Energy Changes & Reactions

1. Energy Changes & Reactions Unit 7 - Chapter 7.3

2. 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

3. Chemical Bonds and Energy • Energy changes in reactions  determined by changes in chemical bonding. • Example: combustion of propane • C3H8 + 5O23CO2 + 4H2O • Bonds in reactants are broken (C3H8 and O2) • Propane molecules have C-C and C-H single bonds • Oxygen molecules have O=O double bonds • Bonds in products are formed (CO2 and H2O) • Carbon dioxide molecules have C=O double bonds • Water molecules have H-O single bonds

4. Chemical Bonds and Energy • Breaking Bonds • In combustionof 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.

5. Chemical Bonds and Energy

6. Chemical Bonds and Energy • Forming Bonds: burning propane • C3H8+ 5O23CO2 + 4H2O • 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

7. 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  lessthan energy released as products formed

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

9. 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.

10. 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. • C3H8 + 5O2  3CO2 + 4H2O + 2220 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

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

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

13. 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.

14. 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.