Thermodynamics and Gibbs

Question 1

What are Joules?

Answer 1

Joules are a measure of energy used to quantify heat. It is measured in N*m ore = 1 kg*m^2/s^2

Question 2

1 Cal = how many calories?

Answer 2

1 kcal or 1000 calories

Question 3

What is the definition of a calorie?

Answer 3

The amount of energy required to raise the temperature of 1 gram of water by 1 degree celcius

Question 4

What equation do we use to calculate heat?

Answer 4

Q=mcdeltaT m= mass of water c=specific heat capacity delta T= temp change

Question 5

What is specific heat capacity (c)?

Answer 5

the amount of energy required to raise the tempurature of 1 gram of a substance by 1 degree celcius.

Question 6

What is the specific heat capacity of water?

Answer 6

4.186 J/gdegreeC or 1 cal/gdegree celcius

Question 7

Enthalpy

Answer 7

Total heat in a system H=U+PdeltaV where U= internal energy of the system H= enthalpy P=Pressure V=Volume So heat of the system is equal to the internal eneryg plus the work required to maintain it Q=deltaH

Question 8

Is H - or + in endothermy?

Answer 8

+

Question 9

Is H - or + in exothermy?

Answer 9

-

Question 10

What are standard conditions?

Answer 10

1 atm and 25*C. Most enthalpy rxns will be conducted at this temp

Question 11

What is the standard enthalpy of any element in its elemental state? (eg O2 is a gas)

Answer 11

deltaHformation=0

Question 12

What is the standard enthalpy of formation?

Answer 12

Enthalpy of change associated with forming one mole of a compound from its elements under standard conditions

Question 13

How do you calculate the enthalpy of a reaction?

Answer 13

deltaH rxn= deltH products= deltH reactants

Question 14

What is the unit for enthalpy? What about change in enthalpy?

Answer 14

Enthlapy= joules, change in enthalpy=joules per mole

Question 15

Bond Dissociation Energy

Answer 15

Enthalpy change associated with breaking bonds dlet H rxn= delt H bonds broken-delt H bonds formed

Question 16

Is bond breakage endo or exothermic?

Answer 16

endo

Question 17

Is bond formation endo or exothermic?

Answer 17

Exo

Question 18

Hess’s Law

Answer 18

The total change in enthalpy is just the sum of the individual steps it took

Question 19

What are the two ways to calculate enthalpy of reactions you will see on the MCAT?

Answer 19

Products- Reactants or Bonds Broken- Bonds formed

Question 20

Gibbs free energy

Answer 20

Energy that can be used to perform work in a reversible reaction

Question 21

What will the gibbs free energy be of a spontaneous reaction?

Answer 21

Negative. A spontaneous reaction is one where the products are lower in energy than the products so we see a negative change in gibbs free energy

Question 22

What is the equation for gibbs free energy?

Answer 22

See picture:

Question 23

What are the 4 major equations to know for gibbs free energy?

Answer 23

deltG=H-TS and deltGrxn=Gproducts-Greactants and delta G naught prime=-RTlnK, and deltG=deltGprim + RTlnQ

Question 24

What is the difference between delta G and delta G naught prime?

Answer 24

Delta G is not in standard conditions, delta G naught prime is at 25C 1 atm and 1M

Question 25

If Keq is less than one, then what is favored?

Answer 25

reactants are favored (Keq=prod/react)

Question 26

What is the difference between k and Q?

Answer 26

K is equilibrium whereas Q is the proportion of reactants to products at anypoint in the rxn

Question 27

WHat is deltG at equilibrium?

Answer 27

0

Question 28

What are the major differences between thermodynamics and kinetics?

Answer 28

Thermodynamics is concerned with stability, spontaneity, and it does not care about time. Kinetics is concerned with the rates of the reactions and the activation energy and it does not care about equilibrium.

Question 29

Describe the difference between the two reactions below:

Answer 29

The blue coordinate describes a reaction that has a more stable product (lower delta G) but a higher activation energy whereas the green coordinate has a lower activation energy but a less stable product (higher delt G). This is the difference between kinetically and thermodynamically favored products. At high tempuratures where there is more energy available, the blue will proceed. At low tempuratures, the green will proceed.

Question 30

If Q=1 and Keq=0.5, will this reaction go twoards reactants or products?

Answer 30

Reactants

Question 31

If Keq=0.8 and Q=0.62, what will be fovored- reactants or products?

Answer 31

Products

Question 32

True or False: lower activation energy, faster rx

Answer 32

true, think about what a catalyst does

Question 33

Will the thermodynamic or kinetic pathway be favored at high temps?

Answer 33

Thermodynamic

Question 34

An object made of silicon (specific heat = 698 J/kg°C) absorbs 3500 J of heat while increasing its temperature from 43°C to 53°C. What is the approximate mass of the object?

Answer 34

The most efficient way to find this answer is by using dimensional analysis and the equation Q (the heat in or out of a system) = mc?T. Plugging in numbers, we get:

3500 J = m x 698 J/kg°C x (53°C - 43°C)

We can rearrange to get:

m = 3500 J / [698 J / kg°C] *10°C ≈ 3500/7000 kg = 0.5 kg = 500 g

Question 35

Sodium carbonate is formed by heating the suspended sodium bicarbonate to 300°C. ΔH° for Reaction 5 is 128 kJ/mol at 25°C.What is ΔS° for the reaction at 25°C?

2NaHCO₃ (s) –> Na₂CO₃ (s) + CO₂ + H₂O (g)

Answer 35

To solve this problem, we use the equation for Gibbs free energy: ΔG° = ΔH° - TΔS°. However, we are only given information for ΔH° and T, so to solve this problem, we need to first find a value for ΔG°. Table 1 provides the values of the free energies of formation at standard state for the compounds involved in Reaction 5. We can use these values to calculate the ΔG° for the reaction based on the reaction stoichiometry. Remember that we need to pay attention to the stoichiometry and the signs of each of the values! We also need to be careful to use the right state of matter – two values are given for H2O, and we need to use the one for gaseous H2O since that is what is given in the reaction.

ΔG° = Σ(ΔGf°products) - Σ(ΔGf°reactants) 
ΔG° = (-1040 kJ/mol + -400 kJ/mol + -240 kJ/mol) – 2(-850 kJ/mol) 
ΔG° = -1680 kJ/mol + 1700 kJ/mol) 
ΔG° = 20 kJ/mol 

Now that we have ΔG°, we can go back to the Gibbs free energy equation, ΔG° = ΔH° - TΔS°, and use it to calculate the ΔS° for the reaction. Be careful to use the right temperature in this equation! Although the reaction occurs at 300°C, the data are provided for 25°C, and the questions asks for 25°C. Also, be sure to use Kelvin instead of Celsius for the calculation!

T = 25 + 273 = 298 K ≈ 300 K

ΔG° = ΔH° - TΔS° 
ΔS° = (-ΔG° + ΔH°)/T 
ΔS° = (-20 kJ/mol + 130 kJ/mol)/300 K = 110/300 = 0.37 kJ/mol•K = 370 J/mol•K

Question 36

True or False: Entropy, like enthalpy, increases with temperature and as a material changes phase from solid to liquid to gas.

Answer 36

True

Question 37

True or False: Reactions that increase the number of moles of substances in the system (or produce more gas particles) typically increase the entropy of the system.

Answer 37

True

Question 38

True or False:

Answer 38

Entropy generally increases when a solid or liquid is dissolved in a solvent.

Question 39

True or False: Entropy increases when the solubility of a gas decreases and it escapes from a solvent.

Answer 39

True

Question 40

True or False:

Entropy generally increases as molecular complexity increases (KOH vs. Ca(OH)2) due to the increased movement of electrons.

Answer 40

True

Question 41

Which of the following represents the enthalpy for the complete combustion of one mole of methane gas?

Answer 41

-840 kJ/mol