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Chemical Reactions and Potential Energy (PE) Chemical reactions involve the breaking and forming of chemical bonds. • • Endothermic reactions require energy (ex: bond breaking) Exothermic reactions release energy (ex: bond forming) Potential energy is energy of position or the energy possessed by something due to its position relative to something else. Potential energy is abbreviated EP or PE. It is measured in Joules (J) or kiloJoules (kJ). For any chemical reaction, there is always a net change in chemical potential energy - it is either lost or gained as a result of bond breaking and bond forming. Kinetic energy is energy of motion. In chemistry, we are interested in the motion of atoms, ions and molecules and the relationship between kinetic energy and potential energy. Kinetic energy is abbreviated Ek or KE. Temperature is an indicator of kinetic energy. The higher the temperature, the higher the KE. In many circumstances, only some of the particles in a sample have enough kinetic energy to collide hard enough to cause bond breaking. This critical kinetic energy level is called the threshold energy. The high energy molecules are the ones that possess enough kinetic energy to result in the breaking of old bonds when they collide. The other molecules will just bounce off each other when they collide. 1 Heat of Reaction ( ∆H) Can be written 2 ways: 1) Written next to the equation > Positive number for endothermic reactions (heat is gained) ex: > Negative number for exothermic reactions (heat is lost) ex: 2) Written as part of the equation > Written as a reactant for endothermic reactions (heat is absorbed) ex: > Written as a product for exothermic reactions ( heat is produced/given off) ex: Potential Energy Diagrams Potential Energy (PE) diagrams illustrate the potential energy changes that occur as reactants become products during a chemical change. It has five distinct regions: 1. The PE of the reactants 2. The PE gain that must take place in order for old bonds to be stretched to the breaking point. This is called the activation energy. 3. The PE of the transition state. This is the activated complex. An activated complex is a species formed at the transition point of a collision. The species is highly unstable - it can become reactants again or turn into products. 4. The PE released as new bonds form during a chemical change. This is the∆H. 5. The PE of the products 2 Important Notes: 1. Reactions with low activation energy are fast, while those with high activation energy are slow. 2. The higher the activation energy, the slower the rate of the reaction. If the potential energy of the reactants is greater than that of the products, the reaction is 3. exothermic - it results in the net release of potential energy as heat. If the potential energy of the reactants is less than that of the products, the reaction is 4. endothermic - it results in the net gain of energy from some external source (e.g. the sun) which is then stored in the products. For exothermic reactions, the potential energy of the reactants is always greater than the 5. potential energy of the products. The potential energy difference between the reactants and products is the heat of reaction (∆H - negative). For endothermic reactions, the potential energy of the reactants is always less than the potential 6. energy of the products. The potential energy difference between the reactants and products is the heat of reaction (∆H - postive). For exothermic reactions, the larger the value of ∆H, the more stable the products are and the 7. lower the likelihood of the reverse reaction occurring. This is because greater ∆H for the forward reaction means a larger activation energy for the reverse reaction. 3 Reversibility of Chemical Reactions Many chemical reactions are reversible. For example, hydrogen and oxygen gas can react to form water The formation of water is exothermic, 286 kJ of heat is released per mole of water formed. By convention, the sign of this value is negative, -286 kJ. The decomposition of water is endothermic, 286 kJ of heat is absorbed per mole of water broken down into hydrogen and oxygen gas. By convention, the sign of this value is positive, +286 kJ. If the sign of the forward reaction is negative, then the sign of the reverse reaction is positive (and vice versa). Given two of the three labelled values in the diagram above, you can calculate the third. 4 Sample Exercise 3 The heat of reaction for the formation of water is -286 kJ. Assume that the activation energy for the formation of water from hydrogen and oxygen gas is 112 kJ. Calculate the activation energy for the decomposition of water (i.e. the reverse reaction) using the information provided in this diagram. 5