Thermodynamics And Kinetics

Question 1

Thermodynamics

Answer 1

Study of energy transfer and change
Can be used to predict how much energy a given process requires
Thermodynamic functions are based on probabilities and are valid only for systems composed of large number of molecules
Cannot usually be applied to specific microscopic phenomena such as a single collision between two molecules

Question 2

Kinetics

Answer 2

Study of reaction rates and mechanisms
Can explain how essential chemical reactions can happen within milliseconds with the help of enzymes in the cell
Can be used to predict the rate of a chemical reaction
Often provides insight into the mechanism of the reaction at the molecular level

Question 3

What can average speed of microscopic atoms and molecules contained in a space tell us?

Answer 3

Manifests as macroscopic observable properties such as pressure, temperature, and volume

Question 4

What are the three categories of thermodynamic systems?

Answer 4

Open, closed, and isolated
System definitions are based on mass and energy exchange within the surroundings
Open: Can exchange both mass and energy with surroundings
Closed: can exchange energy, but not mass
Isolated: Cannot exchange energy or mass

Question 5

What are two types of properties which can be used to describe the macroscopic state of a system?

Answer 5

Extensive: proportional to size of system (Volume, n moles)
Intensive: independent of size of system (Pressure P, Temperature T)
If two identical systems are combined and a property is the same for both the single and combined systems, then intensive
If property doubles when systems are combined, then extensive
Dividing one extensive property by another gives an intensive property

Question 6

Temperature

Answer 6

Represents the amount of molecular movement in a substance
Motion of molecules divided into translational, rotational, and vibrational energies, which together describe overall energy of molecular motion
Sum of these energies is the thermal energy
Increase in thermal energy increases temperature
Temp can be thought of as the thermal energy per mole of molecules
- So temp is intensive property
- Combine two systems and thermal energy and moles double, temp same

Question 7

How does the temperature of a gas or liquid relate to the translational kinetic energy of its molecules?

Answer 7

Directly proportional relationship

Question 8

Translational motion

Answer 8

One of three energies of molecular motion
Can be divided into three degrees of freedom, or modes: 1. Along the x-axis, 2. Along the y-axis, and 3. Along the z-axis
Equipartition theory states that in a normal system each mode of motion will have the same average energy
- Energy of each mode is equal to 1/2 kT, k is Boltzmann’s constant (1.38 x 10^-23 J/K)

Question 9

Boltzmann constant

Answer 9

Symbol k
1.38 x 10^-23 J/K
Related to ideal gas constant R by Avogadros’ number N_A
R = N_A k
 OR k = R/N_A

Question 10

What is the average kinetic energy of a single molecule in any fluid? A mole of molecules in a fluid?

Answer 10

KE_molecule = 3/2 kT
KE_mole = 3/2 RT
T is temperature in Kelvin

Question 11

Absolute Zero

Answer 11

Lowest possible temperature
-273 deg C
Or 0 deg K
Temp at which no molecular movement

Question 12

At a pressure of 1 atm what temperatures does water freeze and boil at?

Answer 12

Freezes at 0 deg C or 273 K

Boils at 100 deg C or 373 K

Question 13

What does Standard temperature and pressure refer to? What does standard ambient temperature and pressure refer to?

Answer 13

Stand temperature and pressure (STP): pressure of 1 atm, temperature of 273 K
Standard ambient temperature and pressure (SATP): pressure of 1 atm and temperature of 298 K (25 deg C)

Question 14

What does the graph of volume vs. temperature look like for any given fixed pressure?

Answer 14

Exactly linear for any given pressure, but different slope
Higher slope for lower pressure, lower slope for higher pressure
Extrapolate line back to x intercept to get absolute zero temperature of 0 K or 273 deg C

Question 15

What are the different ideal gas constants, R?

Answer 15

R = 0.0821 L atm / mol K
R = 8.314 J atm / mol K

Question 16

Pressure

Answer 16

Proportional to random translational kinetic energy of a group of molecules per volume occupied
At microscopic level, pressure results from molecules pushing against their container as they move randomly, exerting force that causes the container to stretch until equilibrium is reached through an opposing force

Question 17

How does the rate of a given reaction usually compare to the frequency of collisions?

Answer 17

Rate of given reaction is usually much lower than frequency of collisions
Most collisions do not result in a reaction
Relative kinetic energies of a colliding compound must be greater than or equal to a threshold energy called activation energy
Atoms of both molecules must align in a specific way for collision to result in a reaction
- When molecules do not properly align, no reaction occurs, even if particles have sufficient kinetic energy to overcome activation energy

Question 18

Arrhenius equation

Answer 18

k = zpe^{-Ea/RT}
Where z: collision frequency, p: fraction of collisions having effective spatial orientation (steric factor), and e^{-Ea/RT} fraction of collisions having sufficient relative energy
K is rate constant of reaction
Often written as k = A e^{-Ea/RT}
Increase in activation energy reduces value of k
Increase in either z or p (A) increases k
Value of rate constant is affected by pressure (relevant for gases), presence of catalysts, and temperature

Question 19

How does increasing temperature affect the rate of a reaction?

Answer 19

Increases the number of possible collisions with sufficient activation energy, therefore increases the rate of a reaction
Higher temperature increases rate of forward and reverse reactions

Question 20

Reaction rate

Answer 20

Describes how quickly the concentration of the reactants or products are changing over the course of the reaction
Rates are most often presented in units of molarity / s (M /s or mol / L s) because represent change in concentration of the reactants and the products over time
Concentration of substances can affect rate of reaction

Question 21

Elementary reaction

Answer 21

Reaction that occurs in a single step
Stoichiometric coefficients of elementary equation give the molecularity of reaction (number of molecules that need to collide at one time for reaction to occur)

Question 22

Molecularity of Reaction

Answer 22

Number of molecules that need to collide at one time for a reaction to occur
E.g. given elementary reaction aA + bB -> cC + dD
Molecularity is a + b (if both are 1, then bimolecular)
Most common molecularities are unimolecular, bimolecular, and termolecular
Most reactions represent the sum of multistep reactions
Average reaction rate over time for example: rate = - delta [A] / a t = - delta [B] / b t = delta [C] / c t = delta [D] / d t

Question 23

Intermediates

Answer 23

Species that are products of one step and reactants of a later step in a multistep reaction
Get used up before end of reaction, so not shown in overall chemical equation
Often present in low concentrations
If sufficiently low, could pretend a reaction is elementary as an approximation for rate law

Question 24

How do reverse reactions complicate calculation of reaction rates?

Answer 24

In early reaction, when reactants are high and concentration of products is zero, formation of products may have a different rate than later on, when there is a reverse reaction going to form reactants from the products
Rates rely on the concentration of reactants, so complicated
Tend to determine reaction rates based only on concentrations observed by experimenter in initial moments of reaction

Question 25

Rate Law

Answer 25

Equation for reaction rate which incorporates only the concentration of reactants Rate_forward kf [A]^{alpha} [B]^{beta} Where kf is rate constant for forward reaction, alpha and beta are reaction order of each reactant, and sum of alpha + beta is overall order of reaction If reaction is elementary then alpha = a and beta = b (stoichiometric coeffs), if not then alpha and beta must be determined experimentally Remember that rate constant kf is not the rate of the reaction (proportional)

Question 26

Reaction Order

Answer 26

Indicates how changes in the reactant concentrations influence the reaction rate Order of each reactant indicates the particular influence of that reactant, while order of overall reaction provides more general information about relationships btwn reactant concentrations and reaction rate Order of overall reaction is sum of exponents in rate law Exponent of reactant in the rate law is reaction order for one reactant

Question 27

What’s an example of a zero-order reaction in biology?

Answer 27

Occurs in enzyme-catalyzed reactions when concentration of substrate far outweighs concentration of enzyme All enzyme catalytic sites are saturated and addition of additional substrate has no effect on reaction rate Or multistep reaction where that particular reactant is in fast step of reaction

Question 28

Rate-determining step

Answer 28

Rate of slowest elementary step determines rate of overall reaction If second step in reaction, the first step also contributes (formation of reactants for slow step)

Question 29

Catalyst

Answer 29

Substance that increases the rate of a reaction without being consumed or permanently altered Increase rate of both the forward and reverse reactions by providing an alternative reaction mechanism that competes with uncatalyzed mechanism - Can enhance product selectivity and reduce energy consumption - May lower activation energy - May increase steric factor Most work by decreasing activation energy Does not change equilibrium ratio of products and reactants

Question 30

Types of catalysts

Answer 30

Heterogeneous or homogeneous Heterogeneous: different phase than reactants and products (gas or aqueous particle react on a solid) - particles can stick to or adsorb to surface of solid (IMFs) - Rate of catalysis depends on strength of attraction btwn reactant and catalyst - Rates can be enhanced by increasing SA of catalyst (grind solid into powder) Homogeneous: Same phase as reactants and products (gas or liquid) - Aqueous acid or base solutions

Question 31

Auto-catalysis

Answer 31

Product of reaction acts as a catalyst for the reaction | E.g. Acid-catalyzed hydrolysis of ester, where product of carboxylic acid creates more acid to further catalyze reaction

Question 32

Rate law of catalyst

Answer 32

Reactions with catalysts require separate rate constants Total rate is given by sum of rates of both reactions (catalyzed and uncatalyzed) Sometimes rate law includes conc of catalyst if catalyst in small concentration in reaction E.g. uncatalyzed, rate = k0 [A] Catalyzed, rate = kH+ [H+] [A] Total rate law, rate = k0[A] + kH+ [H+][A]

Question 33

Biological example of a catalyst

Answer 33

Enzyme: protein catalyst that speeds up almost every chemical reaction in human body Enzymes mostly far more effective than catalysts found in lab due to specificity Turnover rate: Number of reactions occurring at a single active site on one enzyme ~1000 / second - Can be tens of thousands of times faster

Question 34

How does a solvent effect the rate of a reaction?

Answer 34

Liquid molecules have 100 x more collisions / s than gas, but most collisions w/ solvent, so do not lead to reaction Rate constant in liquid is function of both solvent characteristics and temperature Solvent characteristics: - dielectric: electrical insulation of reactants, making more stable - solvation: reactants spread out and surrounded by solvent, decreasing collisions - viscosity: reactants get trapped in solvent cages (cage effect), making rate of collisions of reactants equal to rate of collision of reactants in gas Stirring or shaking liquids increases number of collisions

Question 35

State

Answer 35

Physical condition of system as described by a specific set of thermodynamic properties Macroscopic state of any one-component fluid system in equilibrium can be described completely by just three properties and all other properties can be derived - at least one property must be extensive

Question 36

State functions

Answer 36

``` Properties that describe the current state of a system Thermodynamic state functions are macroscopic properties of a system Internal energy (U), Temperature (T), Pressure (P), Volume (V), Enthalpy (H), Entropy (S), Gibbs energy (G) ```

Question 37

Path Functions

Answer 37

Properties that do not describe the state of a system, but rather depend on the pathway used to achieve that state E.g. Work and heat

Question 38

How do we relate thermodynamic functions to a system on a molecular scale?

Answer 38

Using statistics Statistically-based thermodynamic functions are highly useful for predictions on a macroscopic scale - Usefulness of predictions increases with number of samples (i.e. molecules)

Question 39

Internal energy

Answer 39

Collective energy of molecules measured on a microscopic scale Energy includes vibrational, rotational, translational, electronic, intermolecular potential, and rest mass energy Does not include macroscopic mechanical energies such as kinetic energy of entire system or potential energy of entire system raised off the ground or does not include bond enthalpy: energy associated with formation of intramolecular chemical bonds Internal energy is state function For ideal gas, any state function can be expressed as function of temperature and volume only, but internal energy depends only on temperature

Question 40

Vibrational energy

Answer 40

Energy created by back-and-forth motion of atoms within a molecule Makes insignificant contributions to internal energy for light diatomic molecules at T < few hundred K Atoms in monatomic gas have no vibrational energy

Question 41

Rotational energy

Answer 41

Energy created by rotation of a molecule around its center of mass Spatial orientation of body changes, but center of mass remains fixed and each point within molecule remains fixed relative to other points Atoms in monatomic gas have no rotational energy

Question 42

Translational energy

Answer 42

Energy created by movement of center of mass of molecule | For monatomic gases, translational energy is sole contributor to kinetic energy

Question 43

Electronic energy

Answer 43

Potential electrical energy created by the attractions between electrons and their nuclei In chemical rxn, changing electronic energy accounts for greatest change in internal energy With no chemical rxn, electronic energy remains nearly constant

Question 44

Intermolecular potential energy

Answer 44

Energy created by intermolecular forces between molecular dipoles For gas at room temp, very small contribution to internal energy At high pressures, contribution becomes significant Makes up substantial portion of internal energy in liquids and solids

Question 45

Rest mass energy

Answer 45

Energy described in Einstein’s equation E = mc^2

Question 46

What component of internal energy can change for an ideal gas?

Answer 46

Translational energy is only component of internal energy that can change for ideal gas As a result, any change in internal energy of ideal gas causes corresponding temperature change Not true for real gases, liquids, or solids bc internal energies can change through changes in other components that do not affect temperature

Question 47

What are the two ways to transfer energy between systems?

Answer 47

Heat (q) and work (w) Heat is spontaneous transfer of energy from a warmer body to a cooler body Any energy transfer that is not heat is work

Question 48

Heat Transfer

Answer 48

Occurs through random collisions between molecules of two systems In each collision, energy is transferred from higher energy molecule to lower energy molecule Higher energy molecule could even be in cooler system (remember averages) Overall, majority of energy transfers from warmer to cooler system Warmer body becomes cooler and cooler body becomes warmer until equilibrium

Question 49

How can mitochondria help with thermoregulation?

Answer 49

When temperature of body begins to fall, mitochondria insert channels in inner membrane that dissipate the proton gradient Energy “lost” as heat warms the body

Question 50

Zeroth law of thermodynamics

Answer 50

Two systems in thermal equilibrium with a third system are in equilibrium with each other Two bodies in thermal equilibrium share a thermodynamic property- temperature, a state function Remember, that when two bodies at different temperatures are placed in thermal contact, on macroscopic level the temperatures of the two become equal

Question 51

What are the three ways that energy transfer through heat can occur?

Answer 51

Conduction, convection, and radiation Conduction: thermal energy transfer via molecular collisions Convection: thermal energy transfer via fluid movements Radiation: thermal energy transfer via electromagnetic waves

Question 52

Conduction

Answer 52

Thermal energy transfer via molecular collisions Requires direct physical contact Higher energy molecules of one system transfer some of their energy to lower energy molecules of other system via molecular collisions Also can be conducted through single object Thermal conductivity (k): object’s ability to conduct heat - depends on composition and temperature

Question 53

Convection

Answer 53

Thermal energy transfer via fluid (liquid or gas) movements | Differences in pressure or density drive warm fluid in direction of cooler fluid

Question 54

Radiation

Answer 54

Thermal energy transfer via electromagnetic waves Heated metal glows red, orange-yellow, white, and finally blue-white as hot metal radiates visible EM waves Before it begins to glow, radiates EM waves at frequency too low to be visible to human eye All objects with T > 0K radiate heat Does not require a medium to pass through (can propagate through vacuum)

Question 55

Newton’s law of cooling

Answer 55

Body’s rate of cooling is proportional to temperature difference between body and environment

Question 56

Emissivity

Answer 56

When radiation strikes an opaque surface, only a fraction is absorbed and the rest is reflected - Fraction absorbed indicated by emissivity of surface (depends on surface composition, between 0 and 1) Blackbody radiator: emissivity of 1, absorb 100% of incident radiation, so appear totally black

Question 57

What are two types of work a chemical system at rest can do?

Answer 57

PV work and non-PV work - Most important non-PV work for MCAT is electrical work Work defined as energy transfer due to a force Done whenever a force is applied over a distance (changes object’s kinetic and / or potential energy)

Question 58

What is an example of PV work?

Answer 58

Gas that expands in a balloon Gas must exert forces to both stretch the balloon and to expand into opposing pressure of atmosphere - energy dissipated in process - Outside stress will initiate PV work (exposure of gas in balloon to heat source)

Question 59

At constant pressure, what is the magnitude of PV work?

Answer 59

P x delta V When system does work on surroundings, work is assigned negative value, because transferring energy out of the system (could be opposite sign on MCAT, look for how it is defined)

Question 60

How can you determine the amount of PV work done from a pressure vs. volume graph?

Answer 60

Area under the curve of a PV diagram | Work is a path function, so the path that was taken determines how much energy was transferred out the system

Question 61

What is the defining feature that distinguishes work from heat on a molecular scale?

Answer 61

Ordered or directional collisions mean work and random collisions from high energy molecules to low energy molecules mean heat Defined in terms of the effect on the surroundings Capacity to do work arises from the constraint of molecules - Unconstrained system such as ideal gas with infinite volume can do no work

Question 62

First law of thermodynamics

Answer 62

Total energy of the system and surroundings is always conserved Any energy change to a system must equal the sum of heat flow into the system and work done on the system Delta E = q + w In a closed system at rest with no electric or magnetic fields, only energy change is internal energy Delta U = q + w Difference in sign of w for physicists vs chemists For MCAT usually: delta E negative when energy transferred out of system during expansion

Question 63

Enthalpy

Answer 63

Defined as an equation: H = U + PV Where U is internal energy Measured in units of energy (joules), but enthalpy is not conserved Enthalpy of the universe does not remain constant Since U, P, and V is a state function, enthalpy is also a state function Enthalpy depends only on T for an ideal gas Extensive property

Question 64

Change in Enthalpy

Answer 64

Delta H = delta U + P delta V (at constant pressure) If a system does work at a constant pressure and in reversible process: Delta H = [w_nonPV - P delta V + q] + p delta V = w_nonPV + q(constant pressure, closed system at rest, PV work only) Then if only PV work is done: Delta H = q (constant pressure, closed system at rest) At constant volume and constant pressure, Change in U equals about change in H

Question 65

Standard State

Answer 65

Not standard temperature and pressure Complicated, varies with phase and other factors and has different values depending on convention chosen For pure solid or liquid, standard state is reference form of a substance at any chosen temperature and pressure of 1 bar (~750 torr, or 10^5 pascals) Reference form usually form that is most stable at T and P, for pure gas must behave like ideal gas Enthalpy is assigned at standard state at 25 deg C to be 0 kJ/mol

Question 66

Standard enthalpy of formation

Answer 66

Delta Hf^{deg} Enthalpy values assigned to compounds based on change in enthalpy when formed from raw elements in standard states at 25 deg C Change in enthalpy for reaction that creates one mol of compound from raw elements in standard state (determined experimentally)

Question 67

Heat of reaction

Answer 67

Since enthalpy approximates heat in many reactions in the lab (constant pressure, constant volume), change in enthalpy from reactants to products is heat of reaction: Delta H deg _ reaction = delta H^{deg}_{fproducts} - delta H^{deg}_{freactants} Remember to include coefficients of each reactant and product when calculating Negative enthalpy change indicates exothermic reaction, usually produces heat flow to surroundings (at constant pressure) Positive enthalpy change indicates endothermic reaction

Question 68

What are five possible ways to know change in enthalpy for a reaction on the MCAT?

Answer 68

1. Passage defines delta H outright 2. Heat measured with calorimeter and so heat is -delta H 3. Sign can be intuited from rxn (melting is endergonic, + delta H) 4. Compute from a table of bond energies by number of bonds broken and formed (breaking bonds requires energy) 5. Calculate using standard enthalpies of formation (delta H^{deg}_{f})

Question 69

Entropy

Answer 69

Universe’s tendency towards disorder Nature’s tendency to create the most probable arrangement that can occur within a system (more ways for that arrangement to occur) State function and extensive property Nature’s effort to spread energy evenly between systems Driving force that dictates whether or not a reaction will proceed (must increase the entropy of the universe, but not necessarily the system in order to proceed)

Question 70

Second Law of thermodynamics

Answer 70

Entropy of an isolated system will never decrease Universe is an isolated system, therefore sum of entropy changes of any system and its surroundings equals the entropy change of the universe, which must be equal to or greater than zero Delta S_system + delta S_surroundings = delta S_universe >= 0

Question 71

Relationship of entropy to other variables

Answer 71

All other factors being equal, entropy increases with number, size, volume, and temperature If reaction increases the number of gaseous molecules, reaction has a positive entropy for the reaction system

Question 72

Third Law of thermodynamics

Answer 72

Assigns a zero entropy value to any pure element or compound in its solid form at absolute zero and in internal equilibrium At absolute zero, atoms have very little motion, can be realized only in theory

Question 73

Entropy change

Answer 73

Delta S = q_rev / T Infinitesimal change in heat (q_rev) per Kelvin in a reversible process Units in J / K Cannot be used to calculate change in S for irreversible process because heat is pathway dependent Delta S for irreversible process can be found by imaging a reversible process between same two states and finding heat transfer for reversible process

Question 74

Reversibility (in thermodynamics)

Answer 74

Reaction that can take place in both the forward and reverse directions must have an entropy change of zero in both directions For reversible reaction in an isolated system, entropy change must be zero Real reversible reactions do not happen - hypothetical reversible reaction would be quasi-static process, with system in equilibrium with surroundings at all times - pressure differences must be infinitesimal, heat transfer occurs at same T, infinite amount of time

Question 75

Alternative definitions of reversibility

Answer 75

Entropy undefined on the microscopic level of molecular reactions All reactions at microscopic level must be considered reversible Collision between individual molecules is exact opposite of reverse process, no law in classical physics that favors one direction over another Equilibrium achieved when rate of forward reaction equals rate of reverse reaction (point of greatest universal entropy)

Question 76

Change in entropy of a molecular reaction

Answer 76

Delta S^{deg}_{reaction} = delta S^{deg}_{products} - delta S^{deg}_{reactants}

Question 77

Gibbs free energy

Answer 77

Incorporates the entropy change in both the system and the surroundings into a single equation that only relies on information about the system Delta G = delta H - T delta S H, T, S are all state functions Delta G < 0 required for spontaneous process, and delta G = 0 required for equilibrium Extensive property and a state function, not conserved Represents the maximum non-PV work available from , or “free” for a reaction Endergonic (positive delta G), exergonic (negative delta G)

Question 78

What does the negative of the Gibbs free energy refer to?

Answer 78

Amount of non-PV work available in a completely reversible process Actual amount of non-PV work that we will get out of the system is always less than magnitude of - delta G

Question 79

Change in Gibbs free energy for standard state?

Answer 79

State function, so same equation applies as for S and H | Delta G^{deg}_{reaction} = delta G^{o}_{f,products} - delta G^{o}_{f,reactants}

Question 80

What can Gibbs free energy tell us about the coupling of non-spontaneous and spontaneous reactions?

Answer 80

If a reaction is non-spontaneous, can couple with a spontaneous reaction to make it favorable E.g. biological non-spontaneous reaction coupled with ATP hydrolysis

Question 81

Hess’s Law of Heat Summation

Answer 81

Sum of enthalpy changes for each step is equal to the total enthalpy change regardless of the path chosen Forward reaction has exactly opposite change in enthalpy as reverse reaction with opposite sign

Question 82

Transition state

Answer 82

State in the middle of a reaction where old bonds are breaking and new bonds are forming Peak of energy hill in reaction Not the same as intermediate, which exists for a longer duration of time (lower energy than transition state)

Question 83

Chemical equilibrium

Answer 83

Condition where the rate of forward reaction equals the rate of the reverse reaction At equilibrium, no net change in concentrations of products or reactants Point of greatest entropy Reactions move towards and then stay at equilibrium

Question 84

Equilibrium constant

Answer 84

For a reaction: aA + bB -> cC + dD The equilibrium constant determines the relative amounts of each species at equilibrium: K = [C]^c [D]^d / [A]^a [B]^b = Products^coeffs / Reactants^coeffs Law of Mass Action: Mathematical relationship between chemical equation and associated equilibrium constant K is unitless, will not change unless temperature changes Concentration of pure liquid or solid given a value of 1 for expression

Question 85

How can a reaction run to completion?

Answer 85

If forward reaction is so much more favorable than reverse reaction, then for all practical purposes, it runs to completion Also with Le Chatelier’s principle, if remove product continually, reaction can run to completion

Question 86

Reaction Quotient

Answer 86

Q = Products^coeff / Reactants^coeff Describes a state perturbed from equilibrium Q will always move toward equilibrium If Q = K, reaction at equilibrium If Q > K, reaction will shift to increase reactants and decrease products (reverse) If Q < K, reaction will shift to increase products and decrease reactants (forward)

Question 87

Le Chatelier’s Principle

Answer 87

When a system at equilibrium is stressed, the system will shift in a direction that will reduce stress Stress that obeys Le Chatelier’s: 1. Addition or removal of product or reactant 2. Changing pressure or volume of system 3. heating or cooling the system

Question 88

Haber Process

Answer 88

Exothermic all gas reaction (creates heat), assume it occurs in rigid container at equilibrium: N2(g) + 3H2(g) -> 2NH3(g) + Heat Adding N2 gas shifts equation to the right, producing more heat Adding heat, reaction is pushed to the left Decreasing size of container shifts reaction to right (less separate molecules)

Question 89

Effect of Concentration, Pressure, and Temperature on Rxn at Equilibrium

Answer 89

Concentration: reaction proceeds in direction of less concentrated side Pressure: Reaction shifts in direction with less moles of gas as pressure increases Temperature: If heat added, reaction shifts away from the side that produces heat

Question 90

Exceptions to Le Chatelier’s

Answer 90

Solvation reactions: solubility of salt generally increases with increasing temp, even when reaction is exothermic (significant entropy increase w/ dissolution) Increased pressure due to addition of a nonreactive gas does not cause any shift in forward or reverse reaction rates

Question 91

How do we relate Gibbs free energy for a particular set of conditions from the standard state Gibbs free energy?

Answer 91

delta G = delta G^{o} + RTln(Q) Q is reaction quotient Standard state free energy occurs at 1 bar with all species starting at one molar concentrations and usually at 25 o C Base 10: Delta G = delta G^{o} + 2.3 RT log(Q) K and delta G^{o} vary with temperature, so necessary to look up for specific temperature At equilibrium, no available free energy to do work, delta G = 0: Delta G^{o} = -RT ln(K), so K > 1 then delta G^{o} < 0