Enthalpy and Entropy

Transcription

1 Tutorial 2 - Solutions Enthalpy and Entropy Answers to questions 1-8 on page 6 & 7 of Tutorial 2: 1. Tell whether each of the following is endothermic or exothermic and state which has minimum enthalpy, the reactants or the products: a. PCl 5(g) Cl 2(g) + PCl 3(g) H = 92.5 kj endothermic and the reactants_ have minimum enthalpy. b. 2NH 3(g) kj N 2(g) + 3H 2(g) endothermic and the reactants have minimum enthalpy. c. CO (g) + 3H 2(g) CH 4(g) + H 2 O (g) kj exothermic and the products_ have minimum enthalpy. d. Cl 2(g) Cl 2(aq) H = -25 kj exothermic and the products have minimum enthalpy. 2. When no other factors are considered, a reaction will move in such a way (left or right) in order to achieve a state of minimum enthalpy. 3. Given the equation: 2NH 3(g) kj N 2(g) + 3H 2(g) If only the enthalpy is considered, the (reactant / products) will be favoured at equilibrium. reactants 4. Given the equation: Cl 2(g) Cl 2(aq) H = -25 kj If only the enthalpy is considered, the (reactant / products) will be favoured at equilibrium. products 5. If the reaction : CO (g) + 3H 2(g) CH 4(g) + H 2 O (g) kj was proceeding to the right, the enthalpy would be decreasing. Is this a favourable change? yes. Page 1

2 6. If the reaction: PCl 5(g) Cl 2(g) + PCl 3(g) H = 92.5 kj was proceeding to the right, the enthalpy would be increasing. Is this a favourable change? no. 7. If the reaction: Cl 2(g) Cl 2(aq) H = -25 kj was proceeding to the right, the enthalpy would be decreasing. Is this a favourable change? yes. 8. If the reaction: 2NH 3(g) kj N 2(g) + 3H 2(g) was proceeding to the right, the enthalpy would be increasing. Is this a favourable change? no. ************************************************* Answer to question 9 a-e on p. 10 of Tutorial For each of the following, decide whether the reactants or the products have greater entropy: a) I 2(s) I 2(aq) The product has greater entropy.((aq) > (s)) b) 2NH 3(g) N 2(g) + 3H 2(g) The products have greater entropy. (Reactants have 2 moles of gas, products have = 4 moles of gas.) c) NH 3(g) NH 3(aq) The reactant has greater entropy. (A gas has more disorder than an aqueous solution. The particles are much farther apart in a gas!) d) CO (g) + Cl 2(g) COCl 2(g) The reactants have greater entropy. Page 2

3 (The reactants have 2 moles of gas, while the product has only 1 moles of gas.) e) MgCO 3(s) + 2HCl (aq) MgCl 2(aq) + H 2 O (l) + CO 2(g) The products have greater entropy. (There is a gas in the products and no gases in the reactants. This means the products have greater entropy.) ************************************************* Answer to question 10 on page 15 of Tutorial For each of the following reactions decide which has minimum enthalpy (reactants or products), which has maximum entropy (reactants or products), and if the reactants are mixed, what will happen?. a) PCl 3(g) + Cl 2(g) PCl 5(g) ; H = kj The product have minimum enthalpy. The reactants have maximum entropy. If PCl 3 and Cl 2 are put together, what should happen?(go to completion/ reach a state of _reach a state of equilibrium (the enthalpy and entropy oppose each other.) b) 2NO 2(g) N 2 O 4(g) + energy The product has minimum enthalpy. The reactants have maximum entropy. If NO 2 was put in a flask, what should happen?(go to completion/ reach a state of _reach a state of equilibrium (the enthalpy and entropy oppose each other.) c) P 4(s) + 6H 2(g) + 37 kj 4PH 3(g) The reactants have minimum enthalpy. Page 3

4 The reactants have maximum entropy. If P?(s) and 6H 2(g) was put in a flask, what should happen?(go to completion/ reach a state of equilibrium/ not occur at all (both enthalpy and entropy favour reactants.) d) 2PbO (s) + 4NO 2(g) + O 2(g) 2Pb(NO 3 ) 2(s) ; H = kj The product has minimum enthalpy. The reactants have maximum entropy. If P 4(s) and 6H 2(g) was put in a flask, what should happen?(go to completion/ reach a state of _reach a state of equilibrium (the enthalpy and entropy oppose each other.) Answers to Self Test on Tutorial 2 - pages What is meant by enthalpy? heat content or chemical potential energy 2. What is meant by entropy? disorder 3. In an endothermic reaction, the reactants have minimum enthalpy. 4. In an exothermic reaction, the products have minimum enthalpy. 5. Arrange the following in order from least entropy to greatest entropy: a) liquids b) gases c) aqueous solutions d) solids solids < liquids < aqueous solutions < gases 6. There is a natural tendency toward minimum enthalpy and maximum entropy. 7. A process in which entropy increases and enthalpy decreases will go to completion Page 4

5 8. A process in which entropy increases and enthalpy increases will reach a state of equilibrium 9. A process in which entropy decreases and enthalpy decreases will reach a state of equilibrium 10. A process in which entropy decreases and enthalpy increases will not occur at all 11. A process in which both the enthalpy and entropy trends favour reactants will not occur at all 12. A process in which both the enthalpy and entropy trends favour products will go to completion 13. A process in which the enthalpy and entropy trends oppose each other will reach a state of equilibrium 14. In each of the following, state which has the maximum entropy, (reactants or products) a) C (s)? O 2(g) CO 2(g) product b) 2Al(s) + 6HCl (aq) 3H 2(g) + 2AlCl 3(aq) products c) 2SO 3(g) 2SO 2(g) + O 2(g) products d) HCl (g) H + (aq) + Cl - (aq) reactant e) KOH (s) K + (aq) + OH - (aq) product Page 5

6 15. For each of the following reactions decide which has minimum enthalpy (reactants or products), which has maximum entropy (reactants or products), and if the reactants are mixed, what will happen?. Assume there is sufficient activation energy to initiate any spontaneous reaction. a) PCl 5(g) PCl 3(g) + Cl 2(g) ; H = kj The reactant has/have minimum enthalpy. The products has/have maximum entropy. If PCl 5 is put in a flask what should happen?(go to completion/ reach a state of reach a state of equilibrium b) 2NO (g) + O 2(g) 2NO 2(g) + energy The products has/have minimum enthalpy. The reactants has/have maximum entropy. If NO and O 2 were put in a flask, what should happen?(go to completion/ reach a state of reach a state of equilibrium c) Na 2 CO 3(s) + 2HCl (aq) 2NaCl (aq) + CO 2(g) + H 2 O (l) kj The products has/have minimum enthalpy. The products has/have maximum entropy. If Na 2 CO 3(s) + 2HCl (aq) were put in a flask, what should happen?(go to completion/ reach a state of go to completion Page 6

7 d) 2Pb(NO 3 ) 2(s) kj 2PbO (s) + 4NO 2(g) + O 2(g) ; The reactant has/have minimum enthalpy. The products has/have maximum entropy. If Pb(NO 3 ) 2 was put in a flask, what should happen?(go to completion/ reach a state of reach a state of equilibrium 16. Reactions which result in a/an decrease in enthalpy and a/an increase in entropy will always be spontaneous. 17. Reactions which result in a/an increase in enthalpy and a/an decrease in entropy will always be non-spontaneous. You have now finished Tutorial 2 Get clarification on anything you don't understand - as soon as possible! ********************************************************** Page 7