UWorld Thermodynamics, Kinetics, and Gas Laws

Ideal Gas Law

PV = nRT

Phase Diagram

Illustrates a substance’s stability in a given phase (solid, liquid, gas) as a function of pressure and temperature. Boundary lines represent where two phases are in equilibrium. Crossing a boundary line represents a change in phase.

Triple Point

The point at which all three boundary lines intersects, and indicates the temperature and pressure at which all three phases exist in equilibrium at the same time. Liquid water cannot exists below the triple point.

The Universal Solvent

Water, because as a polar compound it has an ability to interact with other polar or charged compounds. Water readily dissolves polar and ionic compounds through electrostatic interactions (hydrogen bonding and ion diipole bonding)

Characteristics that contribute to water’s ability to be a solvent:

1. Oxygen is more electronegative than hydrogen which allows water to have interactions with other charged particles

2. The bent geometry of water contributes to its polarity by grouping positive charges and negative charges on either side of the molecule, attracting other opposite charges from polar molecules.

3. Water’s small size allows for the formation of a hydration shell around solutes which evenly distributes and isolates solutes.

Surface Tension

A force induced at the interface between a liquid and a gas. Molecules interact more strongly in the liquid then they do in the gas causing the surface to behave as a thin film. Water has a high surface tension.

Freezing Point

The temperature at which solid and liquid phases of a substance are in equilibrium. Freezing occurs when the kinetic energy of a molecule can no longer overcome the intermolecular force binding it to nearby molecules. If a molecule experiences strong intermolecular forces, the kinetic energy is higher and the freezing point is higher.

Boiling Point

The temperature at which the vapor pressure is equal to ambient pressure. The addition of a solute lowers the vapor pressure of a solution at all temperatures and therefore raises the boiling point.

Law of Mass Action

States that at an equilibrium constant Keq can be expressed as a ratio of the molar concentrations of products over reactants, each raised to the power of its respective balanced reaction coefficient. Solids, pure liquids, and solvent species are omitted from the Keq expression.

Equilibrium Expression Kp R

Ratio of equilibrium partial pressures of products to reactants, each raised to the power of their balanced stoichiometric coefficients.

ICE Table

Initial, Change, and Equilibrium

Change in Enthalpy of the Reaction (Delta H)

The amount of heat released or absorbed by a chemical reaction measured under constant pressure.

Exothermic Reaction

Release heat (heat is product) and results in negative delta H. During equilibrium, a temperature decrease causes the reaction to shift towards the products to compensate for heat loss.

Endothermic Reaction

Absorb heat (heat is reactant) and results in a positive delta H.Treat heat as a reactant.

Open System

Both heat and matter are freely exchanged with surroundings

Closed System

Only heat is exchanged with surroundings

Isolated System

Neither heat nor matter is exchanged with the surroundings

Compressibility

Compressibility = PV/nRT. Real gases under very low temperatures or very high pressures deviate from ideal gas conditions

1. A real gas with compressibility < 1 indicates attraction between molecules. These attractions decrease the force of the molecular collisions with the wall, giving real gas a lower pressure and compressibility than an ideal gas.

2. A real gas with compressibility > 1 diminishes space in which gas can move. Gives a real gas a higher compresibility and volume than an ideal gas.

Latent Heat

The amount of heat 1 mole of a substance must absorb/release to produce a change in phase while maintaining a constant temperature.

Second Law of Thermodynamic

The total entropy of an isolated system will always increase over time during any spontaneous process.

Entropy

Indicates the disorder of a system and measures how much energy in the system is unavailable to do work.

Change in Entropy (Delta S) Equation

Delta S = q/T (heat = q, absolute temperature = T)

Thermal Energy Transfer

Some of the energy in the system becomes distributed in the random, thermal motion of the atoms and the molecules. As the disorder increases, part of the energy is made unavailable as dispersed heat and cannot be converted to work.

Steps to solving for Heat Energy

1. Use equation q = mCp (T final - T initial) = mCp (delta T) (Cp = Specific Heat; heat needed to raise temperature of 1 gram of the substance by 1 C)

2. Cp = 4.2 J / (g)(C)

Heat transfer occurs from:

Hotter objects to cooler objects

First Law of Thermodynamics

Heat energy is conserved and the heat lost from a sample must be equal to the sum of the heat gained by substances in thermal contact with the sample

Entropy Increases:

Solids < Liquids < Gases

Gibbs Free Energy Equation

Delta G = Delta H - T Delta S

Delta H

Describes the flow of heat in a reaction

Delta S

Describes the disorder of the products

Delta G

A state function that describes the spontaneity of the reaction (the likelihood for it to occur without additional intervention)

Delta G < 0 (negative)

Spontaneous and thermodynamically favorable

Delta G > 0 (positive)

Non spontaneous and thermodynamically unfavorable

Change in gibbs standard free energy of a reaction

Delta G = - RT ln(Keq) (Equilibrium constant and absolute temperature)

Thermodynamic Values

Indicate if a reaction is spontaneous or not but do not indicate the rate of a reaction. Enthalpy, entropy, and gibbs free energy are state functions and are independent of the chemical pathway that a reaction takes to get from reactants to products.

Catalyst

A chemical species that is added to a reaction to increase the rate of the reaction by stabilizing the transition state and decreasing the activation energy of the reaction. A catalyst has no effect on the amount of products produced or change the energies of the reactants and products.

Delta H Equation

Delta H Products - Delta H Reactants

Dissociation Reaction

Typically endothermic processes because energy must be added to the system (as heat) to dissociate (break) the bonds in the reactants. Dissociation reactions tend to increase entropy because a larger molecule is broken into two or more smaller molecules which increase entropy.

1 atm proportions

1 atm = 760 mmHg = 760 torr = 101, 325 Pa = 101.325 kPa

Dalton’s Law of Partial Pressures

P Elemental Gas = (X elemental Gas)(P Total)

X Elemental Gas = mol / mol total

P tot = P1 + P2 + P3

Xi Mol Fraction = ni / n1 + n2 + n3

Moles are found from balanced chemical equation

Rate Law

A reaction relates the rate of the reaction to the concentration of each reactant. When two reactants are present the rate law is expressed by the equation:

Rate = k[A]^m[B]^n

( k = the constant, m and n are the respective orders of the reactants, and m + n is the overall reaction order)

Exothermic (Temperature)

Causes heat to increase because more heat is produced then consumed

Endothermic (Temperature)

Causes heat to decrease because more heat is consumed then produced

Hess’ Law

The overall enthalpy change (Delta H) of a reaction is equal to the sum changes in each enthalpy component

Steps to solving Molar Enthalpy formation of a compound

1. Change signs of reactants to positive because they are consumed not produced

2. Multiply the new standard enthalpies by their stoichiometric coefficients from the balanced equation

3. Use x and add stoichiometric coefficient if necessary for the unknown compound you are solving for

4. Solve for x

Overall Enthalpy Equation

Overall Enthalpy = Enthalpy of products - enthalpy of reactants

Entropy

A measure of disorder of a system

Positive Entropy (Delta S) vs Negative (Delta S)

When entropy/delta S is positive then entropy is increasing (more disorder and less order) and when entropy/delta S is negative then entropy is decreasing (less disorder and more order)

If Delta G is negative:

The reaction is spontaneous

If ln(Keq) is positive:

The reaction is spontaneous

If ln(Keq) is negative:

The reaction is nonspontaneous

Products / Reactants < Keq

Reactants are consumed and products are formed

Products / Reactants > Keq

Products are consumed and reactants are formed

Reaction Equilbrium State

Set of concentrations of products and reactants at which no new products or reactants are formed

Reaction Quotient

[C]^c[D]^d / [A]^a[B]^b

Example: (HbO2)/[Hb][O2]^4

O2^4 = 16

Calorimetry

Technique that measures the amount of heat exchanged between a substance undergoing a chemical or physical change and a calibrated object.

Equilibrium Constant Keq

Ratio of the equilibrium molar concentrations.

[Products]^stoichiometric coefficient / [Reactants]^stoichiometric coefficient

Specific Heat Capacity (Cp)

(J/(g)(C)) is the amount of heat q required to raise the temperature of 1 gram of a substance by 1 degree Celsius,

Boiling Point of a Liquid

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the ambient pressure of the surroundings.

Low Vapor Pressure equals:

a higher boiling point

High Vapor Pressure equals:

a lower boiling point

Intermolecular Forces and Boiling Points

Structures with less intermolecular forces have lower boiling points because less energy is required to overcome attractions between molecules. Boiling points decrease as hydrogen bonding groups decrease.

The Reaction Order

For each reactant in a chemical process can be determined by comparing kinetic data at different concentrations , from which the rate law can be written. The rate constant k can be evaluated from the rate law.

Reaction Free Energy Diagram

A plot of the gibbs free energy (y axis) versus the the progress of a reaction pathway. The reaction progress is represented abstractly by the reaction coordinate (x axis).

Transition State

An activated complex with elevated energy caused by increased geometric strain and charge repulsion. Are graphically written as higher energy maxima/peaks

The Activation Energy

The minimum energy needed to reach the highest energy transition state that enables the completion of a reaction. In a reaction energy free diagram, the activation energy is the energy difference between the tallest peak and the reactants/

Arrhenius Equation

k = Ae^(-Ea/RT)

Rate Determining Step

The step with the largest energy barrier/the slowest step

Change in Gibbs Free Energy Equation

Delta G = Gibbs free energy products - Gibbs free energy reactants

(If energy of products is bigger than the delta g is positive (endergonic) and if the reactants are bigger than delta g is negative (exergonic))

If equilibrium favors the formation of products

Keq > 1

If equilibrium favors the formation of reactants

1 > Keq

Kinetic Product

Compound with the lowest energy transition state that forms faster. The major product formed at lower temperatures.

Thermodynamic Product

The compound with the higher energy transition state that forms more slowly. (most stable product) The major product formed at higher temperatures.

Avogadro’s Law

V1 / n1 = V2 / n2

Bond Enthalpy (Delta H)

Bond dissociation energy, the amount of energy needed to break 1 mole of a bond between 2 atoms in the gas phase at a temperature of 298K.

Forming a bond is an:

Exothermic process (Delta H bonds formed < 0)

Breaking a bond is an:

Endothermic process (Delta H bonds formed > 0)

Delta H reaction equation

Delta H reaction = Delta H bonds broken + Delta H bonds formed

If H rxn > 0

Then H Bonds Broken > H Bonds Formed (endothermic/positive)

If H rxn < 0

Then H Bonds Broken < H Bonds Formed (exergonic/negative)

(State Functions) When I feel dense and under pressure I watch TV and get HUGS

Density, pressure, temperature, volume, enthalpy, internal energy, entropy, and gibbs free energy.

Rate of a Reaction

Indicates how quickly the reactant molecules are consumed and the product molecules are produced.

Catalyst

A substance that increases the reaction rate (change in concentration per unit time) but is not consumed in the overall reaction. Forms a more stable transition state and lowers the activation energy of the rate determining step . Increases the number of productive collisions between reactant molecules

Partial pressure for 4CO2 + 2H2O + 2N2 when CO2 = 0.4

CO2 = 0.4 and H2O and N2 = 0.2 so Ptot = 0.8

Removing a reactant will:

Shift the reaction towards the reactants

Removing a product will:

Shift the reaction towards the products

Kinetic Molecular Theory of Gases

The average kinetic energy of gas particles’ velocity in the system increases as temperature increases.

Charle’s Law

V1 / T1 = V2 / T2

Boyle’s Law

V1P1 = V2P2

Irreversible Reactions

Tend to be under kinetic control, which favors products who has the lowest activation energy.

Reversible Reactions

Tend to be under thermodynamic control, which favors the highest activation energy/most stable product

-G vs +G (thermodynamic favorability)

-G is thermodynamically favorable and +G is thermodynamically unfavorable

Temperature and Keq

An exothermic reaction will increase heat and will shift reaction towards the reactants (heat is treated as a product) (keq decreases) and an endothermic reaction will absorb heat (heat is treated as a reactant) and the reaction shifts towards the products (keq increases) with temperature increase.

Dissolution Reaction

The amount of heat q released or absorbed by the reaction is equal in magnitude but opposite in sign to the amount of heat absorbed or lost during a reaction.

Specific Heat Capacity

q(solution) = m(solution)C(s) Delta T

(Delta T = T final - T initial)

Intermediates

A chemical species that is formed in one step and consumed in the next step. Is neither a reactant or product.

Characteristics of an Ideal Gas

1. No attractive or repulsive interactions

2. Negligible molecular volume

3. Elastic collisions

4. Kinetic energy increases with higher temperatures

5. Pressure produced by combined force of collisions

Deviations from Ideal Gas Behavior

Deviations Less Likely (No Intermolecular Interactions):

1. High Temperature

2. Low Pressure

3. Low molecular volume

Deviations More Likely (Intermolecular Interactions):

1. Low Temperature

2. High Pressure

3. High molecular volume

Van Der Waals Equation

( Pr + a(n²/V²)(Vr - nb) = nRT

Pi = Ideal Pressure, Pr = Real Pressure, a = attraction constant, n = moles, V = Volume)

Small Rate Constant/Slow Step

Has a large activation energy because more energy is required to initiate the reaction.