Energy Diagram Endothermic Reaction Draw the energy diagram for exothermic and endothermic reactions. Label each part.

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1 CP Chapter 18 Notes A Model for Reaction Rates Expressing Reaction Rates Average Rate = Δquantity Δtime The amount of increase or decrease depends on their mole ratios Units = or mol/ls Expressing Reaction Rates Cont. As a reaction proceeds, there is a decrease in concentration of and an increase in the concentration of the products. N 2 + 3H 2 à 2NH 3 N 2 and H 2 decrease in concentration over time, while NH 3 increases in concentration over time. Expressing Reaction Rates Average reaction rate Concentration M 2 - Concentration M 1 t 2 - t 1 Reaction rates must always be If you get a negative reaction rate, change the sign to positive. The Collision Theory Summary 1. Reacting substances (atoms, ions, or molecules) must. 2. Reacting substances must collide with the correct. 3. Reacting substances must collide with sufficient to form the activated complex. Activation Energy The amount of energy that reacting particles must have to form the activated complex and lead to a reaction Symbol: influence on the rate of a reaction Energy Diagram Endothermic Reaction Draw the energy diagram for exothermic and endothermic reactions. Label each part. exothermic endothermic 1

2 Calculating Average Reaction Rates Given H 2 + Cl 2 à 2HCl Time (s) [H 2 ] (M) [Cl 2 ] (M) [HCl] (M) Calculate the average reaction rate expressed in moles H 2 consumed per liter per second. Calculate the average reaction rate expressed in moles Cl 2 consumed per liter per second. Reaction Rates An expression that relates rate of reaction and concentration Rate of reaction on reactant concentrations Never includes Given: 2A + 3B 2C o In general, rate = k [A] X [B] Y Rate Constant Symbol: A constant specific and for every reaction If k is large, products form If k is small, products form slowly Order of reaction in rate law (x and y) Determine how much the rate depends on the respective concentration(s) Can ONLY be determined by Overall order of a reaction is the sum of all orders (x + y) Example 1: 2A + 3B 2C Skeleton rate: Overall order of reaction (x + y) o If x=1, o If x=2. o Overall order + = Example 2 Write the skeleton rate law for 2H 2 O(l) 2H 2 (g) + O 2 (g) 2

3 Relationship of order and rate Example 1: rate = k [A] o If concentration of A doubles, rate o If concentration of A triples, rate o If concentration of A is halved, rate Example 2: rate = k [A] 2 o If concentration of A doubles, rate o If concentration of A triples, rate is times as fast Example 3: rate = k [A] 0 o Recall: anything raised to the 0 th power = o If the concentration of A changes in any way happens to the rate o Rate is not dependent on concentration o Rate = k Factors Affecting Reaction Rates Factors Affecting Reaction Rates There are several factors affect reaction rates o Concentration o (Particle size) o Temperature o Catalysts o Inhibitors Catalysts A catalyst is a substance that the rate of a chemical reaction itself being used up in the reaction. A catalysts provide an alternative energy pathway for the reaction. The different pathway the activation energy allowing more molecules to overcome the activation energy and produce products at a faster rate ΔH Inhibitor An inhibitor is a substance that down, or inhibits reaction rates. Uses o A preservative o A weed killer 3

4 Equilibrium: A State of Dynamic Balance Reversible Reactions When a reaction results in complete conversion of reactants to products chemists say it goes to. Not all reactions go to completion. They appear to stop because they are. Reversible reactions can occur in both the and direction. Reversible Reactions cont... N 2 + 3H 2 2NH 3 o In the forward reaction, the reactants are and ; the product is NH 3 o In the reverse reaction, the reactant is ; the products are N 2 + 3H 2 o The forward and reverse reactions are happening at the same time. Chemical Equilibrium Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal. o Rate forward reaction = Rate reverse reaction This does not mean the concentrations of the products and reactants are the Equilibrium is a state of action, not inaction. This process is dynamic; dynamic equilibrium. Equilibrium Expression and Constant (Keq) Law of chemical equilibrium states that at a given, a chemical system may reach a state in which a particular ratio of reactant and product concentrations has a value known as Keq or equilibrium constant. Equilibrium Constant Keq Keq is the ratio of concentrations to concentrations. Keq only includes compounds that are in the or aqueous state, not solid or liquid A Keq, Keq > 1 means the products are over the reactants A Keq, Keq < 1 means the reactants are favored over the. Homogenous vs. Heterogeneous Equilibrium Homogenous equilibrium means all reactants and products are in the physical state Ex: H 2 (g) + O 2 (g) D H 2 0(g) Heterogeneous equilibrium is when the reactants and products are in than one physical state. Ex: H 2 O(g) + C(s) D H 2 (g) + CO(g) Chemical Equilibrium - Keq 4

5 Homogenous Equilibria Given N 2 (g) + 3H 2 (g) 2NH 3 (g). Write the equilibrium constant expression for the reaction. Given 2SO 2 (g) + O 2 (g) 2SO 3 (g). Write the equilibrium constant expression for the reaction. Heterogeneous Equilibria Given 2NaHCO 3 (s) Na 2 CO 3 (s) + CO 2 (g) + H 2 O(g). Write the equilibrium constant expression. Calculating the Value of Equilibrium Constants At equilibrium and 100 o C a flask contains: [ PCl 5 ] = M [H 2 O ] = M [HCl] = M [POCl 3 ] = M Calculate the Keq for the reaction PCl 5 (g) + H 2 O (g) 2HCl (g) + POCl 3 (g). 5

6 Day 1: Le Chatelier s Principle Concentration & Temperature Day 1: Le Chatelier s Principle If a stress is applied to a system at equilibrium, the system shifts in the direction that the stress. Stresses include: o Change in o Change in volume/pressure o Change in Le Chatelier Change in Concentration PCl 5 (g) PCl 3 (g) + Cl 2 (g) ΔH = -47 kj Stress PCl 5 (g) PCl 3 (g) Cl 2 (g) [PCl 5 ] [Cl 2 ] [Cl 2 ] PRACTICE [PCl 5 ] [PCl 3 ] [PCl 3 ] TEMPERATURE temperature heat Day 2: Le Chatelier s Principle Volume/Pressure Volume and pressure are related. As the volume goes up the pressure goes and vice versa. (Boyles Law; P 1 V 1 =P 2 V 2 ) REVIEW FROM CH.14 Only are affected by a change in volume/pressure You need to count the number of of gas on each side of the reaction. 2H 2 O (g) D 2 H 2 (g) + O 2 (g) The left side has 2 molecules of gas, while the right side has 3 molecules of gas If the stress is pressure, shift toward the side with the least number of moles of gas. If the stress is pressure decreasing, shift toward the side with the number of moles of gas. Le Chatelier and Pressure - YouTube Change in Volume/Pressure PCl 5 (g) PCl 3 (g) + Cl 2 (g) Stress PCl 5 (g) PCl 3 (g) Cl 2 (g) pressure volume pressure volume PRACTICE CO(g) + 3H 2 (g) CH 4 (g) + H 2 O(g) Stress CO(g) H 2 (g) CH 4 (g) H 2 O(g) pressure volume N 2 O 4 (g) 2NO 2 (g) Stress N 2 O 4 (g) 2NO pressure volume 6