chem102workshop3 - NAMES Chem 102 Thermodynamics Workshop 1...

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1 NAMES__________________________________________________________ Chem 102 Thermodynamics Workshop 1. What is the molar heat of reaction ( Δ H°) at 298 K for the combustion of C 2 H 2 gas in pure oxygen to produce carbon dioxide and water gas? C 2 H 2 (g) + O 2 (g) 2CO 2 (g) + H 2 O(g) Δ H° = Δ f(products) - Δ f(reactants) = -242 + 2(-393.5) – 227 - 5/2(0) = -1256 kJ/mol This reaction has been balanced for 1 mol of C 2 H 2 (g) because the problem is asking for the molar heat of the combustion of C 2 H 2 (g). 2. A 5.0 g piece of potassium metal (at 20 °C) is tossed in 150 ml of water (at 20 °C) held in an insulated container. The potassium reacts with some of the water to produce potassium hydroxide and hydrogen gas. Given that the heat capacity of both water and the KOH solution is 4.18 J/g-deg, what is the final temperature of the KOH solution? HINT: This is a 3 part problem. 1) You must find the Δ H ° for the reaction 2) You must use the Δ H ° to solve for the heat (Q) that is being transferred. 3) You must use Q = mc Δ T to solve for the final temperature The reaction to consider is: K(s) + H 2 O(l) KOH(aq) + ½ H 2 (g) 1) First find Δ H ° = Δ f(products) - Δ f(reactants) = ½ (0) + -481 – 0 - -286 = -195 kJ/mol (this corresponds to 1 mol of K) 2) We now need to find Q. Because this is constant P and PV work only we know that Q will be equal to Δ H ° , but we need a Δ H ° that corresponds to the correct amount of K. We have been asked about a reaction with 5g of K: moles K = 5 g / 39.1 g/mol = 0.13 moles K Thus the heat transferred for this reaction (Q) = 0.13 mol(-195 kJ/mol) = -25 kJ.
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