Chapter 6 - Energy Changes Reaction Rates and Equilibrium

Chapter Six: Energy Changes, Reaction Rates, and Equilibrium Energy energy: the capacity to do work potential energy: stored energy kinetic energy: the energy of motion the law of conservation of energy states that the total energy in a system does not change; energy cannot be created or destroyed. a compound with lower potential energy is more stable than a compound with higher potential energy. reactions that form products having lower potential energy than the reactants are favored. breaking a chemical bond needs energy, and forming a bond releases energy. The Units of Energy a calorie (cal) is the amount of energy needed to raise the temperature of 1 g of water by 1° C. 1 g carbohydrate = 4 cal; 1 g protein = 4 cal, 1 g fat = 9 cal a joule (J) is another unit of energy. 1 cal = 4.184 J both joules and calories can be reported in the larger units kilojoules (kJ) and kilocalories (kcal). 1,000 J = 1 kJ 1,000 cal = 1 kcal 1 kcal = 4.184 kJ Energy Changes in Reactions when molecules come together and react, bonds are broken in the reactants and new bonds are formed in the products. bond breaking always requires an input of energy. endothermic to cleave this bond, 58 kcal/mol must be added. bond formation always releases energy. to form this bond, 58 kcal/mol is released. Bond Dissociation Energy exothermic ΔH is the energy absorbed or released in a reaction; it is called the heat of reaction or enthalpy change. when energy is absorbed, the reaction is said to be endothermic, and ΔH is positive. when energy is released, the reaction is said to be exothermic and ΔH is negative. the bond dissociation energy is the ΔH for breaking a covalent bond by equally dividing the electrons between the two atoms. bond dissociation energies are positive values because bond breaking is endothermic (ΔH > 0). bond formation always has negative values because bond formation is exothermic (ΔH < 0). the stronger the bond, the higher its bond dissociation energy. in comparing bonds formed from elements in the same group, bond dissociation energies generally decrease going down the column. Calculations Involving ΔH Values ΔH indicates the relative strength of the bonds broken and formed in a reaction. when ΔH is negative: • more energy is released in forming bonds than is needed to break the bonds. • the bonds formed in the products are stronger than the bonds broken in the reactants when ΔH is positive: • more energy is needed to break bonds than is released in the formation of new bonds. • the bonds broken in the reactants are stronger than the bonds formed in the products. example: using the given value of ΔH and the balanced equation for the reaction of Mg with H2O, how many kilocalories of energy are released when 4.00 g of Mg are used in this reaction? 1. convert 4.00 g Mg to mol 2. convert the mole number of Mg to the kilocalorie energy released Energy Diagrams for a reaction to occur, two molecules must collide with enough kinetic energy to break bonds. the orientation of the two molecules must be correct as well. Ea, energy of activation: the difference in energy between the reactants and the transition state. It is: • the minimum amount of energy that the reactants must possess for a reaction to occur • called the energy barrier, and the height of the barrier determines the reaction rate When the Ea is high, few molecules have enough energy to cross the energy barrier, and the reaction is slow. When the Ea is low, many molecules have enough energy to cross the energy barrier, and the reaction is fast. the difference in energy between the reactants and the products is the ΔH. if ΔH is negative, the reaction is exothermic: if ΔH is positive, the reaction is endothermic: How Concentration and Temperature Affect Reaction Rate increasing the concentration of the reactants: • increases the number of collisions • increases the reaction rate increasing the temperature of the reaction: • increases the average kinetic energy of the molecules • increases the reaction rate Catalysts catalyst: a substance that speeds up the rate of a reaction • recovered unchanged in a reaction and does not appear in the product catalysts accelerate a reaction by lowering Ea without affecting ΔH. the uncatalyzed reaction (higher Ea) is slower. the catalyzed reaction (lower Ea) is faster. ΔH is the same for both reactions. Focus on the Human Body: Biological Catalysts enzymes: biological catalysts (usually protein molecules) held together in a very specific three-dimensional shape the active site binds a reactant, which then undergoes a very specific reaction with an enhanced rate. the enzyme lactase converts the carbohydrate lactose into the two sugars glucose and galactose. people who lack adequate amounts of lactase suffer from intestinal issues; they cannot digest lactose when it is ingested. Equilibrium a reversible reaction can occur in either direction. a forward reaction proceeds to the right of a chemical equation, and the reverse reaction proceeds to the left. the system is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. the net concentrations of the reactants and products do not change at equilibrium. The Equilibrium Constant the relationship between the concentration of the products and the concentration of the reactants is the equilibrium constant, K. brackets, [ ], are used to symbolize concentration in moles per liter (mol/L), M. for the following balanced equation: the coefficient becomes the exponent. The Magnitude of the Equilibrium Constant when K is much greater than 1, equilibrium lies to the right and favors the products. when K is much less than 1, equilibrium lies to the left and favors the reactants. when K is around 1 (0.01 < K < 100), both reactants and products are present in similar amounts. for the reaction: the product is favored because K > 1; the equilibrium lies to the right. Calculating the Equilibrium Constant example: calculate K for the reaction between the general reactants A2 and B2. The equilibrium concentrations are as follows. the unit of concentration is always mol/L (or M). 1. write the expression for the equilibrium constant from the balanced equation. 2. substitute the given concentrations in the equilibrium expression and calculate K. since the units of the concentrations are always mol/L, they are omitted during calculation. Le Chatelier’s Principle if a chemical system at equilibrium is disturbed or stressed, the system will react in a direction that counteracts the disturbance or relieves the stress. some possible disturbances include: • concentration changes • temperature changes • pressure changes Concentration Changes what happens if [CO (g)] is increased? • the concentration of O2 (g) will decrease • the concentration of CO2 (g) will increase what happens if [CO2 (g)] is increased? • the concentration of CO (g) will increase • the concentration of O2 (g) will increase what happens if a product is removed? • the concentration of ethanol will decrease • the concentration of the other product (C2H4) will increase Temperature Changes when the temperature is increased, the reaction that absorbs heat is favored. an endothermic reaction absorbs heat, so increasing the temperature favors the forward reaction. an exothermic reaction releases heat, so increasing the temperature favors the reverse reaction. conversely, when the temperature is decreased, the reaction that adds heat is favored. Pressure Changes when pressure increases, equilibrium shifts in the direction that decreases the number of moles in order to decrease pressure. when the pressure decreases, equilibrium shifts in the direction that increases the number of moles in order to increase pressure.