Chapter 11 – Equilibrium

Chapter 11 – Equilibrium 11.1 Collision Theory Collision Theory: Molecules must collide with each other in order to react!  If the collision is successful, reactant bonds are broken and new bonds are formed; thus, a chemical reaction has occurred. Old bonds start to break and new bonds start to form. This is considered the transition state. Factors that affect the number of Successful Collisions 1) Collision frequency Increase the # of collisions  more chances for successful collisions to occur. 2) Collision Energy  For a reaction to occur, the molecules must collide with enough energy to break the existing reactant bonds and form new bonds.  Ea, the activation energy, is the minimum amount of energy needed for a reaction to occur. In order to react, molecules have to surmount this activation energy barrier. 3) Collision Geometry  Molecules must be in the correct orientation for a reaction to occur 11.2 Rates of Reaction Rate of Reaction is how fast the reactant molecules are turned into product molecules.  The speed at which the reaction occurs. How can we increase the rate of reaction? 1) Increase reactant concentration Increase concentration of a reactant  more particles are present, which collide more frequently, so the reaction rate increases. 2) Increase the temperature Increase T causes molecules to move faster and collide more frequently. Increase T increases the energy of the reactants, so the molecules collide with more energy.  There are a higher number of successful collisions, so the reaction rate increases. 3) Add a Catalyst - a catalyst is a substance that speeds up a reaction without being consumed.  A catalyst increases the number of successful collisions by providing a more favorable collision geometry that creates an alternative path with lower activation energy. page 1 11.3 Equilibrium Equilibrium is defined as the point a reaction reaches when the rate of the forward reaction equals the rate of the reverse reaction. A + B FORWARD REVERSE C + D  Forward Reaction: Reactant molecules A + B becoming product molecules C + D.  Reverse Reaction: As product molecules C + D start to accumulate, they can react and move in the reverse direction, which allows them to be converted back to reactant molecules A + B. 11.4 Exothermic and Endothermic Reaction Profiles A reaction profile shows the energy of reactants and products during a reaction. TS transition state: arrangement of atoms at the top of the energy barrier, where reactant bonds are breaking and new product bonds are being formed. Very high energy and unstable species. Ea, Activation Energy: energy required for reactant molecules to achieve the transition state. H, the heat of reaction: the difference in energy between the reactants and products. Endothermic reaction: reactants absorb heat from the surroundings, heat is a reactant. Reactants + heat  Products Exothermic reaction: reactants release heat to the surroundings, heat is a product. Reactants  Products + heat  Reaction profile for endothermic reaction: page 2  Reaction profile for exothermic reaction:  Reaction profile for a catalyzed reaction: Ea is lower for catalyzed reaction  CHAPTER 11 PRACTICE PROBLEMS 1. State three things that can increase the rate of a chemical reaction: a. _______________________ b. _______________________ c. _______________________ 2. Indicate if the following statements are true or false: T/F a. In an exothermic reaction, heat is a reactant. T/F b. In an endothermic reaction, the heat of the products is higher than the heat of reactants. T/F c. When a reaction reaches equilibrium, there is no net change in concentration of reactants and products. T/F d. The addition of a catalyst lowers the overall heat of reaction, H. T/F e. The transition state occurs when the reaction is over and only products are left. page 3 3. Draw an endothermic reaction profile. Label the x and y-axis, the transitions state, the reactants, products, Ea, and H. Answers to Practice Problems 1. State three things that can increase the rate of a chemical reaction: a. Increasing Concentration b. Increasing Temperature c. Addition of a Catalyst 2. Indicate if the following statements are true or false: False a. In an exothermic reaction, heat is a reactant. Heat is released so heat is a product. True b. In an endothermic reaction, the heat of the products is higher than the heat of reactants. True c. When a reaction reaches equilibrium, there is no net change in concentration of reactants and products. The reaction does not stop, the product and reactant concentrations are constantly increasing and decreasing at the same rate so there is no NET change. False d. The addition of a catalyst lowers the overall heat of reaction, H. The addition of a catalyst lowers the activation energy, Ea. False e. The transition state occurs when the reaction is over and only products are left. The transition state is at the top of the energy barrier. Old bonds are breaking and new bonds are forming simultaneously! 3. Draw an endothermic reaction profile. Label the x and y-axis, the transitions state, the reactants, products, Ea, and H. page 4