1 A. That the reaction is endothermic when proceeding in the left to right direction as written.

Transcription

1 1 Q. If Δ r H is positive, what can you say about the reaction? 1 A. That the reaction is endothermic when proceeding in the left to right direction as written. 2 Q If Δ r H is negative, what can you say about the reaction? 2 A That the reaction is exothermic when proceeding in the left to right direction as written. 3 Q For an exothermic reaction, what can you say about the enthalpy of products of the reaction when compared to the enthalpy of the reactants? 3 A That the products have less enthalpy (energy) than the reactants.

2 4 Q For an endothermic reaction, what can you say about the enthalpy of products of the reaction when compared to the enthalpy of the reactants? 5 Q In terms of activation energy, what is the effect of a catalyst on a reaction, and how does it do this? 6 Q When ammonium nitrate dissolves in water, an endothermic reaction takes place. What would you observe to happen, and what is happening? 4 A That the products have more enthalpy (energy) than the reactants. 5 A A catalyst lowers the activation energy for a reaction by providing an alternative reaction pathway for the reaction. 6 A The solution would cool down (and so its container would get cold) because it is absorbing energy from its surroundings.

3 7 Q When potassium hydroxide dissolves in water, an exothermic reaction takes place. What would you observe to happen, and what is happening? 7 A The solution would heat up (and so its container would get hot) because it is releasing energy from its surroundings. 8 Q What is the unit of Enthalpy? 8 A J, or kj, or J mol -1 or kj mol Q For the reaction C (s) + O 2(g) CO 2(g) Δ r H = -400 kj/ mol -1. How much heat would be released when 88 g of CO 2 is produced? Where do we commonly see this reaction used? 9 A M CO2 = 44, so 2 moles produced, so 400 kj heat produced. A common use of this reaction is in charcoal BBQs.

4 10 Q. In terms of reactants and products, what happens in a reaction which is in dynamic equilibrium? 10 A. The forward and background reactions are going on continuously but at the same rate, so no change is observed. 11 Q. How would you distinguish a process which is in physical equilibrium with one which is in chemical equilibrium? 12 Q. Is the dissolution of an ionic salt in water a chemical or physical process? 11 A. In a physical process the atoms or molecules are not altered, only their physical state is. In a chemical process the atoms or molecules are changed into different combinations. 12 A. Chemical as the bonds in the salt are broken and hydrolysed ions are formed.

5 13 Q. What happens to the concentrations of all species in a system which is in dynamic equilibrium? 13 A. Nothing, they stay the same. 14 Q. What is the equilibrium constant for the reaction: H 2(g) + I 2(g) 2HI (g)? 14 A. K c = [HI] 2 /[H 2 ][I 2 ] 15 Q. What is the equilibrium constant for the reaction: N 2(g) + 3F 2(g) 2NF 3(g)? 15 A. K c = [NF] 2 /[N 2 ][F 2 ] 3

6 16 Q. What is the value of [C] in the equilibrium constant expression for the reaction and what is K c : C (s) + O 2(g) CO 2(g)? 16 A. 1, so it doesn t appear in the expression. so K c = [CO 2 ]/[O 2 ] 17 Q. What is the effect of an increase in pressure in the reaction: 2NO 2(g) N 2 O 4(g) + heat? 17 A. Equilibrium moves to the right i.e. more N 2 O 4 forms. 18 Q. What is the effect of an increase in temperature in the reaction: 2NO 2(g) N 2 O 4(g) + heat? 18 A. Equilibrium moves to the left i.e more NO 2 forms.

7 19 Q. What is the effect of an increase in [N 2 O 4 ] in the reaction: 2NO 2(g) N 2 O 4(g) + heat? 19 A. Equilibrium moves to the left i.e. more NO 2 forms. 20 Q. What is the effect of an increase in [NO 2 ] in the reaction: 2NO 2(g) N 2 O 4(g) + heat? 20 A. Equilibrium moves to the right i.e. more N 2 O 4 forms. 21 Q. For the reaction CH 3 COOH + H 2 O CH 3 COO - + H 3 O + K c = 1.8 x 10-5 If [CH 3 COOH] = 0.1 mol L -1, then what is [CH 3 COO - ]? 21 A.K c = [CH 3 COO - ][H 3 O + ]/[CH 3 COOH] = 1.8 x 10-5 [CH 3 COO - ] = [H 3 O + ] So [CH 3 COO - ] 2 = (1.8 x 10-5 /0.1) And [CH 3 COO - ]= 4.24 x 10-3

8 22 Q. a. What is an acid? b. What is the definition of ph? 22 A. a. A proton donor. b. ph = -log[h 3 O + ] 23 Q. What is a base? What is the formula for K w and what is its value? 23 A. a. A proton acceptor. b. K w = [H 3 O + ][OH - ] = Q. Define a weak and a strong base and explain how they differ from concentrated and dilute bases. 24 A. A weak base does not completely dissociate in reaction with water: a strong one does. A concentrated base has more of the base dissolved in water than a dilute one, but has nothing to do with the extent of dissociation.

9 25 Q. Explain why the ammonium ion is acidic. 26 Q. Explain why ammonia is basic. 25 A. The ammonium ion reacts with water to produce an hydronium ion. NH 4 + (aq) + H 2 O NH 3(aq) + H 3 O + (aq) A proton is transferred from the ammonium ion to the water molecule. 26 A. Ammonia reacts with water to produce an hydroxide ion. NH 3 + H 2 O NH OH - 27 Q. Explain why a. the salt Na 2 CO 3 is basic, b. the salt NH 4 Cl is acidic. 27 A. a. NaOH is a strong base, H 2 CO 3 is a weak acid, so the base dominates to give a basic solution in water. b. HCl is a strong acid but NH 4 OH is a weak base so the acid dominates to give an acidic solution in water.

10 28 Q. What is the ph of a 0.05 mol L -1 solution of sulfuric acid in water? 28 A. ph = 1 29 Q. What is the ph of a mol L -1 solution of nitric acid? 29 A. ph = Q. What is the [OH - ] in a solution of mol L -1 HCl? 30 A. [OH - ] = mol L -1