Thermodynamics Exam Review

Enthalpy

100

The standard enthalpy of formation of C(s, graphite) is: a. zero b. negative c. positive d. unable to determine

What is zero (a)? The enthalpy of formation for elements in their standard state is zero.

200

Determine the enthalpy of reaction for: PCl3(g) + Cl2(g) -> PCl5(g) Given: dHf PCl5 = -374.9 kJ/mol dHf PCl3 = -287 kJ/mol

What is dH(rxn) = -87.9 kJ?? Sum of products - sum of reactants. dHf Cl2 = 0 (standard state).

300

Calculate the reaction enthalpy for the formation of dihydrogen sulfide given the following information: SO3(g) + H2O(g) -> H2SO4(l) dH = -205kJ SO3(g) + H2O(l) -> H2S(g) + 2O2(g) dH = 690kJ H2O(g) -> H2O(l) dH = -110kJ H2SO4(l) -> H2S(g) + 2O2(g) dH(rxn) = ??

What is dH(rxn) = + 785kJ? (Use Hess's Law)

400

Final Jeopardy!!! A bomb calorimeter is set up with 500 mL of water as a heat sink for the combustion of 5 grams of methane (CH4). The heat capacity of the calorimeter is 12.2 kJ/K and the temperature change is 48 C, what is the molar enthalpy of combustion of methane?

What is -2.19x10^6 J/mol??? dH(surr) = mCdT + CdT dH(surr) = (500 g)(4.184 J/gK)(48 K) + (12200 J/K)(48 K) dH(surr) = - dH(rxn)/0.313 mol CH4

Entropy

100

Use the Boltzman formula to calculate the entropy of H2O at absolute zero. Is this value valid? Why or why not?

What is 2.47x10^-23 J/K?? S = (1.38x10^-23 J/K)ln(6) Draw out the geometry. 6 possible orientations. Not valid since this is the MAX positional entropy and is more of a theory than the actual amount. H2O also has strong IMFs which decreases entropy.

200

Calculate the standard entropy of condensation of Br2 at its boiling point of 58.78 C when the standard molar enthalpy of vaporization at its boiling point is 43 kJ/mol.

What is -0.1296 kJ/molK? dG = 0 at equillibrium, therefore dS = dH/T Hvaporization = -Hcondensation

300

Double Jeopardy!!!! The signs for dS(universe), dS(system), and dS(surroundings), respectively, for a spontaneous, exothermic reaction are: (Hint: Two different answers) a. -,-,- b. +,+,+ c. +,-,+ d. +,-,- e. -,+,-

What are b (temperature independant) and c (temperature dependant)? dG = neg if spontaneous dHsys = neg, then dSsurr = positive (dSsurr = -dH/T). dSuniv is always increasing if spontaneous (positive). Two scenarios: 1. Temp dependant: dG = dH - TdS (-, -, -) <--dSsys neg (c). 2. Temp independant: dG = dH - TdS (-, -, +) <--dSsys positive (b).

Energy

100

The measurement of the heat produced when a know amount of a marshmallow burns in a constant volume calorimeter would allow us to calculate... a. The molar entropy change from the combustion of the marshmallow. b. The molar Gibb's Free Energy change from the combustion of the marshmallow. c. The molar enthalpy change from the combustion of the marshmallow. d. The molar internal energy change from the combustion of the marshmallow.

What is the molar internal energy change from the combustion of the marshmallow (d)? Closed containers (constant volume) measure delta E (though it's often approximated as dH) dH is measured under constant pressure (open containers).

200

Calculate the change in Gibb's free energy of reaction from the change in entropy and enthalpy of the following reaction at 298K: NH3(g) + HCl(g) -> NH4Cl(s) dH = -46.11 -92.31 -314.43 kJ/mol dS = 192.45 186.91 94.6 J/molK

What is dG = -91.15 kJ?? products minus reactants dHrxn = -176.01 kJ dSrxn = -284.76 J/K dG = dH - TdS

300

At what temperature will a reaction with dH = 0.001591114 kJ and dS = 1.1 J/K be spontaneous?

What is 1.446 K?? 0 < dH - TdS dH < TdS dH/dS < T

Heat

100

Heat flow is considered negative when heat flows _____ the system and work is negative when work is done _____ a system.

out of, by

200

A system did 320 kJ of work and the internal energy increased by 50 kJ. How much energy was gained or lost as heat?

What is +370 kJ?? (Heat was gained)

300

6 grams of methane are combusted in a calorimeter with 400 mL of water. If the heat capacity of the calorimeter is 82J/C and the temperature change is 4C, how much heat is evolved per gram of methane burned? What is dH(rxn) per gram of methane burned?

What is dH(surr) = 1170.5 J/g and dH(rxn) = -1170.5J/g? dH(surround) = (mCdT + cdT)/6 grams

Work

100

For the following reaction, estimate how much PV work is being done at 27 C: 2H2O(g) -> O2(g) + 2H2(g)

What is w= -2.5kJ/mol? PdV = w = -dnRT w = -nRT = -RT

200

A gas in a 3 L container at 5 atm is cooled and the pressure drops to 2.5 atm. Assuming no volume change, how much work is being done? Next, the volume is allowed to expand from 3 L to 8 L when heated at the constant pressure of 2.5 atm. How much work is now being done? (Answer in J) 1 atm = 101.325 kPa 1 kPaL = 1 J

What is w=0 (part a) and w=-1266.56 J (part b)?? a) No volume change = no PV work! b) dV = 5 L w = -(5 L)(2.5 atm)(101.325 kPa/atm) w is done on the surroundings = negative!!

300

A piston with a surface area of 30cm^2 pushes against a pressure of 203 Pa and is forced 20 mm outward. Calculate w. (Answer in J) 1kPaL = 1J

What is -0.012 J?? Lots of conversions!!! 200 Pa = 0.2 kPa 20 mm = 2 cm (2 cm)(30 cm^2) = 60 cm^3 = 60 mL = 0.06 L work is done on surroundings = negative!!