6.3 Nature of Reactions6.3 Nature of Reactions

ObjectivesObjectivesDemonstrate factors that influence Demonstrate factors that influence the directions of a reaction.the directions of a reaction.Classify factors that influence the Classify factors that influence the rate of a reaction.rate of a reaction.

Reversible ReactionsReversible Reactions

Many reactions can change direction.Many reactions can change direction. These reactions are called reversible These reactions are called reversible

reactions.reactions. They are reversible based on energy flow.They are reversible based on energy flow. However, not all reactions are reversibleHowever, not all reactions are reversible

– Formation of caves, drying of paint and the Formation of caves, drying of paint and the burning of fuel.burning of fuel.

Automobile BatteryAutomobile Battery

Pb(s) + PbOPb(s) + PbO22(s) + 2 H(s) + 2 H22SOSO44(aq) 2 PbSO(aq) 2 PbSO44 (s) + 2 H (s) + 2 H22O(l) + energyO(l) + energy

When a car isn’t running the battery releases the When a car isn’t running the battery releases the energy as it moves to the right.energy as it moves to the right.

If you need to jump start the battery the reaction If you need to jump start the battery the reaction moves to the left while the car engine runsmoves to the left while the car engine runs

EquilibriumEquilibrium

EquilibriumEquilibrium – term for a system where no – term for a system where no net change occurs in the amount of net change occurs in the amount of reactants or productsreactants or products

Dynamic Equilibrium Dynamic Equilibrium – term describing a – term describing a system in which opposite reactions are system in which opposite reactions are taking place at the same rate.taking place at the same rate.

Reactions at EquilibriumReactions at Equilibrium

Reactants are never used up, b/c they are Reactants are never used up, b/c they are constantly being formed by the productsconstantly being formed by the products

Reactants and products are being formed at Reactants and products are being formed at the same rate.the same rate.

Example-LimeExample-Lime

CaCOCaCO33(s) CaO(s) + CO(s) CaO(s) + CO22(g)(g) Reactions at equilibrium do not necessarily Reactions at equilibrium do not necessarily

have the same amount of reactants and have the same amount of reactants and productsproducts

Changing DirectionChanging Direction

Le Chatelier’s principle-disturbing an Le Chatelier’s principle-disturbing an equilibrium will make a system readjust to equilibrium will make a system readjust to reduce the disturbance and regain reduce the disturbance and regain equilibrium.equilibrium.

Chemical engineers apply this to obtain a Chemical engineers apply this to obtain a larger product yieldlarger product yield– They will remove some product so the reaction They will remove some product so the reaction

will produce additional product in order to will produce additional product in order to reach equilibrium. reach equilibrium.

CaCOCaCO33(s) CaO(s) + CO(s) CaO(s) + CO22(g)(g)

If carbon dioxide is removed as it is produced the If carbon dioxide is removed as it is produced the reaction will produce more to achieve equilibriumreaction will produce more to achieve equilibrium

When one produce is a gas and the other products and When one produce is a gas and the other products and reactants are not, it is easy to see how the gas can be reactants are not, it is easy to see how the gas can be removedremoved

A product that does not dissolve in water can be A product that does not dissolve in water can be removed if all other products and reactants dissolves in removed if all other products and reactants dissolves in water.water.

Soluble-it dissolves in a liquidSoluble-it dissolves in a liquid Insoluble-does not dissolve in a liquidInsoluble-does not dissolve in a liquid An insoluble product will form a precipitate and sink to An insoluble product will form a precipitate and sink to

the bottomthe bottom

Adding Reactants or EnergyAdding Reactants or Energy

Adding more reactants has the same effect Adding more reactants has the same effect as removing productsas removing products

Adding or removing energy (usually as Adding or removing energy (usually as heat) can also influence the direction of a heat) can also influence the direction of a reactionreaction– Because energy is part of any reaction it can Because energy is part of any reaction it can

thought of as either a reactant or a productthought of as either a reactant or a product

3 C + 2 Al3 C + 2 Al22OO33(s) + energy (s) + energy 4 Al(l) + 3 CO 4 Al(l) + 3 CO22(g)(g)

If more energy is added then the reaction If more energy is added then the reaction goes to the right forming more aluminum goes to the right forming more aluminum and carbon dioxideand carbon dioxide

NN22(g) + 3 H(g) + 3 H22 (g) (g) 2 NH 2 NH33(g) + energy(g) + energy

Adding energy to the right will cause the Adding energy to the right will cause the reaction to move to the leftreaction to move to the left

This will produce more nitrogen and This will produce more nitrogen and hydrogenhydrogen

Reaction RateReaction Rate

The rate at which a reaction occursThe rate at which a reaction occurs Activation EnergyActivation Energy – the amount of energy – the amount of energy

the particles in a reaction must have when the particles in a reaction must have when they collide for the reaction to occurthey collide for the reaction to occur

Reaction can be sped up or slowed down.Reaction can be sped up or slowed down.

TemperatureTemperature

Temperature can speed up or slow down Temperature can speed up or slow down reactionsreactions

A rise in temperature caused the baking of a A rise in temperature caused the baking of a cake to speed upcake to speed up

Decreasing temperature can slow down a Decreasing temperature can slow down a reaction such as keeping film cool so that reaction such as keeping film cool so that film has a better self lifefilm has a better self life

ConcentrationConcentration

Concentration Concentration – the amount of a substance – the amount of a substance present in a unit volume present in a unit volume

Concentration also effects the reaction rate Concentration also effects the reaction rate A rise in concentration will increase the rate A rise in concentration will increase the rate

of a reactionof a reaction A decrease in concentration will slow down A decrease in concentration will slow down

the reactionthe reaction

Limiting ReactantLimiting Reactant

Limiting ReactantLimiting Reactant – the reactant of which – the reactant of which there is not enough; when it is used up, the there is not enough; when it is used up, the reaction stops and no new product is formedreaction stops and no new product is formed

Limiting reactants control the amount of Limiting reactants control the amount of product that a reaction produces, once it is product that a reaction produces, once it is used up the reaction ceasesused up the reaction ceases

CatalystsCatalysts

CatalystsCatalysts- a substance that speeds up the rate of a - a substance that speeds up the rate of a reaction without being permanently changed or reaction without being permanently changed or used up itselfused up itself

Catalysts do not change the position of Catalysts do not change the position of equilibrium and therefore does not affect how equilibrium and therefore does not affect how much product is formedmuch product is formed

EnzymesEnzymes – biological catalysts – biological catalysts– Enzymes are involved in almost every process in a cellEnzymes are involved in almost every process in a cell

– Figure 6.22 page 222 Jello and PineappleFigure 6.22 page 222 Jello and Pineapple

Slowing Down ReactionsSlowing Down Reactions

InhibitorInhibitor – a substance that slows down a – a substance that slows down a reactionreaction

Inhibitors do not completely stop a reactionInhibitors do not completely stop a reaction An inhibitor is placed in bottles of hydrogen An inhibitor is placed in bottles of hydrogen

peroxide to prevent the decompositionperoxide to prevent the decomposition

ReviewReview

Define: Define: – EquilibriumEquilibrium– Dynamic EquilibriumDynamic Equilibrium– Activation EnergyActivation Energy

What does a catalyst do to a reaction?What does a catalyst do to a reaction? What does a inhibitor do to a reaction?What does a inhibitor do to a reaction?

HomeworkHomework

Page 223 #1-3Page 223 #1-3 Page 225 # 7Page 225 # 7

Extra Credit:Extra Credit: Read page 216-217 and answer #1-2Read page 216-217 and answer #1-2 Read page 221 and answer #1-2Read page 221 and answer #1-2