Chemistry: Gibs Free Energy
What is the equation for enthalpy?
H = E + PV
Equation for heat
q=mc∆T
What is the equation for Gibbs Free Energy under nonstandard conditions?
Delta G = Delta G Prime + RTLogQ R = 8.314 J/mol*k (gas constant) T = 298 K (25C)
Given a reaction, how is the enthalpy change for the overall reaction calculated?
Delta H for a reaction is calculated by subtracting the sum of the heat of formation of the products by the sum of the heat of formation of the reactants.
Consider the following reaction: C3H8 + 5O2 -> 3CO2 + 4H2O Heat of formation: C3H8: -104.7 kJ O2 (g): 0 kJ CO2: -393.5 kJ H2O: -285.83 kJ Calculate the Delta H for the reaction:
Delta H of reaction = Heat of formation of products minus heat of formation of reactants Delta Hrxn = [3(-393.5) + 4(-285.830)] - [-104.7] = -2219 kJ
Consider the following reaction: 4NH3 + 5O2 -> 4NO + 6H2O Heat of formation: NH3: -45.8 kJ O2 (g): 0 kJ NO: +90.29 kJ H2O: -285.83 kJ Calculate the Delta H for the reaction:
Delta H of reaction = Heat of formation of products minus heat of formation of reactants Delta Hrxn = [4(90.29) + 6(-285.830)] - [4(-45.9)] = -1170 kJ
Explain how the change in entropy (delta S) of an overall is calculated.
Delta S is calculated by subtracting the total entropy of the products by the total entropy of the reactants.
exergonic vs endergonic reactions
Exergonic reaction a chemical reaction that releases energy. Endergonic reactions yield products that are rich in potential energy.
TRUE or FALSE When a reaction is thermodynamically spontaneous, this means that it will go fast.
FALSE It only means that it will process eventually without external energy input.
True or False: The rate of reaction depends on Gibb's free energy
FALSE The rate of reaction depends on the activation energy.
TRUE OR FALSE The enthalpy of a reaction gives information on the spontaneity of a reaction.
FALSE Enthalpy only tells us if a reaction is endothermic or exothermic. It does not tell us whether a reaction is spontaneous.
TRUE OR FALSE: An exothermic reaction is ALWAYS spontaneous
FALSE: An exothermic reaction CAN be nonspontaneous if the change in entropy is negative enough. An endothermic reaction CAN be spontaneous if the change in entropy is positive enough.
Important standard states of a substance includes?
H2 (g) H20 (l) NaCl (s) O2 (g) C (s, graphite)
Heat vs. Temperature
Heat is a specific form of energy that can enter or leave a system. (process function) Temperature is a measure of the kinetic energy of the particles in a system.
Explain the difference between heat and temperature.
Heat is the amount of energy transferred due to a change in temperature. This transfer is between a system and its surroundings. Temperature is the average kinetic energy in molecules
What are the units of Gibbs Free Energy?
Joules/mol
Consider the following reaction: CaCO3 (s) <-> CaO (s) + CO2 (g) What happen you add CaCO3(s)
No shift because adding a solid will not effect the reaction. The equilibrium expression is only determined by the CO2 concentrations.
State functions mnemonic: When I'm under PRESSURE and feeling DENSE, all i want to do is watch TV and get HUGS
Pressure, density, temperature, volume, enthalpy, internal energy, Gibbs free energy and entropy
Enthalpy is a state function. What does this mean in regards to how its calculated?
Since enthalpy is a state function, it can be calculated by comparing the enthalpy of the final state to the enthalpy of the initial state.
What is the temperature during Standard Conditions and during Standard Temperature and Pressure (STP)?
Standard Conditions: 25C STP: 0C
Standard conditions vs. STP
Standard Conditions: 25C (298 K), 1 atm pressure, 1 M concentration. -used for kinetics, equilibrium, and thermodynamics problems STP: 0C (297 K), 1 atm pressure -used for ideal gas calculations
Explain how the change in Gibbs Free Energy can be explained as a State Function
State functions are a property whose value does not depend on the path taken to reach that specific value. Similarly, the change in Gibbs Free Energy does not depend on the path taken, however, it is only dependent on the products and the reactants of the reaction. Not the path taken.
TRUE OR FALSE: On a heating curve, temperature does NOT change during phase changes.
TRUE When melting an ice cube, all the heat added during the process is used to overcome the intermolecular forces between the water molecules in ice, forming liquid water.
What type of information does the Gibbs Free Energy (Delta G) of a reaction tell us?
The Gibbs Free Energy tells us simply whether a reaction is spontaneous or not spontaneous.
Consider the following reaction: A -> B -> C How can the change in free energy be calculated?
The Gibbs free energy can be calculated from A->B and from B->C. Since the change in Gibbs Free Energy is a state function, the individual change in free energy can be added to obtain the overall Delta G of A->C
What is the difference between exergonic/endergonic reactions and exothermic/endothermic reactions?
The classifications endothermic and exothermic refer to transfer of heat q or changes in enthalpy ΔRH. The classifications endergonic and exergonic refer to changes in free energy (usually the Gibbs Free Energy) ΔRG.
What is the Standard enthalpy of formation?
The enthalpy change when one mole of a substance is formed from its elements
what does it mean for a substance to be in its standard state
The standard state of a substance is its most stable form under standard conditions.
What does Hess's Law state?
The total enthalpy change for a reaction is independent of the route by which the chemical change takes place
What does it mean to be thermodynamically favored versus kinetically favored? Draw out a reaction coordinate diagram for both.
Thermodynamically favored: -Gibbs Free energy -Spontaneous -Stability of products versus reactants. -The bonds of the products are more stable than the reactants. Kinetically favored: -Activation Energy is low -faster reaction. -Stability of transition state.
What is significant about the critical point on a phase diagram?
This is the point at which the phase boundary between the liquid and gas phases terminate. This is when the temperature and pressure at which the two densities become equal and there is no distinction between the two phase.
On a phase diagram, the point at which the temperature and pressure of each of the three phases are in equilibrium is?
Triple Point
When the standard heat of formation (Delta Hf Prime) is positive, was heat inputted or released to make that compound? what about when it is negative
When the standard heat of formation of a compound is positive, then an input of heat is required to make that compound from its constitutive elements. When it is negative, this means that making the compound from its elements gives off energy.
Which phase change between the three phases of matter exhibit the greatest increase in entropy?
gas
Consider the following reaction: C3H8 + 5O2 -> 3CO2 + 4H2O Delta Hrxn = -2219 kJ How much heat will be released if 33g of C3H8
mol C3H8 = 44g/mol 33 g x (1 mol/44g) = .75 mole C3H8 (.75)(2219) = 1664 kJ of heat is released
What is the Gibbs free energy equation?
∆G = ∆H - T∆S
Draw a diagram with Delta G as the y-axis and rxn coordinate at the x-axis. Draw a spontaneous and non spontaneous reaction
DO IT
How is the the change in enthalpy of reaction calculated when given the Bond Dissociation Energies (BDE) of the reactants and products?
Change in Enthalpy of a reaction = Sum(BDE bonds broken) - Sum(BDE bonds formed)
Create a table of Delta H and Delta S to determine what the overall Delta G would be.
DO IT
The three principles used to calculate enthalpy include: 1. Bond dissociation energy 2. Heat of formation 3. Hess's Law Describe how enthalpy is calculated with each method.
1. Bond Dissociation energy: Enthalpy = Sum of Enthalpy (bonds broken) minus Sum of Enthalpy (bonds formed) 2. Heat of formation: The heat of formation of a substance at its elemental form is zero. e.g. O2 gas has a 0 kj/mol heat of formation. 3. Hess's Law -add partial reactions -when reversing a partial reaction, flip sign of deltaH -when you multiply by a coefficient, multiply DeltaH by the same.
Bond formation is (endothermic/exothermic) while bond breaking is (endothermic/exothermic).
Bond formation is exothermic. Bond dissociation is endothermic.
Describe the relationship between the change in free energy (Delta G) and the equilibrium constant (Keq) when: a) Delta G < 0 b) Delta G = 0 c) Delta G > 0
a) Delta G < 0; Keq >1 Products are favored b) Delta G = 0; Keq =1 Products and reactants are present in roughly equal amounts at equilibrium c) Delta G > 0 reactants are favored at equilibrium
Determine the effects on spontaneity based on the signs of H, S, and T. a) H = + and S = + b) H = + and S = - c) H = - and S = + d) H = - and S = -
a) Spontaneous at high T b) Nonspontaneous at all T c) Spontaneous at all T d) Spontaneous at low T
When standard state conditions no longer apply, one can predict the free energy change by looking at the ratio of Q over Keq. What does the following tell about the equilibrium: a) Q/Keq > 1 b) Q/Keq < 1 c) Q/Keq =1
a) The natural log will be positive, and the free energy change will be positive. In that case, the reaction will spontaneously move in the reverse direction until equillbrium b) Natural log will be negative, and vice versa of above. c) Natural log will be 0, thus, the free energy change is zero and the reaction is at equilibrium.
What is used to determine the heat transferred during the phase change when: a) at the Solid-liquid boundary b) at the Liquid-gas boundary
a) enthalpy (or heat) of fusion b) enthalpy (or heat) of vaporization
Explain what it means for the change in enthalpy (delta H), change in entropy (delta S), and the change in Gibb's Free Energy (delta G) to be: a) equal to zero b) greater than zero c) less than zero
a) equal to zero H: S: G: Thermodynamic Equilibrium b) greater than zero H: endothermic S: more order/less disorder G: endergonic c) less than zero H: exothermic S: less order/more disorder G: spontaneous, exergonic
When Gibbs Free Energy (DeltaG) is: a) positive b) negative c) zero What does it sign mean?
a) nonspontaneous => endergonic b) spontaneous => exergonic c) the system is in a state of equililbrium; H = TS
What does it mean for: a) Delta G = 0 b) Delta G > 0 c) Delta G < 0a
a) rxn is in EQ b) rxn is spontaneous c) rxn is nonspontaneous
