General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 6.9 Energy Changes in Chemical Reactions Chapter 6 Chemical Reactions and Quantities © 2013 Pearson Education, Inc. Lectures

© 2013 Pearson Education, Inc. Chapter 6, Section 9 2 Reaction Conditions There are three conditions required for a chemical reaction to occur. 1. Collision: The reactants must collide. 2. Orientation: The reactants must align properly to. break and form bonds. 3. Energy: The collision must provide the energy of. activation.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 3 Activation Energy  Activation energy is the amount of energy required to break the bonds between atoms of the reactants.  If the energy of a collision is less than the activation energy, the molecules bounce apart without reacting.  Many collisions occur, but only a few actually lead to the formation of product.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 4 Activation Energy The activation energy is the energy needed to convert reacting molecules into products.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 5 Heat of Reaction The heat of reaction  is the amount of heat absorbed or released during a reaction.  is the difference between the energy of breaking bonds in the reactants and forming bonds in the products.  has the symbol ΔH. ΔH = ΔH products − ΔH reactants

© 2013 Pearson Education, Inc. Chapter 6, Section 9 6 Exothermic Reactions In an exothermic reaction,  the energy of the reactants is greater than that of the products and heat is released along with the products.  the heat of reaction (ΔH) value is written with a negative sign (–) indicating heat is released.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 7 Endothermic Reactions In an endothermic reaction,  the energy of the reactants is lower than that of the energy of the products.  heat is absorbed and used to convert the reactants to products and written with a (+) sign.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 8 Heat of Reaction Summary Reaction Energy Heat Sign of TypeChange in Reaction ΔH Endothermic Heat absorbed Reactant side + Exothermic Heat released Product side –

© 2013 Pearson Education, Inc. Chapter 6, Section 9 9 Learning Check Identify each of the following reactions as exothermic or endothermic.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 10 Solution Identify each of the following reactions as exothermic or endothermic. Heat is a product. The reaction is exothermic. Heat is a reactant. The reaction is endothermic. Heat is a product. The reaction is exothermic.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 11 Guide to Calculations Using Heat of Reaction (  H)

© 2013 Pearson Education, Inc. Chapter 6, Section 9 12 Calculating the Heat in a Reaction If 15.0 g of NO are produced according to the following reaction, how many kJ were absorbed? Step 1 State the given and needed quantities. Analyze the Problem.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 13 Calculating the Heat in a Reaction If 15.0 g of NO are produced according to the following reaction, how many kJ were absorbed? Step 2 Write a plan using heat of reaction and any molar mass needed. g of NO molar moles of NO heat of kilojoules mass reaction molar mass Heat of reaction

© 2013 Pearson Education, Inc. Chapter 6, Section 9 14 Calculating the Heat in a Reaction If 15.0 g of NO are produced according to the following reaction, how many kJ were absorbed? Step 3 Write the conversion factors including heat of reaction.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 15 Calculating the Heat in a Reaction If 15.0 g of NO are produced according to the following reaction, how many kJ were absorbed? Step 4 Set up the problem to calculate the heat.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 16 Learning Check How many grams of O 2 reacted if 306 kJ were released in the following reaction? A g of O 2 B g of O 2 C g of O 2

© 2013 Pearson Education, Inc. Chapter 6, Section 9 17 Solution How many grams of O 2 reacted if 306 kJ were released in the following reaction? Step 1 State the given and needed quantities. Analyze the Problem.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 18 Solution How many grams of O 2 reacted if 306 kJ were released in the following reaction? Step 2 Write a plan using heat of the reaction and any molar mass needed. kilojoules heat of moles of O 2 molar grams of O 2 molar mass Heat of reaction

© 2013 Pearson Education, Inc. Chapter 6, Section 9 19 Solution How many grams of O 2 reacted if 306 kJ were released in the following reaction? Step 3 Write the conversion factors including heat of reaction.

© 2013 Pearson Education, Inc. Chapter 6, Section 9 20 Solution How many grams of O 2 reacted if 306 kJ were released in the following reaction? Step 4 Set up the problem to calculate mass of O 2. Answer is A.