3 Objectives Reversible reactions Equilibrium – classification Le Chatelier’s Principle
4 When the system reaches the equilibrium no net change can be observed. Reversible ReactionsSome 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: 2SO2(g) + O2(g) ↔ 2SO3(g)When the system reaches the equilibrium no net change can be observed.
5 the rates of the forward and reverse reactions are equal 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 equalthe concentrations of the reactants and products remain constantChemical equilibriumN2O4 (g)2NO2 (g)14.1
6 Physical equilibrium Phase Equilibrium When a substance is changing phase an equilibrium between phases can be establishedH2O (s) H2O (l)
8 Solution EquilibriumWhen 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[B] = concentration of BAs 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 Chatelier’s Principle The French chemist Henri Le Chatelier ( ) studied how the equilibrium position shifts as a result of changing conditions
11 Le Chatelier’s Principle 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!
12 Le Chatelier’s Principle What items did he consider to be stress on the equilibrium?ConcentrationTemperaturePressureEach of these will now be discussed in detail
13 Stress I - CONCENTRATION Case 1Increasing the concentration of reactantsConsequence: more products will be formed.Equilibrium shifts to the right
14 Case 2 – Increasing concentration of products ConsequenceThe system will react to decrease the concentration of products by forming more reactants.Equilibrium will shift to the left or the reactants.
15 Case 3: Decreasing the concentration of reactants ConsequenceThe system will react to increase the concentration of reactants by using more products.Equilibrium will shift to the left of the reactants
16 Case 4: Decreasing the concentration of products Consequence: more products will be formed.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!
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.A decrease in temperature favors the exothermic reactionC + O2(g) → CO2(g) kJ
18 Increase in temperature favors endothermic 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.
20 EXOTHERMIC APPARATUSHeaters 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 volumeN2(g) + 3H2(g) ↔ 2NH3(g)this equilibrium shifts to the right with an increase in pressure because the product side occupies a smaller volume4 mol of gas in reactants vs 2 in product
22 Effect of a CatalystThe 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 [N2]What is the effect on the concentration ofa. [ NH3 ]b. [ H2 ]
24 Heat + 2NH3(g) N2(g) + 3H2(g) 2 Change: increase in temperatureWhat is the effect on the concentration ofa. [ N2 ]b. [ NH3]
25 Heat + 2NH3(g) N2(g) + 3H2(g) 3: Increase in pressureWhat is the effect on thea. number of moles of N2B. number of NH3
26 Is the forward reaction endo or exo? The Concept of EquilibriumConsider colorless frozen N2O4. At room temperature, it decomposes to brown NO2:N2O4(g) <- > 2NO2(g).Clear brownAt some time, the color stops changing and we have a mixture of N2O4 and NO2.Challenge questionIs the forward reaction endo or exo?
27 The Concept of Equilibrium As the substance warms it begins to decompose:N2O4(g) 2NO2(g)When enough NO2 is formed, it can react to form N2O4:2NO2(g) N2O4(g).At equilibrium, as much N2O4 reacts to form NO2 as NO2 reacts to re-form N2O4The double arrow implies the process is dynamic.
29 Demonstration- Do now Copy reaction in your notes and predict what would be the consequence of each change. Then record your observationsCo(H2O) Cl- <--> CoCl H2OPink BlueChange 1 – Increase [Cl-] by adding HClChange 2- Decrease [Cl-] by adding AgNO3
31 Answers to MC questions handout Le Chatelier (8741) 1 B2 B3 D4 B5 B6 C7 A8 D9 B10 B
32 SAMPLE ANSWERS TO LE CHATELIER’S PRINCIPLE 1 Equilibrium shifts toward the fewer number of moles of gas . OrThe reaction shifts to the side with the smaller volume.2. Removing CO2 shifts the equilibrium towards the right lowering [ H2CO3 ]
33 3.-An increase in Temperature favors the endothermic reaction, which produces SO2 4.- A higher  causes more collisions, more effective collisions form more product, decreasing [SO2]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 KNO3 is equal to the rate of recrystallizing KNO3
35 10. a) If T increases N2 increases because the endothermic reaction will be favored. b) If pressure is increased H2 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.
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 D H < 0 the change is favorable in terms of energyDelta H = Heat of reaction = EnthalpyD 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.
43 Gas = high entropyLiquid = less entropySolid = 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 areLow enthalpy and high entropy.
45 Demonstration- write your observations in your notes Fe3+ (aq) + SCN- (aq) <--> FeSCN2+ (aq)Pale yellow dark redIn your notebook predict what would happen to the equilibrium if we Decrease [Fe3+]