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Presentation on theme: "Great site to explore!"— Presentation transcript:

1 Great site to explore!

2 EQUILIBRIUM TOPIC 8 Review Book Chapter 19 Textbook

3 Objectives Reversible reactions Reversible reactions Equilibrium – classification Equilibrium – classification Le Chateliers Principle Le Chateliers Principle

4 Reversible Reactions Some reactions occurs simultaneously, forming and decomposing the product. The two equations can be combined into one, by using a double arrow, which tells us that it is a reversible reaction: 2SO 2(g) + O 2(g) 2SO 3(g) When the system reaches the equilibrium no net change can be observed.

5 Equilibrium is a state in which there are no observable changes as time goes by. Chemical equilibrium is achieved when: the rates of the forward and reverse reactions are equal the concentrations of the reactants and products remain constant Chemical equilibrium N2O4 (g)N2O4 (g) NO 2 (g)

6 Physical equilibrium Phase Equilibrium Phase Equilibrium When a substance is changing phase an equilibrium between phases can be established When a substance is changing phase an equilibrium between phases can be established H2O (s) H2O (l) H2O (s) H2O (l)

7 Physical Equilibrium

8 Solution Equilibrium When a solution is saturated, if there is solid present, the solid solute and the solute in solution are at equilibrium. When a solution is saturated, if there is solid present, the solid solute and the solute in solution are at equilibrium.

9 The Concept of Equilibrium [A] = concentration of A [A] = concentration of A [B] = concentration of B [B] = concentration of B As the reaction progresses –[A] decreases to a constant, –[B] increases from zero to a constant. –When [A] and [B] are constant, equilibrium is achieved.

10 Le Chateliers Principle The French chemist Henri Le Chatelier ( ) studied how the equilibrium position shifts as a result of changing conditions

11 Le Chateliers Principle IF A STRESS (CHANGE) IS APPLIED TO A SYSTEM AT EQUILIBRIUM THE SYSTEM WILL REACT IN THE DIRECTION THAT COUNTERACTS THE STRESS. IF A STRESS (CHANGE) IS APPLIED TO A SYSTEM AT EQUILIBRIUM THE SYSTEM WILL REACT IN THE DIRECTION THAT COUNTERACTS THE STRESS. In other words : the system is a REBEL! In other words : the system is a REBEL!

12 Le Chateliers Principle What items did he consider to be stress on the equilibrium? Concentration Temperature Pressure Each of these will now be discussed in detail

13 Stress I - CONCENTRATION Case 1 Case 1 Increasing the concentration of reactants Increasing the concentration of reactants Consequence: more products will be formed. Consequence: more products will be formed. Equilibrium shifts to the right Equilibrium shifts to the right

14 Case 2 – Increasing concentration of products Consequence Consequence The system will react to decrease the concentration of products by forming more reactants. The system will react to decrease the concentration of products by forming more reactants. Equilibrium will shift to the left or the reactants. Equilibrium will shift to the left or the reactants.

15 Case 3: Decreasing the concentration of reactants Consequence Consequence The system will react to increase the concentration of reactants by using more products. The system will react to increase the concentration of reactants by using more products. Equilibrium will shift to the left of the reactants Equilibrium will shift to the left of the reactants

16 Case 4: Decreasing the concentration of products Consequence: more products will be formed. Consequence: more products will be formed. Equilibrium shifts to the right or the products will be favored. Equilibrium shifts to the right or the products will be favored. This is a way to make a reaction go to completion. As products form they are taken away! This is a way to make a reaction go to completion. As products form they are taken away!

17 Stress II - Temperature If temperature is increased the system will favor the reaction that lowers the temperature (absorbing heat) then the ENDOTHERMIC RECTION will be favored.If temperature is increased the system will favor the reaction that lowers the temperature (absorbing heat) then the ENDOTHERMIC RECTION will be favored. A decrease in temperature favors the exothermic reactionA decrease in temperature favors the exothermic reaction C + O 2 (g) CO 2 (g) kJC + O 2 (g) CO 2 (g) kJ

18 Increase in temperature favors endothermic reaction Increase in temperature favors endothermic reaction Decrease in temperature favors exothermic reaction Decrease in temperature favors exothermic reaction

19 ENDOTHERMIC APPARATUS Air conditioners absorb heat of a room and they LOWER the temperature of the room they are in. Air conditioners absorb heat of a room and they LOWER the temperature of the room they are in.

20 EXOTHERMIC APPARATUS Heaters are exothermic, they release heat into a room and increase the temperature of the room they are in. Heaters are exothermic, they release heat into a room and increase the temperature of the room they are in.

21 Stress III - Pressure – Changes in pressure will only effect gaseous equilibria. Increasing the pressure will favor side with the smaller volume N 2(g) + 3H 2(g) 2NH 3(g) this equilibrium shifts to the right with an increase in pressure because the product side occupies a smaller volume 4 mol of gas in reactants vs 2 in product

22 Effect of a Catalyst The addition of a catalyst changes the rate of both forward and reverse reactions equally. It causes the equilibrium to be established more quickly but it does not change the equilibrium concentrations. The addition of a catalyst changes the rate of both forward and reverse reactions equally. It causes the equilibrium to be established more quickly but it does not change the equilibrium concentrations.

23 Heat + 2NH3(g) N2(g) + 3H2(g) 1.- Change : increase in [N 2 ] 1.- Change : increase in [N 2 ] What is the effect on the concentration of What is the effect on the concentration of a. [ NH 3 ] a. [ NH 3 ] b. [ H 2 ] b. [ H 2 ]

24 Heat + 2NH3(g) N2(g) + 3H2(g) 2 Change: increase in temperature 2 Change: increase in temperature What is the effect on the concentration of What is the effect on the concentration of a. [ N 2 ] a. [ N 2 ] b. [ NH 3 ] b. [ NH 3 ]

25 Heat + 2NH3(g) N2(g) + 3H2(g) 3: Increase in pressure 3: Increase in pressure What is the effect on the What is the effect on the a. number of moles of N 2 a. number of moles of N 2 B. number of NH 3 B. number of NH 3

26 The Concept of Equilibrium Consider colorless frozen N 2 O 4. At room temperature, it decomposes to brown NO 2 : Consider colorless frozen N 2 O 4. At room temperature, it decomposes to brown NO 2 : N 2 O 4 (g) 2NO 2 (g). Clear brown At some time, the color stops changing and we have a mixture of N 2 O 4 and NO 2. Challenge question Is the forward reaction endo or exo?

27 The Concept of Equilibrium As the substance warms it begins to decompose: As the substance warms it begins to decompose: N 2 O 4 (g) 2NO 2 (g) When enough NO 2 is formed, it can react to form N 2 O 4 : When enough NO 2 is formed, it can react to form N 2 O 4 : 2NO 2 (g) N 2 O 4 (g). At equilibrium, as much N 2 O 4 reacts to form NO 2 as NO 2 reacts to re-form N 2 O 4 At equilibrium, as much N 2 O 4 reacts to form NO 2 as NO 2 reacts to re-form N 2 O 4 The double arrow implies the process is dynamic. The double arrow implies the process is dynamic.

28 Smog…

29 Demonstration- Do now Copy reaction in your notes and predict what would be the consequence of each change. Then record your observations Co(H 2 O) Cl - CoCl H 2 O Co(H 2 O) Cl - CoCl H 2 O Pink Blue Pink Blue Change 1 – Increase [Cl - ] by adding HCl Change 1 – Increase [Cl - ] by adding HCl Change 2- Decrease [Cl - ] by adding AgNO 3 Change 2- Decrease [Cl - ] by adding AgNO 3

30 Co(H 2 O) Cl - CoCl H 2 O Pink Blue ndothermic Reaction! ndothermic Reaction! Change 3 – Increasing temperature Change 3 – Increasing temperature Consequence Consequence Change 4 – Decreasing temperature Change 4 – Decreasing temperature Consequence Consequence

31 Answers to MC questions handout Le Chatelier (8741) 1 B 2 B 3 D 4 B 5 B 6 C 7 A 8 D 9 B 10 B

32 SAMPLE ANSWERS TO LE CHATELIERS PRINCIPLE 1 Equilibrium shifts toward the fewer number of moles of gas. Or The reaction shifts to the side with the smaller volume. 2. Removing CO 2 shifts the equilibrium towards the right lowering [ H 2 CO 3 ]

33 3.-An increase in Temperature favors the endothermic reaction, which produces SO A higher [0 2 ] causes more collisions, more effective collisions form more product, decreasing [SO 2 ] 6.- As T increases the solubility of the gas decreases, the gas comes out the soda.

34 7. As the pressure decreases the solubility decrease, and the gas comes out of the soda. 8.- Increasing temperature favors the endothermic reaction. 9.- The rate of dissolving KNO 3 is equal to the rate of recrystallizing KNO 3

35 10. a) If T increases N 2 increases because the endothermic reaction will be favored. b) If pressure is increased H 2 will decreased, because the equilibrium moves to the side with the smaller volume. C) No effect because the catalyst does not affect the equilibrium position. It makes both (forward and reverse reaction ) faster.

36 Review Book Answers P146 18) 2 19) 2 20) 4 21) 2 22) 3 23) 4 24) 3 25) 4 26) 1 27) 3 28) 1 29) 1 30) 3 31) 4 32) 1 33) 3 34) 3 35) 4

37 Review Book p Entropy 36) 1 37) 1 38) 2 39) 4 40) 2 41) 2 42) 2 43) 2 44) 3

38 ENDOTHERMIC REACTION NH 4 SCN(s) + Ba(OH) 2 (s) NH 3 (g)+ H 2 O(l) +Ba 2 + (aq)+NH 4 + (aq) From solid to liquid entropy increases

39 Why does any change occur? Factors that determine if a physical or a chemical change would take place: Tendency to lower the energy of the system. Changes tend to occur if they result in a system that is more stable. Stability implies low energy.

40 1.-Energy factor - ENTHALPY All exothermic reactions are favorable in terms of energy, because their products have less energy than the reactants and therefore are more stable. Then if H < 0 the change is favorable in terms of energy Delta H = Heat of reaction = Enthalpy H

41 2.- Tendency to greater disorder. ENTROPY There is a tendency in nature to change to a state of greater randomness or disorder. Entropy is a measure of the disorder or randomness in a system. A system with high entropy is messy and has a lack of structure.

42 Entropy = Disorganization, chaos, randomness High entropy Randomness, messiness Low Entropy Organized, neat

43 Gas = high entropy Liquid = less entropy Solid = little entropy

44 Spontaneous change A change that occurs naturally. When a change results in an stable and more disorganized product the reaction will be spontaneous. The tendencies that favor a spontaneous reaction are Low enthalpy and high entropy.

45 Demonstration- write your observations in your notes Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) dark red Pale yellow dark red In your notebook predict what would happen to the equilibrium if we Decrease [Fe 3+ ]


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