general chemistry problem set – Energy Relationships in Chemical Reactions

1. Consider the dissolving of ammonium nitrate, NH4NO3 (s), for which ∆H° = +25.3 kJ/mol. If 133 grams of ammonium nitrate are dissolved in 367 grams of water, and the initial temperature is 25.0°C, what will be the final temperature of the system? You should assume that the specific heat of the entire system (the ammonium nitrate and water combined) is 4.18 J/g°C.

2. Citric acid, represented by H3Cit, is a triprotic acid. A student mixes together the following solutions in a styrofoam coffee cup: • 5.00 mL of 0.64 M citric acid • 45.00 mL of 0.77 M NaOH The two solutions start at a temperature of 25.5°C, and reach a final temperature of 27.8°C. The final, combined mixture has a mass of 51.6 grams and a specific heat of 4.0 J/g°C. Assume that no heat is transferred to the cup or surroundings. a) Write a complete, balanced equation for the neutralization of citric acid (H3Cit) and NaOH. b) Determine the total heat absorbed by the solution during this reaction. c) Calculate the number of moles of citric acid that reacted. d) Calculate the enthalpy of reaction, ∆H, for this reaction in kJ/mol. Is the reaction exothermic or endothermic?

3. a) Using only the following information: • ∆H°f for NO (g) is +90.4 kJ/mol • ∆H° = –56.6 kJ/mol for the reaction: NO (g) + 1/2 O2 (g) à NO2 (g) Determine ∆H°f for NO2 (g). b) Using only your answer to (a) and the following information: •∆H° = –283.0 kJ/mol for the reaction: CO (g) + 1/2 O2 (g) à CO2 (g) Determine ∆H° for the reaction: 4 CO (g) + 2 NO2 (g) à 4 CO2 (g) + N2 (g) c) A 10.0-L vessel contains 5.0 atm of CO and 3.0 atm of NO2 at 25°C. How much heat (in Joules) will be liberated if this is allowed to react to completion according to the reaction in part (b)? d) In a separate experiment using a very large reaction vessel with a movable piston, 6.00 moles of CO2 (g) reacts completely with 3.00 moles of nitrogen gas according to the following equation at 25°C and with a constant external pressure of 2.00 atm: 4 CO2 (g) + N2 (g) à 4 CO (g) + 2 NO2 (g) This reaction proceeds to completion. Calculate ∆U, q, and w for this reaction under these conditions.

4. Given 100 g of hot tea at 90°C, what mass of ice at –10°C must be added in order to obtain a final temperature of 10°C? Useful Info: specific heat of ice = 2.09 J/g°C specific heat of tea = specific heat of liquid water = 4.18 J/g°C normal freezing point of water = 0°C normal boiling point of water = 100°C ∆H° = +6.01 kJ/mol for the melting of ice: H2O (s) → H2O (l) Note: This problem is very similar to a problem in the Practice Problems. But watch out—it is not identical!

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