# Exam 4 - Version 155 – Exam 4 – Mccord –(52385 1 This...

This preview shows pages 1–3. Sign up to view the full content.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Version 155 – Exam 4 – Mccord – (52385) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCord CH301 This exam is only for McCord’s TTH CH301 class. 001 10.0 points For a given transfer of energy, a greater change in disorder occurs when the temperature is high. 1. False correct 2. True Explanation: From Δ S = q T since T is in the denomina- tor, Δ S will be larger (more positive) when- ever T is smaller . 002 10.0 points Calculate the average H S bond enthalpy in H 2 S(g) given the standard enthalpies of for- mation for H 2 S(g), H(g), and S(g) as- 20 . 1, 218, and 223 kJ · mol − 1 , respectively. 1. 231 kJ · mol − 1 2. 679 kJ · mol − 1 3. 10 . 1 kJ · mol − 1 4. 340 kJ · mol − 1 correct 5. 461 kJ · mol − 1 Explanation: Write an equation which represents the bond energy (or a multiple of it): H 2 S(g) → 2 H(g) + S(g) Δ H ◦ r = 2 (BE H − S ) Reactants: Δ H f H 2 S(g) =- 20 . 15 kJ/mol Products: Δ H f H(g) = 218 kJ/mol Δ H f S = 223 kJ/mol Δ H rxn = summationdisplay n Δ H f prod- summationdisplay n Δ H f rct = [2 (218 kJ / mol) + 223 kJ / mol]- (- 20 . 1 kJ / mol) = 679 . 1 kJ / mol rxn , so H ◦ rxn = 2 × BE BE = 679 . 1 kJ mol rxn 2 = 339 . 55 kJ mol rxn . 003 10.0 points Using the provided bond energy data, cal- culate the change in enthalpy for the reaction below. 6HCl(g) + N 2 (g)-→ 2NH 3 (g) + 3Cl 2 (g) Bond Energy (kJ · mol − 1 ) H- Cl 432 N ≡ N 945 N- H 391 Cl- Cl 199 1.- 2 , 357 kJ · mol − 1 2. 787 kJ · mol − 1 3.- 594 kJ · mol − 1 4. 2 , 357 kJ · mol − 1 5. 594 kJ · mol − 1 correct Explanation: Δ H = Σ BE reactants- Σ BE products = (6 · 432 + 945)- (6 · 391 + 3 · 199) = 594 kJ · mol − 1 Version 155 – Exam 4 – Mccord – (52385) 2 004 10.0 points Which of the following statements concerning internal energy is/are true? I) If the expansion work is small, Δ H and Δ U are close in value. II) The internal energy of a system is equal to q at constant volume. III) Assuming no heat is exchanged, when pressure-volume work is done on the sys- tem, Δ U is positive. 1. I, II 2. II, III 3. I only 4. III only 5. I, II, III correct 6. II only 7. I, III Explanation: Statement I follows from the identity Δ H = Δ U + p Δ V , because p Δ V is the expansion work. Statement II follows from Δ U = q + w , because w = 0 for processes that occur at constant volume. Statement III also follows from Δ U = q + w , because w is the pressure- volume work. (Note: for reversible processes, expansion work and pressure-volume work are identical.) 005 10.0 points Heat absorbed by a system at constant vol- ume is equal to 1. ΔV 2. Δ S 3. Δ E correct 4. Δ H 5. Δ G Explanation: Δ E = q + w q = Δ H w =- P Δ V When Δ V = 0, w = 0....
View Full Document

{[ snackBarMessage ]}

### Page1 / 10

Exam 4 - Version 155 – Exam 4 – Mccord –(52385 1 This...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online