Thermodynamics and Gibbs Flashcards
What are Joules?
Joules are a measure of energy used to quantify heat. It is measured in N*m ore = 1 kg*m^2/s^2
1 Cal = how many calories?
1 kcal or 1000 calories
What is the definition of a calorie?
The amount of energy required to raise the temperature of 1 gram of water by 1 degree celcius
What equation do we use to calculate heat?
Q=mcdeltaT m= mass of water c=specific heat capacity delta T= temp change
What is specific heat capacity (c)?
the amount of energy required to raise the tempurature of 1 gram of a substance by 1 degree celcius.
What is the specific heat capacity of water?
4.186 J/gdegreeC or 1 cal/gdegree celcius
Enthalpy
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
Is H - or + in endothermy?
+
Is H - or + in exothermy?
-
What are standard conditions?
1 atm and 25*C. Most enthalpy rxns will be conducted at this temp
What is the standard enthalpy of any element in its elemental state? (eg O2 is a gas)
deltaHformation=0
What is the standard enthalpy of formation?
Enthalpy of change associated with forming one mole of a compound from its elements under standard conditions
How do you calculate the enthalpy of a reaction?
deltaH rxn= deltH products= deltH reactants
What is the unit for enthalpy? What about change in enthalpy?
Enthlapy= joules, change in enthalpy=joules per mole
Bond Dissociation Energy
Enthalpy change associated with breaking bonds dlet H rxn= delt H bonds broken-delt H bonds formed
Is bond breakage endo or exothermic?
endo
Is bond formation endo or exothermic?
Exo
Hess’s Law
The total change in enthalpy is just the sum of the individual steps it took
What are the two ways to calculate enthalpy of reactions you will see on the MCAT?
Products- Reactants or Bonds Broken- Bonds formed
Gibbs free energy
Energy that can be used to perform work in a reversible reaction
What will the gibbs free energy be of a spontaneous reaction?
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
What is the equation for gibbs free energy?
See picture:
What are the 4 major equations to know for gibbs free energy?
deltG=H-TS and deltGrxn=Gproducts-Greactants and delta G naught prime=-RTlnK, and deltG=deltGprim + RTlnQ
What is the difference between delta G and delta G naught prime?
Delta G is not in standard conditions, delta G naught prime is at 25C 1 atm and 1M
If Keq is less than one, then what is favored?
reactants are favored (Keq=prod/react)
What is the difference between k and Q?
K is equilibrium whereas Q is the proportion of reactants to products at anypoint in the rxn
WHat is deltG at equilibrium?
0
What are the major differences between thermodynamics and kinetics?
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.
Describe the difference between the two reactions below:
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.
If Q=1 and Keq=0.5, will this reaction go twoards reactants or products?
Reactants
If Keq=0.8 and Q=0.62, what will be fovored- reactants or products?
Products
True or False: lower activation energy, faster rx
true, think about what a catalyst does
Will the thermodynamic or kinetic pathway be favored at high temps?
Thermodynamic
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?
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
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)
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
True or False: Entropy, like enthalpy, increases with temperature and as a material changes phase from solid to liquid to gas.
True
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.
True
True or False:
Entropy generally increases when a solid or liquid is dissolved in a solvent.
True or False: Entropy increases when the solubility of a gas decreases and it escapes from a solvent.
True
True or False:
Entropy generally increases as molecular complexity increases (KOH vs. Ca(OH)2) due to the increased movement of electrons.
True
Which of the following represents the enthalpy for the complete combustion of one mole of methane gas?
-840 kJ/mol
