Advanced Chemistry Practice Problems

Transcription

1 Thermodynamics: Review of Thermochemistry 1. Question: What is the sign of DH for an exothermic reaction? An endothermic reaction? Answer: ΔH is negative for an exothermic reaction and positive for an endothermic reaction. 2. Question: Is heat gained or lost by the system in an exothermic process? By the surroundings? Answer: Heat is lost from the system in an exothermic process and gained by the surroundings. 3. Question: What is the enthalpy of the reaction given a. H 2 (g) + ½ O 2 (g) H 2 O(g) b. H 2 O(g) H 2 (g) + ½ O 2 (g) 2H 2 (g) + O 2 (g) 2H 2 O(g) ΔH = kj? Answer: a. H 2 (g) + ½ O 2 (g) H 2 O(g) Since the reaction is one-half the original reaction, ΔH rxn will be one-half the enthalpy of the original reaction which is kj. b. H 2 O(g) H 2 (g) + ½ O 2 (g) Since the reaction is the reverse of the original reaction and one-half of each substance, the enthalpy of this reaction will be one-half that of the original reaction and have the opposite sign so it will be kj.

2 4. Question: What is DH rxn for 2KCl + 3O 2 2KClO 3? Substance DH f (kj/mol) KCl 436 KClO Answer: To find the enthalpy of the reaction, we need to use the enthalpies of formation for each substance. Remember that elements in their standard state, such as O 2 (g) have an enthalpy of formation of zero. What is ΔH rxn for 2KCl + 3O 2 2KClO 3? Δ H = n ΔH n ΔH rxn products f,products reactants f,reactants = $ & 2ΔH % f,kclo 2 H 3' $ & Δ f,kcl + 3ΔH % f,o2' $ ( kj )% $ ( kj ) ( kj )% =, 2* mol -, * + + * + mol mol - &. /' &. /. /' $ kj % $ kj ( kj )% =, & mol-, 3* 0 + ' mol mol - &. /' kj kj = mol mol kj kj = mol mol kj = 90. mol

3 Thermodynamics: Spontaneous vs. Nonspontaneous 1. Question: Are spontaneous processes always endothermic? Always exothermic? Answer: The sign of ΔH rxn indicates whether heat is gained or lost by the system but does not predict the spontaneity of the reaction. Both exothermic and endothermic reactions can be spontaneous and they can both be nonspontaneous. 2. Question: How are spontaneity of a reaction and its rate related to one another? Answer: The spontaneity of a reaction and its rate are not related to one another. Spontaneous reactions run at a variety of rates depending on the reaction conditions. 3. Question: If the reaction A + B C is spontaneous, what do we know about the spontaneity of the reaction C A + B? Answer: If a reaction is spontaneous in the forward direction then it must be nonspontaneous in the reverse direction. In this problem, the second reaction is nonspontaneous because it is the reverse of the first reaction, which is spontaneous.

4 Thermodynamics: The Second Law of Thermodynamics 1. Question: Describe the First and Second Laws of Thermodynamics. Answer: The first law of thermodynamics states the energy of the universe is constant. The second law of thermodynamics states that the entropy of the universe increases for a spontaneous process. 2. Question: Which processes result in an increase in the entropy of the system? a. Steam condensing to water. b. Melting of iron. c. Vaporization of ethanol. Answer: Increases in entropy result from an increase in the disorder of the components of a system. a. Steam condensing to water is decreasing the entropy because the liquid phase of a substance has less entropy than does the gas phase. b. Melting of iron is increasing the entropy because the sample is going from solid to liquid which is going from a less disordered system in the solid to a more disordered system of the liquid. c. Vaporization of ethanol is increasing the entropy because we are going from the liquid phase to the gas phase which is accompanied by an increase in the disorder of the molecules and therefore an increase in the entropy.

5 Thermodynamics: Entropy 1. Question: What is the sign on ΔS for a system that is increasing the entropy? Answer: ΔS is positive for a system that is increasing in entropy. One way to think about this is that ΔS rxn follows the same plan as ΔH rxn which uses products minus reactants. If the products have more entropy than the reactants, the value of ΔS will be positive. 2. Question: Which system(s) will have an increase in entropy? a. 2H 2 O 2 (l) 2H 2 O(l) + O 2 (g) b. N 2 (g) + 3H 2 (g) 2NH 3 (g) c. CaCO 3 (s) CaO(s) + CO 2 (g) Answer: a. 2H 2 O 2 (l) 2H 2 O(l) + O 2 (g); Entropy will increase because the reactants are in the liquid phase and the products contain a substance in the gas phase. b. N 2 (g) + 3H 2 (g) 2NH 3 (g); Entropy will decrease because there are four moles of gaseous reactants and only two moles of gaseous products. c. CaCO 3 (s) CaO(s) + CO 2 (g); Entropy will increase because the reactant is in the solid phase and the products contain a substance in the gas phase.

6 Thermodynamics: Entropy of the Universe 1. Question: If ΔS sys = 432 J/K and ΔS surr = -659 J/K, will ΔS universe be positive or negative? Will the process be spontaneous or not? Answer: ΔS universe will be negative and the process will not be spontaneous. 2. Question: If the signs on the entropy values for the system and surroundings in the previous problem are reversed, will ΔS universe be positive or negative? Will the process be spontaneous or not? Answer: ΔS universe will be positive and the process will be spontaneous. 3. Question: Find the entropy change (ΔS surr ) of the surroundings for the decomposition of H 2 O 2 (ΔH rxn = 98.2 kj/mol) at each of the following temperatures. a. 15 C b. 0 C c. 25 C ΔHrxn Answer: The value of ΔS surr is found using Δ Ssurr = where T is in units of T Kelvin. kj 98.2 a. Δ Ssurr = mol 1000= 381 J/mol. K K kj 98.2 b. Δ Ssurr = mol 1000= 360 J/mol. K K kj 98.2 c. Δ Ssurr = mol 1000= 330J/mol. K K