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ws4s08key

# ws4s08key - CH 302 Spring 2008 Worksheet 4 Answer Key...

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CH 302 Spring 2008 Worksheet 4 Answer Key Practice Exam 1 1. Predict the signs of Δ H and Δ S for the sublimation of CO 2 . a. Δ H > 0, Δ S > 0 b. Δ H > 0, Δ S < 0 c. Δ H < 0, Δ S > 0 d. Δ H < 0, Δ S < 0 Answer: This reaction happens only at higher temperatures, which means Δ H > 0 and Δ S > 0. 2. Vapor pressure increases ________ with temperature. a. Linearly b. Exponentially c. Logarithmically d. Quadratically Answer: Recall the Clausius-Clapeyron equation. Pressure is related exponentially to temperature. 3. Which of the following salts will dissolve most easily in water? a. LiF b. MgO c. BN d. KBr Answer: The salt with the lowest charge density will dissolve the easiest. BN and MgO both have multiple charges, so they have high charge density. Li+ and F- are smaller than K+ and Br-, so LiF has a higher charge density than KBr.

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4. For this question, refer to the phase diagram shown above. What is the phase of this substance at -56°C and 5.1 atm? a. Solid b. Liquid c. Gas d. Mixture of solid and gas e. Mixture of solid, liquid, and gas f. Supercritical fluid Answer: This is the triple point, which is the point at which solid, liquid, and gas are all at equilibrium with each other. 5. For this question, refer to the phase diagram shown above question 4. The substance is originally held in a container at -60°C and 20 atm. It is then heated to room temperature, and next allowed to expand to atmospheric pressure. What happens to the substance? 6. 1 kg of water starts at 200°C and is allowed to cool to room temperature. For water, the specific heats are c ice = 2.093 J/g°C, c water = 4.186 J/g°C, and c steam = 2.009 J/g°C. The enthalpy changes are Δ H fusion = -335.5 J/g and Δ H vaporization = 2.26 kJ/g. What is Δ H sys for this process? H = mc steam T = (1000g)(2.009 J/g°C)(-100°C) = -201 kJ Condense steam: H = m H melting = -m H vaporization = (1000 g)(2.26 kJ/g) = -2260 kJ Cool water 100°C 25°C: H = mc water T = (1000g)(4.186 J/g°C)(-75°C) = -314 kJ H tot = -201 kJ + -2260 kJ + -314 kJ = -2775 kJ

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ws4s08key - CH 302 Spring 2008 Worksheet 4 Answer Key...

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