Reaction Kinetics and Equilibrium

Why do chemical reactions occur between some substances and not in others?

Chemical reactions occur so that the atoms in the elements involved attain a more stable state of being.

Collision Theory Collision theory – molecules must collide with the proper orientation and sufficient energy to react.

Activation Energy The activation energy is the amount of energy required to break the bonds between the atoms of the reactants.

Condition Necessary for Reactions to Occur 1)Collision: Reactants must collide. 2) Orientation: The reactants must align properly to react. 3) Energy: The activation energy must be attained to react.

Energy in Chemical Reactions Many chemical reactions also produce energy changes. Definitions: System – the reactants and products in the reaction Surroundings – everything else around the reaction (eg air in the room, reaction flask)

Heat of Reaction Heat of Reaction (ΔH) – the amount of heat lost or gained in a reaction Heat of Reaction: ΔH = H products – H reactants

Exothermic Reactions Exothermic Reactions – energy is produced by a reaction; energy flows from the system to the surroundings ΔH is negative because the reaction loses heat.

Exothermic Reaction Graph

Endothermic Reactions Endothermic Reactions – energy is gained by a reaction; energy flows from the surroundings into the system ΔH is positive because the reaction gains heat.

Endothermic Reaction Graph

Classify the reaction: 2H 2 O + 572kJ -> 2H 2 + O 2 1.Exothermic 2.Endothermic

ΔH of the reaction = -560kJ 1.Exothermic 2.Endothermic

After the reaction, your hand gets burnt from the heat. The reaction must be: 1.Exothermic 2.Endothermic

Heat Values in Chemical Reactions Heat of Reaction is a stoichiometric value and is proportional to the coefficients of the reactants and products. 2H 2 O + 572kJ -> 2H 2 + O 2 Therefore, for every 2 moles of water that react, 572kJ of energy are required.

Reaction Rate Reaction rate – rate of disappearance of one of the reactants ( or the rate of appearance of one of the products) -Measured in = ΔMolarity s Change in concentration (molarity) over time.

Factors Affecting Reaction Rate What factors could affect the rate that a reaction takes place?

Factors affecting reaction rate Temperature Pressure Surface Area exposed Concentration of reactants and products Catalyst

Temperature Temperature – as temperature goes up, the average kinetic energy increases. As velocity goes up, more collisions occur. Temperature↑, rate ↑

Factors Affecting Reaction Rate Pressure – as the volume of a closed container decreases, the pressure of a gas increases, causing more collisions (P↑, rate ↑)

Surface Area Surface area exposed As more surface area is exposed, more collisions can occur. Surface Area ↑, rate ↑ Note: crunching up a piece of candy or crushing a tablet into a powder makes smaller pieces, which increases the surface area.

Concentration of Reactants If the reactants are more concentrated, there are more reactants present = more collisions. Concentration of Reactants ↑, rate ↑

Presence of a Catalyst – a substance that increases the rate without being permanently changed - lowers activation energy

Also used: inhibitors – “tie up” a reaction so that it does not occur (opposite of a catalyst) - preservatives - anti-rust agents

Properties of an Equilibrium Reaction Equilibrium systems are DYNAMIC (in constant motion) DYNAMIC (in constant motion) REVERSIBLE REVERSIBLE can be approached from either direction can be approached from either direction Equilibrium systems are DYNAMIC (in constant motion) DYNAMIC (in constant motion) REVERSIBLE REVERSIBLE can be approached from either direction can be approached from either direction

The Concept of Equilibrium Chemical equilibrium occurs when a forward reaction and its reverse reaction proceed at the same rate.

Chemical Equilibrium Fe 3+ + SCN - FeSCN 2+ After a period of time, the concentrations of reactants and products are constant. After a period of time, the concentrations of reactants and products are constant. The forward and reverse reactions continue after equilibrium is attained. The forward and reverse reactions continue after equilibrium is attained.

The Concept of Equilibrium As a system approaches equilibrium, both the forward and reverse reactions are occurring. At equilibrium, the forward and reverse reactions are proceeding at the same rate.

A System at Equilibrium Once equilibrium is achieved, the amount of each reactant and product remains constant even though the reaction is still occurring.

Depicting Equilibrium In a system at equilibrium, both the forward and reverse reactions are being carried out; as a result, we write its equation with a double arrow N 2 O 4 (g) 2 NO 2 (g)

Le Châtelier’s Principle “If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.”

Look at the equation: N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + energy How can changes in conc., temp, or pressure affect this reaction?

N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + energy Increase reactant concentration, there are more collisions between reactants, so reaction shifts to the right, towards the products.

N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + energy Increase in temperature affects both sides of reaction, but it will increase one side more than the other. Think of energy as a product, if we increase its concentration, the reaction will lean in the other direction to regain equilibrium.

N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + energy Increase in pressure will demand the fewest moles of gas. Occurs with a reduction in volume. -In this example, products will be favored -If number of moles of gas are equal, pressure has no effect. -Only has an effect on gases. -Decrease in pressure will have the opposite effect.

N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + energy Decrease in volume will demand the fewest moles of gas. -In this example, products will be favored -If number of moles of gas are equal, pressure has no effect. -Only has an effect on gases. -Increase in volume will have the opposite effect.