) + 3 H2O (l) iv. N2O4 (g) + 4 H2 (g) → N2 (g) + 4 H2O (g)

i) ΔHrxn° = { ΔHf° [CaCl2 (s)] + ΔHf° [H2O (g)] } - { ΔHf° [CaO (s)] + 2 × ΔHf° [HCl (g)] } Use Appendix C to pick the above enthalpy of formation values. [Those will be given in the test] ΔHrxn° = { [-795.8] + [-241.82] } - { [-635.5] + [2 × (-92.30)] } = -1037.62 + 820.1 = -217.52 kJ = -217.5 kJ ii) ΔHrxn° = { 2 × ΔHf° [Fe2O3 (s)] } - { 4 × ΔHf° [FeO (s)] + ΔHf° [O2 (g)] } = { 2 × [-822.16] } - { 4 × [-271.9] + [0] } = -556.7 kJ

How much heat is needed to warm 172 g of water from 23.0 ˚C to 97.0 ˚C?

q = 53254 J = 53.3 kJ