Equilibrium and Thermodynamics (Chem)

Question 1

How can you tell that a reaction favors the products from a Keq equation?

Answer 1

The numerator is greater than the demonimator,thus it will have a large Keq

Question 2

How can you tell that a reaction favors the reactants from a Keq equation?

Answer 2

The numerator is smaller than the demonimator,thus it will have a small Keq

Question 3

True or False

You can substitute any concentration into a Keq expression and expect to obtain an equilibrium constant

Answer 3

False

The concentrations used MUST be equilibrium concentrations

Question 4

Using the equilibrium equation K_eq = k_forward / k_reverse , write the Keq expression for the reverse reaction of a balanced equation of 2X(g) + 4Y(g) ⇌ 3Z (g)

Answer 4

K_eq = X² [Y]⁴ / [Z]³

where the exponents are derived from the coefficients from the balanced equation

Question 5

Using the equilibrium equation K_eq = k_forward / k_reverse , write the Keq expression for the of a balanced equation of 2X(g) + 4Y(g) ⇌ 3Z (g)

Answer 5

K_eq = [Z]³ / X² [Y]⁴

Question 6

Write the general forumula of the Keq of the reverse reaction

Answer 6

1/K_eq

Question 7

True or False

Keq can be reported in negative values

Answer 7

False

since concentrations are given in units of moles/liter or molarity, it cannot be negative

Question 8

In the chemical equation SnO2 (s) + 2CO (g) ⇋ Sn (s) + 2 CO2 (g) , what molecules would we exclude from the equilibrium expression and why?

Answer 8

SnO2 and Sn, because they are solids (s)

We do not include pure solids or liquids in equilibrium expressions

link KA equilibrium page

Question 9

When do we use the reaction quotient (Q) instead of the equilibrium constant Keq?

Answer 9

When we’re uncertain or do not know the equilibrium concentrations

The equation for the reaction quotient is the same as the equation used for Keq, but the concentrations can be from any moment in time, not necessarily at equilibrium. Similarly, the Q expression should not include pure solids, liquids, or solvents

Question 10

When Q < Keq…

Answer 10

The ratio of products to reactants is lower than it would be in equilibrium

The reaction will proceed forward

pg. 52 of chem book for examples

Question 11

When Q > Keq…

Answer 11

The ratio of products to reactants is* greater* than it would be in equilibrium

The reaction will proceed in the reverse

pg. 52 of chem book for examples

Question 12

When Q = Keq…

Answer 12

The reaction is at equilibrium

pg. 52 of chem book for examples

Question 13

Combustion and neutralization reactions are ____ thermic

Answer 13

Exothermic (-ΔH)

releases heat

Question 14

Given Le Chatlier’s principle, an increase in temperature has the effect of:

Answer 14

A shift towards products if ΔH > 0, and a shift towards reactants if ΔH < 0

Question 15

Given Le Chatlier’s principle, an decrease in temperature has the effect of:

Answer 15

A shift towards reactants if ΔH > 0, and a shift towards products if ΔH < 0

Question 16

True or False

Catalysts increases the rate of both a forward and reverse reactions, causing it to reach equilibrium more quickly

Answer 16

True

Note: this is only before the reaction is already at equilibrium!

Question 17

True or False

Catalysts change the relative amounts of products and reactants

Answer 17

False

They only change the reaction rates

Question 18

Explain the equation ΔE = Q -W . Which law of thermodynamics does this correspond to?

Answer 18

Net change in total energy = Heat added - work done

first law of thermodynamics

if the system is being heated, Q is positive, but if heat is being lost from the system Q is negative. When work is being done by the system on its surroundings, W is positive. This is because work done by te system causes the system’s internal energy to decrease. Work done on the system is negative in the above equation. i.e, when work is doneon a system, its internal energy increases since -(-W) is a positive number.

Question 19

What formula can be used to calculate the necessary quantity of heat to raise the temperature of a material?

Answer 19

q = mcΔT

aka the calorimetry equation

Where:

q = heat required (J)
m = mass of substance present (g)
c = substance’s specific heat (J/g*ºC)
ΔT = temperature change (ºC)

Question 20

Write the equation for calculating the changes in enthalpy

Answer 20

ΔH = ΔE + PΔV

Where ΔE = change in internal energy of the system

Enthalpy is measured in Joules (J) or kilojoules (kJ)

Question 21

What is considered “standard conditions” for standard enthalpy/heat of formation?

Answer 21

1 atm and 298 Kelvins

Question 22

Name the equation

ΔH_rxn = ΔH_products - ΔH_reactants

ΔH_rxn = ΔH (bonds broken) - ΔH (bonds formed)

Answer 22

Standard Enthalpy of formation

Question 23

ΔS_rxn = ΔS_products - ΔS_reactants

Answer 23

Standrard Entropy

think: second law of thermodynamics

Question 24

Name the formula

quantifies the amount of usable [free] energy (energy that can do work) in a system; the change in free energy of a system as it goes from some initial state, such as all reactants, to some other, final state, such as all products

measures the spontaneity of a rxn

Answer 24

Gibbs Free Energy

T = temperature (in kelvin)
S = entropy

H = enthalpy

+ ΔG nonspontaneous (endergonic) -ΔG spontaneous (exergonic)

units: kJ/mol rxn
​

Question 25

ΔG°rxn = ΔG°products - ΔG°reactants

Answer 25

Gibbs free energy equation for standard conditions

standard formation of Gibbs free energy

Question 26

The ΔG of a reaction at equilibrium is…

Answer 26

Zero

because there is no overall change in the system (reactants arent moving to products and vice versa), there is no change in free energy

Question 27

ΔG°rxn = -RTlnK_eq

Answer 27

Gibbs free energy wrt the equilibrium constant

R = gas constant
T = temp in Kelvins

note: ln is natural log; ln of 1 is 0, ln of a number greater than 1 is positive, and the ln of a number less than 1 is negative

Question 28

ΔGrxn = ΔG°rxn + RTlnQ

Answer 28

can be used to assess a rxn under non-standard conditions

Question 29

A reaction in which the product ratio is determined by the relative stability of the products

Answer 29

Thermodynamic control

Question 30

At low temperature, the reaction is under ____ (rate, irreversible conditions) and the major product is that from the fastest reaction.

Answer 30

Kinetic control

pg. 65

Question 31

the kinetic product forms more quickly/slowly , is less/more thermodynamically favorable, has a lower/higher activation energy, and is favored at low/high temperatures

Answer 31

forms more quickly
less thermodynamically stable
lower activation energy
favord at low temperatures

Question 32

the thermodynamic product forms more quickly/slowly , is less/more thermodynamically favorable, has a lower/higher activation energy, and is favored at low/high temperatures

Answer 32

forms more slowly
less thermodynamically stable
lower activation energy
favored at low temperatures