Energy Changes & Reactions Unit 7 - Chapter 7.3

Chemical Bonds and Energy  Chemical reactions:  Breaking chemical bonds in the reactants  Forming new chemical bonds in the products.  Chemical energy: Energy stored in the chemical bonds of a substance.  True for both reactants and products

Chemical Bonds and Energy  Energy changes in reactions  determined by changes in chemical bonding.  Example: combustion of propane  C 3 H 8 + 5O 2  3CO 2 + 4H 2 O  Bonds in reactants are broken (C 3 H 8 and O 2 )  Propane molecules have C-C and C-H single bonds  Oxygen molecules have O=O double bonds  Bonds in products are formed (CO 2 and H 2 O)  Carbon dioxide molecules have C=O double bonds  Water molecules have H-O single bonds

Chemical Bonds and Energy  Breaking Bonds  In combustion of propane, all chemical bonds in both reactants (propane and oxygen) must be broken.  Breaking chemical bonds requires energy.  A spark provides enough energy to get the reaction started.

Chemical Bonds and Energy

 Forming Bonds: burning propane  C 3 H 8 + 5O 2  3CO 2 + 4H 2 O  Each molecule of propane burned forms:  3 molecules of carbon dioxide  6 C=O (double) bonds formed  4 molecules of water.  8 O-H (single) bonds formed  Forming of chemical bonds releases energy.  Heat and light given off because new chemical bonds formed

Chemical Bonds and Energy  Exothermic and Endothermic Reactions  During a chemical reaction, energy is either released or absorbed.  Exothermic Reaction – Activity 3  Releases energy to its surroundings  “Exo” means Energy “exits” from the reaction  Energy required to break reactant bonds  less than energy released as products formed

Chemical Bonds and Energy  Exothermic reaction: chemical energy of reactants is greater than chemical energy of products.

Chemical Bonds and Energy  Chemical energy peaks before reactants change into products.  Peak Height = energy required to break the chemical bonds of the reactants.  Particles must collide with enough energy to break these bonds, or the reaction will not occur.

Chemical Bonds and Energy  Combustion is an extremely exothermic reaction.  Example: 1 mole of propane + 5 moles of oxygen  2220 kJ (kilojoules) of heat is released.  C 3 H 8 + 5O 2  3CO 2 + 4H 2 O kJ  1 joule = 1watt for 1 second  100 watt light bulb for 10 seconds needs 1 kJ  44g propane releases enough energy for >6 hrs

Chemical Bonds and Energy  Endothermic Reaction – Activity 1 & 2  Absorbs energy from surroundings  “Endo” means Energy goes “into” the reaction  Energy required to break reactant bonds  greater than energy released by formation of products.

Chemical Bonds and Energy  Endothermic reaction - energy of products is greater than energy of reactants.

Chemical Bonds and Energy  Endothermic Reaction  Example: Emergency Cold Packs  Liquid inside the cold pack is water.  In the water is another plastic bag or tube containing ammonium-nitrate fertilizer.  When you hit the cold pack, it breaks the tube so that the water mixes with the fertilizer.  This mixture creates an endothermic reaction -- it absorbs heat.  The temperature of the solution falls to about 35 F for 10 to 15 minutes.

Chemical Bonds and Energy  Conservation of Energy:  Total Energy is same before and after a reaction  Exothermic reaction:  Chemical energy of reactants = heat + chemical energy of products.  Endothermic reaction:  Chemical energy of reactants + heat = chemical energy of products.