Thermodynamics Flashcards Preview

MCAT Chemistry > Thermodynamics > Flashcards

Flashcards in Thermodynamics Deck (17):

In a piston, as gas is expanding, does the gas heat or cool?

The gas cools as it expands.
Because the gas/our system is doing work when it expands, the system loses energy. And because internal energy is proportional to temperature, the gas will cool.


In a piston, as a gas is being compressed, is it gaining internal energy or releasing internal energy?

Gains internal energy.
Work is being done on the system by the surrounding, so internal energy increases and gas warms as it is compressed.


Expanding gases cool or warm?



Compressed gas cools or warm?



What is the first law of thermodynamics?

total energy of universe is constant
Delta(E) = q + w


When is q(heat) positive or negative?

q = + when system gains heat/energy
q = - when system loses heat/energy


When is W(work) positive or negative?

W = + when work id done on system by surrounding
W = - when work is done by system


Does change in enthalpy depend on the pathway of a reaction?

Nope! It only depends on final and initial values.


What is the difference between standard state conditions and stnadard temperature and pressure (STP)?

Standard state is when T = 298 K (25 degrees Celsius) and P = 1 atm and all solids and liquids are assumed to be pure and concentrations are assumed to be at 1 M, values determined at standard state have a superscript (like a degree sign)

STP is when T = 273 K (O degrees Celsius!!!)


Examples of elements in their natural or standard state, so heat of formation equals 0

C(s), graphite


What are the standard states of the halides?

F2 (g)
Cl2 (g)
Br2 (l)
I2 (s)


When H = + and S = -, what is G? Is the reaction spontaneous?

G is positive and reaction is not spontaneous.

Think of it this way, H = + means it's endothermic and requires more energy to break a bond than the energy released, that is not very favorable because you want a final product with a lower energy state so it will release energy. The 2nd law of thermodynamics also says that disorder if favored, that mean S = - is not as favorable because it would be a decrease in entropy


At high temperatures with H = + and S = +, is the reaction spontaneous?

Reaction is spontaneous and G is negative.

This is because higher temperatures makes entropy (S) have a bigger effect on G, so during high temperatures, G will follow what S is.

At low temperatures, S has little effect and you pay attention more to what H likes.


When equilibrium constant equals 1, reaction is driven by?

Le Chatelier's Principles because at equilibrium, the concentration of products is equal to the concentration of reactants


What is the difference between deltaG and deltaG°?

deltaG tells you if a reaction will be spontaneous in one direction or the other direction, this value is affected by changing reaction composition until it reaches 0, equilibrium

deltaG° is the relative proportions of products and reactants present at equilibrium, at constant temperatures this value is constant

deltaG = deltaG° + RTlinQ
deltaG° = - RTlnKeq


What is happening during an adiabatic process?

No heat is exchanged between the system and surrounding so q = 0 and the change in internal energy of the system is equal to the work done. If a gas is compressed, work is being done on the gas so that would lead to a positive W and positive internal energy which is correlated with temperature so the temperature will increase.


How is heat and moles related to enthalpy?

deltaH = q/n [J/mol]