Reversible Reactions Main Concept In many classes of

Reversible Reactions Main Concept: In many classes of reactions, it is important to consider both the forward and the reverse reaction.

Reversible Reactions Reversible Processes/Reactions Examples

- Many readily observable processes are reversible - Reversible reactions are ones which not only proceed from reactants to products but also from products back to reactants - Examples: evaporating and condensing water, absorption of a gas, or dissolving and precipitating a salt

- Other environmental examples: transfer of carbon between atmosphere and biosphere and transfer of dissolved substances between atmosphere and hydrosphere - Other biological examples: binding of oxygen to hemoglobin and attachment of molecules to receptor sites in nose

- Chemical examples: Dissolution of a solid, transfer of protons in acid-base reactions, and transfer of electrons in redox reaction - Reversible processes will often reach a state of dynamic equilibrium: when the rate of the forward process/ reaction is equal to rate of reverse process/reaction

**NOTE: None of these will include pure solids (s) or pure liquids (l).

Equilibrium Constant Main Concept: When a system is at equilibrium, all macroscopic variables, such as concentrations, partial pressures, and temperature, do not change over time. Equilibrium results from an equality between the rates of the forward and reverse reactions, at which point Q=K.

Equilibrium Constant Defined K>Q K=Q K<Q

- When equilibrium is reached, no observable changes occur in the system known as Kc – Reactant and product molecules are present – Concentration of all species remains constant

- If rate of forward reaction > rate of reverse reaction, net conversion of reactants to products - If rate of the reverse reaction > rate of forward reaction, net conversion of products to reactants - An equilibrium state is reached when rates balance, where progress of reaction, Q, becomes equal to equilibrium constant, K

- K > Q reaction will move towards products - K = Q reaction is at equilibrium - K < Q reaction will move toward reactants - Comparing Q to K allows the determination of whether reaction is at equilibrium, or will proceed toward products or reactant to reach equilibrium - Equilibrium constants can be determined from experimental measurements of concentrations of the reactants and products at equilibrium

- Given a single reaction, initial concentrations, and K, the concentrations at equilibrium may be predicted - Graphs of concentration over time for simple chemical reactions can be used to understand the establishment of chemical equilibrium **

Reaction Quotient Main Concept: The current state of a system undergoing a reversible reaction can be characterized by the extent to which reactants have been converted to products. The relative quantities of reaction components are quantitatively described by the reaction quotient, Q.

Reaction Quotient ICE Table Reaction Quotient What’s NOT Included Reversing and Adding Reactions

positive change = increase negative change = decrease - Given an initial set of reactant and product concentrations, only sets of concentrations that are consistent with stoichiometry can be attained for the purposes of equilibrium - ICE (initial, change, equilibrium) tables are useful for determining which sets of concentration values are possible

- The reaction quotient, Q, provides a measure of current progress of a reaction - Q does not include substances whose concentrations are independent of the amount of substance (ex: a solid in contact with a liquid solution or with a gas, or for a pure solid or liquid in contact with a gas)

- The value of Q (and so also K) changes when a reaction is reversed - When reactions are added together through the presence of a common intermediate, Q (and so also K) of the resulting reaction is a product of the values of Q (or K) for the original reactions

K 3 = K 1 K 2 = (2. 0 × 10− 25)(6. 4 × 109) = 1. 3 × 10− 15

Equilibrium Sides Meaning of Numerical Value of K Q vs K Relationship between Q and K

- For many aqueous reactions, K is either very large or very small, and this may be used to reason qualitatively about equilibrium systems - Particulate representations can be used to describe relationship between numbers of reactant and product particles at equilibrium, and value of equilibrium constant

Q vs K Main Concept: A disturbance to a system at equilibrium causes Q to differ from K, thereby taking the system out of the original equilibrium state. The system responds by bringing Q back into agreement with K, thereby establishing a new equilibrium state.

- Some stresses, such as changes in concentration, cause a change in Q - A change in temperature causes a change in K - In either case, reactions shift to bring Q and K back into equality Q > K, reaction shifts towards reactants Q = K, reaction at equilibrium Q < K, reaction shifts towards products