6.3 Nature of Reactions Objectives Demonstrate factors that influence the directions of a reaction. Classify factors that influence the rate of a reaction.

Reversible Reactions Many reactions can change direction. These reactions are called reversible reactions. They are reversible based on energy flow. However, not all reactions are reversible Formation of caves, drying of paint and the burning of fuel.

Automobile Battery Pb(s) + PbO2(s) + 2 H2SO4(aq) 2 PbSO4 (s) + 2 H2O(l) + energy When a car isn’t running the battery releases the energy as it moves to the right. If you need to jump start the battery the reaction moves to the left while the car engine runs

Equilibrium Equilibrium – term for a system where no net change occurs in the amount of reactants or products Dynamic Equilibrium – term describing a system in which opposite reactions are taking place at the same rate.

Reactions at Equilibrium Reactants are never used up, b/c they are constantly being formed by the products Reactants and products are being formed at the same rate. Example-Lime CaCO3(s) CaO(s) + CO2(g) Reactions at equilibrium do not necessarily have the same amount of reactants and products

Changing Direction Le Chatelier’s principle-disturbing an equilibrium will make a system readjust to reduce the disturbance and regain equilibrium. Chemical engineers apply this to obtain a larger product yield They will remove some product so the reaction will produce additional product in order to reach equilibrium.

CaCO3(s) CaO(s) + CO2(g) If carbon dioxide is removed as it is produced the reaction will produce more to achieve equilibrium When one produce is a gas and the other products and reactants are not, it is easy to see how the gas can be removed A product that does not dissolve in water can be removed if all other products and reactants dissolves in water. Soluble-it dissolves in a liquid Insoluble-does not dissolve in a liquid An insoluble product will form a precipitate and sink to the bottom

Adding Reactants or Energy Adding more reactants has the same effect as removing products Adding or removing energy (usually as heat) can also influence the direction of a reaction Because energy is part of any reaction it can thought of as either a reactant or a product

3 C + 2 Al2O3(s) + energy  4 Al(l) + 3 CO2(g) If more energy is added then the reaction goes to the right forming more aluminum and carbon dioxide

N2(g) + 3 H2 (g)  2 NH3(g) + energy Adding energy to the right will cause the reaction to move to the left This will produce more nitrogen and hydrogen

Reaction Rate The rate at which a reaction occurs Activation Energy – the amount of energy the particles in a reaction must have when they collide for the reaction to occur Reaction can be sped up or slowed down.

Temperature Temperature can speed up or slow down reactions A rise in temperature caused the baking of a cake to speed up Decreasing temperature can slow down a reaction such as keeping film cool so that film has a better self life

Concentration Concentration – the amount of a substance present in a unit volume Concentration also effects the reaction rate A rise in concentration will increase the rate of a reaction A decrease in concentration will slow down the reaction

Limiting Reactant Limiting Reactant – the reactant of which there is not enough; when it is used up, the reaction stops and no new product is formed Limiting reactants control the amount of product that a reaction produces, once it is used up the reaction ceases

Catalysts Catalysts- a substance that speeds up the rate of a reaction without being permanently changed or used up itself Catalysts do not change the position of equilibrium and therefore does not affect how much product is formed Enzymes – biological catalysts Enzymes are involved in almost every process in a cell Figure 6.22 page 222 Jello and Pineapple

Slowing Down Reactions Inhibitor – a substance that slows down a reaction Inhibitors do not completely stop a reaction An inhibitor is placed in bottles of hydrogen peroxide to prevent the decomposition

Review Define: What does a catalyst do to a reaction? Equilibrium Dynamic Equilibrium Activation Energy What does a catalyst do to a reaction? What does a inhibitor do to a reaction?

Homework Page 223 #1-3 Page 225 # 7 Extra Credit: Read page 216-217 and answer #1-2 Read page 221 and answer #1-2