Chemistry Unit 3 (1/2)

Physical Processes

atomic-scale structure of the substance remains before and after

Chemical processes

processes that change one (or more) substance into one (or more) new substance; atoms are neither created nor destroyed

Oxidation-reduction reactions

involve the movement of electrons

Acid-base reaction

involve the redistribution of lone pairs of elections and breaking/forming covalent bonds

Dynamic chemical equilibrium

Where the rate of the forward reaction and reverse reaction are equal; results in the concentrations of the reactants and products remaining constant even though the reactions continue to occur

Chemical equilibrium characteristics

Forward and reverse reactions occur at the same rate, concentrations remain constant; The same eq. constant can be achieved by starting with only reactants, only products, or a mixture of both

Equilibrium constant (K)

A ratio of equilibrium partial pressures or concentrations of products and reactants that has a specific value for a given reaction at a given temperature

This represents:

Equilibrium equation for a gas-phase reaction

This represents:

Equilibrium equation for a solution-phase reaction

T/F: K expressions contain terms for pure solids or pure liquids

False

T/F: For dilute solutions, the solvent is not included in the K expression

True

Homogenous equilibrium

all of the reactants and products are present in a single phase

T/F: In aqueous equilibrium systems, the solvent (H2O) is included in the equilibrium expression

False

Heterogeneous equilibrium

Reactants and products are found in two or more phases

Product-favored reaction

A reaction that proceeds spontaneously in the forward direction when all concentrations (or partial pressures) have the value 1M (or 1 bar)

Reactant-favored reaction

A reaction that proceeds spontaneously in the reverse direction when all concentrations (or partial pressures) have the value 1M (or 1 bar)

If K > 1, the reaction is ________ favored at equilibrium

Product

If K < 1, the reaction is ________ favored at equilibrium

Reactant

If K = 1

Neither the products nor reactants are favored at equilibrium

Does the ratio of products to reactants depend on initial concentrations?

No

Spontaneous (in the forward direction)

When reactants change to products

Not spontaneous (in the reverse direction)

When products change to reactants

Standard state

A commonly accepted set of conditions used as a reference point; For chemists: Pressure = 1 bar (0.987 atm); Concentration = 1M; Pure solids; Pure liquids

Product favored

When all substances are at the standard-state conditions, reactants change to products

Reactant Favored

If products change to reactants under standard-state conditions

ICE Table

Describes initial concentration/partial p., change in concentration/partial p., and equilibrium concentration/partial p.O

If a reaction starts with only reactants present…

Reaction must initially be spontaneous in the forward direction

If a reaction starts with only products present…

Reaction must proceed in the reverse direction

Reaction Quotient (Q)

Used when determining which direction a reaction proceeds when both reactants and products are present

Q = 0 when…

Only reactants are present

When Q = K

Equilibrium has been reached because Q no longer changes over time

When Q < K

the reaction proceeds spontaneously in the forward direction (from left to right; to the product side)

When Q > K

the reaction proceeds spontaneously in the reverse direction (from right to left; to the reactant side)

To calculate equilibrium concentrations…

Use an equilibrium constant and ICE tables

Le Chatelier’s Principle helps…

Enable prediction of which direction an equilibrium will shift when conditions change; Used when we don’t know exact concentration/partial pressure of reactants and products

Le Chatelier’s Principle states:

When a chemical system is at equilibrium and conditions are changed so that the reaction is no longer at equilibrium, the chemical system reacts to achieve new equilibrium concentrations or partial pressures in a way that partially counteracts the change in conditions

If a product concentration is increased and all other concentrations remain the same, the reaction equilibrium shifts _____ (toward/away from) the reactant side

toward

Two general rules for predicting whether a chemical reaction releases energy:

If there are more bonds in the product molecules than in the reactant molecules and the bonds has about the same strength, the reaction is likely exothermic; If there are stronger bonds in the product molecules than in the reactant molecules and the number of bonds is the same in reactants and molecules, the reaction is likely exothermic

Thermochemical expression

A balanced chemical equation together with the value of Δ_rH°, the standard-state reaction enthalpy change, and a temperature

Standard-state reaction enthalpy change, Δ_rH°

The standard enthalpy of pure, unmixed products minus the standard state enthalpy of pure, unmixed reactants; that is, the enthalpy change for a reaction under standard-state conditions; doesn’t specify temperature; units (kJ/mol and J/mol)

A negative Δ_rH° indicates

an exothermic reaction

A positive Δ_rH° indicates

an endothermic reaction

Hess’s law

If a process can be written as the sum of several stepwise processes, the enthalpy change of the total process equals the sum of the enthalpy changes of the various steps; reaction enthalpy change is the same whether the reaction occurs in one or two steps

Standard formation of enthalpy, Δ_fH°

The enthalpy change for a reaction in which exactly one mole of a pure substance in specified state is formed from free elements in their most stable states under standard conditions; Equals zero for an element in its most stable form under standard conditions (kJ/mol)

Entropy

How energy is distributed; Better understood as a measure of energy dispersal (how energy is spread across the various motions and configurations available to particles in a system)

Systems with higher energy are…

more probable, making them more stable

List the three fundamental ways a molecule can move

Translational, rotational, vibrational motion

Translational motion

involves whole molecules moving through space(dominant form of motion in gases)

Rotational motion

involves molecules spinning around their axes

Vibrational motion

more localized: atoms within a molecule stretch and bend the bonds the connect them. Larger, more complex molecules have more _______ nodes than smaller or more rigid ones

Solids mostly allow what type of motion?

Vibrational

Liquids mostly allow what type of motion?

Vibrational, rotational and limited transitional

Gases mostly allow what type of motion?

Vibrational, rotational, and translational

Microstates

Unique combination of the three motions; as substances move from solid to liquid to gas, the number of accessible _________ increases dramatically (why entropy increases across phase changes)

Entropy change during dissolution depends on…

the dispersal of the solute and the restructuring of the solvent

Third Law of Thermodynamics

Defines the entropy of a perfect crystal at absolute zero (0K) as exactly zero; Δ_rS° = ∑S°(products) − ∑S°(reactants)

Spontaneous Process

One that, once started, can occur as written without continuous inputs of energy from the surroundings

Second Law of Thermodynamics

In any spontaneous process, the total entropy of the universe increases; ΔS_universe = ΔS_system + ΔS_surroundings > 0; ΔS_{surroundings =} Q_universe/T (the heat lost by the system becomes the entropy gaines by the environment)