7.3 Chapter 7

Chemical Energy the energy stored in the chemical bonds of a substance Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.

Breaking Bonds It takes energy to break chemical bonds. In some cases the energy comes from an igniter (produces a spark-like in a lighter). In other cases the energy may come from a heat source (like boiling a compound to break it apart).

Breaking Bonds The more bonds that hold a compound together, the more energy you need to break them apart. Double bonds are stronger than single bonds. Triple bonds are stronger than double bonds.

Forming Bonds The formation of chemical bonds releases energy. Sometimes heat and/or light are given off as new bonds form. (This is how a propane grill works.)

Chemical Reactions During a chemical reaction, energy is either released or absorbed.

Exothermic Reactions Reaction that releases energy More energy is released by the product formation than is used by the reactant break down. Combustion reactions are exothermic. See page 208 in your book for energy diagrams.

Heat/Energy You can show the release of heat or energy in a chemical equation like this: C3H8 + 5O2 → 3CO2 + 4H2O + 2220 kJ The 2220 kJ represents the energy produced by the reaction. kJ stands for kilo joules Joules are the units that we measure heat in.

Endothermic Reactions Reactions that absorb energy from the surroundings More energy is required to break the bonds in the reactants than is released by the formation of the products. 2HgO + 181.7 kJ → 2Hg + O2

Law of Conservation of Energy Energy can neither be created nor destroyed It can only change forms. Forms: Heat Chemical energy Stored or potential energy Movement or kinetic energy

7.4 Reaction Rate The rate at which reactants change into products over time (how fast the reaction is going)

What Affects Reaction Rate? Temperature Surface area (the exposed surface space) Concentration Stirring Catalysts Leave a little room under each topic to add notes.

Temperature Increased heat increases reaction rate Heat ↑ = reaction rate ↑ This is because heat makes particles move faster. The faster they go, the more likely they are to bump into each other and react. This is how food cooks and why we keep dairy products in the refrigerator.

Surface Area Smaller particles generally have larger surface areas. ↑ surface area = ↑ reaction rate

Stirring Stirring increases the reaction rate. Stirring makes the particles interact more. Think about how a washing machine works.

Concentration Concentration is the number of particles is a given volume. The more particles there are, the more likely they are to interact. ↑ concentration = ↑ reaction rate

Catalysts Catalysts are substances that affect reaction rates without being used up in the reaction. Catalysts increase reaction rates. The catalyst in a reaction is written over the arrow between the reactants and products, since it is neither. (page 215)

7.5 Equilibrium Equilibrium is when the forward and reverse actions change at the same rate. It may appear that nothing is happening but it is because the rate of change is constant in each direction.

Physical Equilibrium When a physical change is not completed, a physical equilibrium is established between the forward and reverse changes. This is represented like this: H2O (l) ↔ H2O (g) This says that water is evaporating at the same rate that it is condensing.

Chemical Equilibrium When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward and reverse reactions. Depends on reversible reactions.

Reversible Reaction A reaction in which the change from reactant to product happens at the same time that products are changing back into reactants.

Chemical Equilibrium When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change. Equilibrium is affected by: Temperature Pressure Concentration

Temperature Adding heat to an equilibrium where an exothermic reaction has occurred will cause a shift toward reactant production. Adding heat to an equilibrium where an endothermic reaction has occurred will cause a shift toward product production.

Pressure Adding pressure will cause a shift toward the side that contains fewer gas molecules.

Concentration The side with the lower concentration will be produced.