Chemical Kinetic and Thermodynamics

Unit 7:Kinetics and Thermodynamics : Unit 7:Kinetics and Thermodynamics Cartoon courtesy of NearingZero.net

Heat (Enthalpy) Change : Heat (Enthalpy) Change The amount of heat energy released or absorbed during a process. Endothermic: Exothermic: Processes in which energy is absorbed as it proceeds, and surroundings become colder Processes in which energy is released as it proceeds, and surroundings become warmer

Units for Measuring Heat : Units for Measuring Heat The Joule is the SI system unit for measuring heat: The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree

A Bomb Calorimeter : A Bomb Calorimeter

A Cheaper Calorimeter : A Cheaper Calorimeter

Specific Heat : Specific Heat The amount of heat required to raise the temperature of one gram of substance by one degree Celsius.

Calculations Involving Specific Heat : Calculations Involving Specific Heat cp = Specific Heat q = Heat lost or gained T = Temperature change OR m = Mass

Table of Specific Heats : Table of Specific Heats

Latent Heat of Phase Change : Latent Heat of Phase Change Molar Heat of Fusion The energy that must be absorbed in order to convert one mole of solid to liquid at its melting point. The energy that must be removed in order to convert one mole of liquid to solid at its freezing point. Molar Heat of Solidification

Latent Heat of Phase Change #2 : Latent Heat of Phase Change #2 Molar Heat of Vaporization The energy that must be absorbed in order to convert one mole of liquid to gas at its boiling point. The energy that must be removed in order to convert one mole of gas to liquid at its condensation point. Molar Heat of Condensation

Latent Heat – Sample Problem : Latent Heat – Sample Problem Problem: The molar heat of fusion of water is 6.009 kJ/mol. How much energy is needed to convert 60 grams of ice at 0C to liquid water at0C? Mass of ice Molar Mass of water Heat of fusion

Heat of Solution : Heat of Solution The Heat of Solution is the amount of heat energy absorbed (endothermic) or released (exothermic) when a specific amount of solute dissolves in a solvent.

Chemical Kinetics : Chemical Kinetics The area of chemistry that concerns reaction rates. However, only a small fraction of collisions produces a reaction. Why? Key Idea: Molecules must collide to react.

Collision Model : Collision Model Orientation of reactants must allow formation of new bonds. Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy).

Reaction Rate : Reaction Rate The change in concentration of a reactant or product per unit of time

2NO2(g)  2NO(g) + O2(g) : 2NO2(g)  2NO(g) + O2(g) Reaction Rates: 2. Can measure appearance of products 1. Can measure disappearance of reactants 3. Are proportional stoichiometrically

Activation Energy : Activation Energy The minimum energy required to transform reactants into the activated complex (The minimum energy required to produce an effective collision) Flame, spark, high temperature, radiation are all sources of activation energy

Endothermic Reactions : Endothermic Reactions

Exothermic Reactions : Exothermic Reactions

Factors Affecting Rate : Factors Affecting Rate Temperature Increasing temperature always increases the rate of a reaction. Surface Area Increasing surface area increases the rate of a reaction Concentration Increasing concentration USUALLY increases the rate of a reaction Presence of Catalysts

Catalysis : Catalysis Catalyst: A substance that speeds up a reaction without being consumed Enzyme: A large molecule (usually a protein) that catalyzes biological reactions. Homogeneous catalyst: Present in the same phase as the reacting molecules. Heterogeneous catalyst: Present in a different phase than the reacting molecules.

Catalysts Increase the Number of Effective Collisions : Catalysts Increase the Number of Effective Collisions

Endothermic Reaction witha Catalyst : Endothermic Reaction witha Catalyst

Exothermic Reaction with a Catalyst : Exothermic Reaction with a Catalyst

Chemical Equilibrium : Chemical Equilibrium Reversible Reactions: A chemical reaction in which the products can react to re-form the reactants Chemical Equilibrium: When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged 2HgO(s)  2Hg(l) + O2(g) Arrows going both directions (  ) indicates equilibrium in a chemical equation

LeChatelier’s Principle : LeChatelier’s Principle When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress.

Le Chatelier Translated: : When you take something away from a system at equilibrium, the system shifts in such a way as to replace what you’ve taken away. Le Chatelier Translated: When you add something to a system at equilibrium, the system shifts in such a way as to use up what you’ve added.

LeChatelier Example #1 : LeChatelier Example #1 A closed container of ice and water at equilibrium. The temperature is raised. Ice + Energy  Water The equilibrium of the system shifts to the _______ to use up the added energy. right

LeChatelier Example #2 : LeChatelier Example #2 A closed container of N2O4 and NO2 at equilibrium. NO2 is added to the container. N2O4 (g) + Energy  2 NO2 (g) The equilibrium of the system shifts to the _______ to use up the added NO2. left

LeChatelier Example #3 : LeChatelier Example #3 A closed container of water and its vapor at equilibrium. Vapor is removed from the system. water + Energy  vapor The equilibrium of the system shifts to the _______ to replace the vapor. right

LeChatelier Example #4 : LeChatelier Example #4 A closed container of N2O4 and NO2 at equilibrium. The pressure is increased. N2O4 (g) + Energy  2 NO2 (g) The equilibrium of the system shifts to the _______ to lower the pressure, because there are fewer moles of gas on that side of the equation. left

Enthalpy and Entropy : Enthalpy and Entropy Reactions tend to proceed in the direction that lowers the energy of the system (H, enthalpy). Reactions tend to proceed in the direction that increases the disorder of the system (S, entropy).

Spontaneity of Reactions : Spontaneity of Reactions Reactions proceed spontaneously in the direction that lowers their free energy, G. G = H - TS If G is negative, the reaction is spontaneous. If G is positive, the reaction is NOT spontaneous.

H, S, G and Spontaneity : H, S, G and Spontaneity G = H - TS H is enthalpy, T is Kelvin temperature

Reaction Mechanism : Reaction Mechanism The series of steps by which a chemical reaction occurs. A chemical equation does not tell us how reactants become products It is a summary of the overall process. Example: has many steps in the reaction mechanism

Rate-Determining Step : Rate-Determining Step In a multi-step reaction, the slowest step is the rate-determining step. It therefore determines the rate of reaction.