AS Chemistry 3.1.6 Chemical Equilibria PDF

AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA DYNAMIC EQUILIBRIA Some chemical reactions are reversible. We refer to these reactions as being in a dynamic equilibrium and are identified by the use of a ⇌ symbol in the equa...

AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA DYNAMIC EQUILIBRIA Some chemical reactions are reversible. We refer to these reactions as being in a dynamic equilibrium and are identified by the use of a ⇌ symbol in the equation. Forward Reaction e.g. A + B ⇌ C + D Reverse Reaction Initially, the reactants (A + B) react together to form the products (C + D). As the reaction proceeds, the products (C + D) also react together to form the reactants (A + B)! Since the reaction works in both directions, the reaction does not go to completion (does not end!), hence why it is called “dynamic”. An equilibrium is reached when the rate of the forward reaction is equal to the rate of the reverse reaction. In an equilibrium, the relative concentrations of the reactants and products remain constant, so long as the system is “closed” (i.e. no reactants or products can escape). So, all in all, a dynamic equilibrium is when… the rate of the forward reaction = the rate of the reverse reaction and the relative concentrations the reactants and products remain constant. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA THE POSITION OF THE EQUILIBRIUM Every dynamic equilibrium reaches a natural position for its equilibrium. In an equilibrium, the relative concentrations of the reactants and products remain constant, so long as the system is “closed” (i.e. no reactants or products can escape). Initially, the concentrations of reactants decrease and the concentrations of the products increase. In ten graphs below, you can see that an equilibrium is reached when their concentrations remain constant (the graph levels off). Equilibrium lies to the LEFT Equilibrium lies to the RIGHT Equilibrium Equilibrium [concentration] [concentration] Reaction Reaction A + B ⇌ C + D A + B ⇌ C + D More reactants than More products than products reactants Equilibrium lies in the MIDDLE Equilibrium [concentration] Reaction A + B ⇌ C + D Reactants and products in equal concentrations AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA le CHATELIER’S PRINCIPLE Whilst every dynamic equilibrium has a natural position for its equilibrium, it is possible to affect this position by changing the conditions in which it is in. This is useful if we wish to increase the amount of a desirable product in the reaction. Le Chatelier’s principle is the underlying theory behind how and why this is possible: e.g. concentration, No reactants or products pressure, or temperature can escape “If the conditions of a dynamic equilibrium in a closed system are changed, the equilibrium shifts to oppose that change” LEFT: to favour the reactants OR RIGHT: to favour the products i.e If a concentration is increased, it shifts to decrease it If the temperature is increased, it shifts to decrease it If the pressure is increased, it shifts to decrease it …and vice versa AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA CHANGING CONCENTRATION [ ] e.g. the Haber process N2(g) + 3H2(g) ⇌ 2NH3(g) The equilibrium will shift to the RIGHT if: a) [Reactant] is increased OR N2(g) + 3H2(g) ⇌ 2NH3(g) b) [Product] is decreased As a consequence we say that the equilibrium yield of product increases. The equilibrium will shift to the LEFT if: a) [Product] is increased OR N2(g) + 3H2(g) ⇌ 2NH3(g) b) [Reactant] is decreased As a consequence we say that the equilibrium yield of product decreases. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA CHANGING TEMPERATURE How a change in temperature affects the position of equilibrium is dependant on the reactions ΔH. ΔH is the enthalpy change of a reaction (more on this in Unit 2). With respect to equilibria, we need to pay attention to the sign of ΔH: +ΔH -ΔH Endothermic reaction Exothermic reaction Absorbs heat energy from the Releases heat energy to the surroundings surroundings Temperature goes down Temperature goes up If the ΔH for the reaction is given, it refers to the forward reaction. ΔH is negative, so the forward reaction is exothermic. e.g. the Haber process EXO N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -92kJ.mol-1 ENDO The ΔH for the reverse reaction is equal and opposite to that of the forward reaction! An increase in temperature favours the ENDOthermic reaction. Since endothermic reactions absorb heat energy from the surroundings, it opposes the increase in temperature. A decrease in temperature favours the EXOthermic reaction. Since exothermic reactions release heat energy to the surroundings, it opposes the decrease in temperature. If a reaction has a ΔH of 0 (zero), changes in temperature have no effect on the position of the equilibrium. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA CHANGING PRESSURE The position of an equilibrium is only affected by changes in pressure if the equilibrium contains at least one reactant or product in the gaseous (g) state. How the equilibrium shifts is dependant on the relative number of moles of gas on either side of the equation. e.g. the Haber process N2(g) + 3H2(g) ⇌ 2NH3(g) 4 MOLES 2 MOLES An increase in pressure favours the side with the FEWEST moles of gas. Since fewer moles of gas have a lower pressure, it opposes the increase. A decrease in pressure favours the side with the GREATEST moles of gas. Since more moles of gas have a higher pressure, it opposes the decrease. If the number of moles of gas on either side of the equation are equal, changes in pressure have no effect on the position of the equilibrium. ADDING A CATALYST Catalysts increase the rate of a chemical reaction. In dynamic equilibria, the rate of the forward reaction = the rate of the reverse reaction. When a catalyst is added, it causes the rate of both reactions to increase equally, so the position of the equilibrium is not affected. Adding a catalyst only causes an equilibrium to be established more quickly. How To Use le Chatelier’s to Answer Equilibria Questions AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA THE EQUILIBRIUM CONSTANT - Kc The Equilibrium Constant, Kc, is a quantitative measure of the position of an equilibrium using concentration (that’s what the “c” stands for). To calculate it, we need: 1. A balanced equation for the reaction 2. The equilibrium concentrations of all reactants and products e.g. aA + bB ⇌ cC + dD c [C] [D] d [A] = the equilibrium concentration (mol.dm-3) Kc = a b a = no. of moles from the equation [A] [B] Kc is essentially a measure of the ratio between the number of moles of products and reactants present in equilibrium. In this respect, the value of Kc can provide a quantitative idea about where the position of the equilibrium lies: If Kc > 1.0 The equilibrium lies to the right (favours the products). The greater the value the further to right it lies. If Kc = 1.0 The equilibrium lies perfectly in the middle. If Kc < 1.0 The equilibrium lies to the left (favours the reactants). The smaller the value the further to left it lies. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.6 CHEMICAL EQUILIBRIA HINTS | TIPS | HACKS Only changes in temperature affect the value of Kc for an equilibrium! If temperature changes, check in which direction the equilibrium will shift using le Chatelier’s Principle. If it shift to the right, the value of Kc will increase. If it shift to the left, the value of Kc will decrease. Be prepared to rearrange the Kc expression to find the equilibrium concentration of a reactant or product when given a value for Kc. How To Calculate Kc & How To Deduce the Rearrange the Expression Units of Kc AQA www.chemistrycoach.co.uk © scidekick ltd 2024