<p> Energy Changes in Chemical Reactions</p><p>Bond length : The distance between nuclei of two bonded atoms</p><p>Reactant: A starting material in a chemical reaction</p><p>Products: A substance formed in a chemical reaction</p><p>Exothermic reaction: Energy (in the form of heat and light) is released during a chemical reaction</p><p>Endothermic reaction: Energy is taken if from its surroundings</p><p>Activation Energy: The energy needed to get a reaction to start.</p><p>Activated Complex: The transition state between the reactant and product.</p><p>Enthalpy change ΔH: The amount of heat energy gained or lost by a reaction. </p><p> Enthalpy change or Heat of the reaction ΔH = energy of products – energy of reactants </p><p> For endothermic reactions, energy of the products is greater than the energy of the </p><p> reactants ΔH > 0 (positive value)</p><p> For exothermic reactions, energy of the products is less than the energy of the reactants</p><p>ΔH < 0 (negative value)</p><p>Bond strength: The greater the number of bonding electron pairs between a pair of atoms, the </p><p> shorter the bond length. (and stronger the bond) </p><p> atoms held together more tightly when there are multiple bonds e.g. N ≡ N</p><p>greater bond energy for molecules with multiple bonds</p><p>Bond Energy  The energy that must be supplied to a gaseous molecule to separate two of its atoms is called the bond dissociation energy or bond energy for short.</p><p> Bond energy represents energy supplied to the molecule from its surroundings.</p><p> The process of breaking bonds in a molecule is endothermic.</p><p> During a chemical reaction, the molecules of the reactants break up and the atoms rearrange themselves to form new molecules (the products). Page 1 of 5  The initial breaking of chemical bonds takes in energy. When new bonds are formed in the products, energy is released.</p><p> Bond energies tell us how much energy will be taken in (or given out) when a mole of particular bonds are broken (or formed).</p><p> Table of average bond energies: (These values might differ from one source to another)</p><p>Bond Energy (kJ/mol) H - H 436 C - H 414 O – H 460 C - C 347 C = O 799 O = O 499 N ≡ N 946</p><p>Examples</p><p>1. How much energy will be given out when one mole of O = O bonds are formed? ……………………..</p><p>2. How much energy will be taken in when 4 moles of O – H bonds are broken? ……………………………</p><p>3. How may C – H bonds occur in one methane molecule? ………………………</p><p>4. How much energy is required to break up one methane molecule? …………………………</p><p>5. Consider the formation of water molecules when hydrogen burns in oxygen.</p><p>2H2 + O2  2H2O</p><p>5.1 Ho w </p><p> many H – H bonds must be broken? …………………………</p><p>5.2 How many O = O double bonds must be broken? ………………………………..</p><p>5.3 How many O – H bonds are formed? ………………………………………………</p><p>Page 2 of 5 2H2 + O2  2H2O</p><p>5.4 How many kilojoules of energy will be needed to break up one mole of oxygen molecules? ………………………………….</p><p>5.5 How many kilojoules of energy will be needed to break up two moles of hydrogen molecules? ………………………………….</p><p>5.6 How many kilojoules of energy will be released when two moles of water molecules are formed? ………………………………..</p><p>5.7 Will the overall reaction be exothermic and endothermic? Why? ………………………………….. ……………………………………………………………………………………………………………… ………………………………………….. ……………………………………………………………………………………………………………… …………………………………………..</p><p>6. Consider the reaction between propane and oxygen (combustion of propane). When propane burns in oxygen or air, the products are carbon dioxide and water.</p><p>6.1 Balance the equation for this reaction:</p><p>……..C3H8 + …….O2  ……..CO2 + ……H2O</p><p>6.2 How many C – H bonds in propane need to be broken? ……………….</p><p>6.3 How much energy is needed to break the C – H bonds? …………………….</p><p>6.4 How many C – C bonds in propane need to be broken? ………………………………..</p><p>6.5 How much energy is needed to break the C – C bonds? …………………….</p><p>6.6 How many O = O bonds need to be broken in total? …………………………………</p><p>6.7 How much energy is needed to break the O = O bonds? …………………….</p><p>Page 3 of 5 6.8 How many carbon dioxide C = O bonds are formed in total? …………………………………………………</p><p>6.9 How much energy will be released when the C = O bonds form? ……………………………………..</p><p>6.10 How many water O – H bonds are formed in total? …………………………………………………</p><p>6.11 How much energy will be released when the O – H bonds form? ……………………………………..</p><p>6.12 How much energy in total is used in breaking bonds? ……………………………</p><p>6.13 How much energy is released when the new bonds are formed? …………………………..</p><p>6.14 Will the overall reaction be exothermic and endothermic? Why? ………………………………….. ……………………………………………………………………………………………………………………… ………………………………….. ……………………………………………………………………………………………………………………… …………………………………..</p><p>Energy profile of an exothermic reaction</p><p>Energy profile of an endothermic reaction</p><p>Page 4 of 5 Page 5 of 5</p>