SCH4U UNIT 4 - CHEMICAL SYSTEMS AND EQUILIBRIUM QUESTIONS AND ANSWERS ALREADY PASSED LATEST UPDATE

SCH4U UNIT 4 - CHEMICAL SYSTEMS AND EQUILIBRIUM QUESTIONS AND ANSWERS ALREADY PASSED LATEST UPDATE Define chemical equilibrium - Answers The state where the concentrations of all reactants and products remain constant with time Why are chemical equilibriums used? - Answers Because not all reactions proceed to completion What does an equilibrium position that lies far to the right indicate? - Answers It favours the products of the system and the reaction seems to react to completion What does an equilibrium position that lies far to the left indicate? - Answers It favours the reactants of the system and the reaction is virtually undetectable Why doesn't a reaction halt when equilibrium is reached? - Answers Because at equilibrium, the forward reaction rate is equal to the reverse reaction rate, meaning there is constant movement but no overall net change Define a reversible reaction - Answers Reaction which consists of two reactions occurring at the same time In a reversible reaction, what is the forward reaction? - Answers When reactants turn into products In a reversible reaction, what is the reverse reaction? - Answers When products form the reactants What is the term used for when the reactions in a reversible reaction eventually reach the same reaction rate? - Answers Dynamic equilibrium; there is no overall net change but the individual particles continue to react in both directions Describe the relationship between equilibrium positions and the equilibrium constant - Answers For a reaction, there is only one equilibrium constant at a particular temperature but there are an infinite number of equilibrium positions (set of equilibrium concentrations) State the Equilibrium Law Expression - Answers for a reaction aW + bX -- cY + dZ, Keq = ([Y]c[Z]d)/([W]a[X]b) Keq = products / reactants The equilibrium constant is only affected by a change in temperature, as a change in the concentration of the products or reactant only leads to a shift in equilibrium as stated in Le Chatelier's Principle. List the significance of the equilibrium constant with a particular value - Answers 1. if Keq 1, the products are favoured in the reaction 2. if Keq 1, the reactants are favoured in the reaction 3. if Keq = 1, the concentrations of both products and reactants are similar (which occurs in most reactions) Define a homogenous equilibrium - Answers All products and reactants are in the same phase Define a heterogenous equilibrium - Answers Reactants and products are in different phases (e.g liquid and solid) Why are solids and liquids omitted in the equilibrium expression / solubility product constant? - Answers Because the density of solids and liquids, contrary to gases, remains constant no matter what State Le Chatelier's principle - Answers If a system at equilibrium is subject to stress, the equilibrium is shifted in the direction that it tends to relieve the stress Define stress in terms of Le Chatelier's principle - Answers Change in a condition of the equilibrium (e.g. changes in concentration of products/reactants, temperature, pressure in gas reactions, etc.) How does pressure affect the equilibrium of a reaction? - Answers Increase in pressure = favours side with more moles = shift to side with fewer moles Decrease in pressure = favours side with fewer moles = shift to the side with more moles How do concentration changes affect the equilibrium? - Answers The system will shift away from the side the addition occured 1. More products = shift to reactants (left) 2. More reactants = shift to the products The system will shift towards the side where a removal occurred 1. Product removed = shift to side of product 2. Reactant removed = shift to side of reactant How does temperature affect the equilibrium? - Answers Endothermic reaction: 1. increasing temperature = increase in available energy on side of reactants = equilibrium shifts to the right = forming more products to restore equilibrium 2. decreasing temperature = less energy available on the reactant side = equilibrium shifts to the left = more reactants are formed to restore equilibrium Exothermic reaction: 1. increasing temperature = equilibrium position will move to lower temperature = system shifts to the left (reactants) to restore equilibrium 2. decreasing temperature = equilibrium position will move to higher temperature = system shifts to the right (products) to restore equilibrium == Increasing the temperature favours the endothermic (often reverse) reaction, whereas decreasing the temperature favours the exothermic reaction; change is counteracted. Which variable does not have an effect on the (dynamic) equilibrium of a reaction? - Answers Adding a catalyst has no effect on the equilibrium of a chemical reaction because a catalyst speeds up both the forward and reverse reactions to the same extent, thus cancelling out its overall effect on the equilibrium What is calculated when the system has reached equilibrium? - Answers The equilibrium constant (Keq) What is calculated when the system has not yet reached the equilibrium / it's value is unknown? - Answers The reaction quotient Q