Exam 3 Pt. 1 - Thermodynamics at Standard Conditions

What is a spontaneous process? Can it be slow?

• Occurs without outside intervention; can be slow (e.g. rusting)

What are examples of spontaneous processes?

• Object falling

• Old iron rusting

What should a spontaneous reaction not be confused with?

• Rate of rxn NOT the same as spontaneous process

What is a non-spontaneous process?

• Requires input of outside energy

What is an example of a non-spontaneous?

• Lifting a book

• Recharging battery

What is entropy (s)?

• Level of disorderness

What does the 2nd law of thermodynamics state?

• For any spontaneous process, entropy of universe increases (ΔSuniv +)

How does entropy change with change in state?

• Entropy increases from (s) to (l) to (g)

When the process is more disordered what is ΔS?

• + ΔS (Sfinal > Sinitial)

When the process is less disordered, what is ΔS?

• - ΔS (Sfinal < Sinitial)

What is condenstion?

• Gas to liquid

What is sublimation?

• Solid to gas

What is evaporation/vaporatization?

• Liquid to gas

What is deposition?

• Gas to solid

When given a chemical equation and told to predict ΔS, what steps do you follow?

1. Check physical states (more important)

2. IF all the same; look at coefficients (more mols of gas on products, higher entropy)

What is the equation for ΔSuniv?

• ΔSuniv = ΔSsys + ΔSsurr

For a process to be spontaneous, what must occur to the entropy of ΔSuniv?

• Entropy of ΔSuniv must increase; for spontaneous 

Can a process still be spontaneous if entropy of ΔSuniv decreases? How? What is an example?

• Yes; when entropy of surroundings increases by greater amount

e.g. freezing water

What are the signs of ΔSsys and ΔSsurr in exothermic reaction?

• Energy released; ΔSsys<0 and ΔSsurr>0

What are the signs of ΔSsys and ΔSsurr in endothermic reactions?

• Energy absorbed; ΔSsys>0 and ΔSsurr<0 

What equation do you use to find the ΔS of surroundings under constant pressure?

• ΔSsurrr = -ΔHsys/T

How do you determine the sign of entropy change of universe? How is each component calculated?

• Use ΔSuniv = ΔSsurr + ΔSsys

ΔSsurr; use equation

ΔSsys; use patterns/look at chemical eq

What entropy must use to decide if a process is spontaneous?

• Entropy of universe (ΔSuniv)

What is Gibbs free energy be used to predict?

• Predict spontaneity 

What equation for gibbs free energy should be used?

• ΔG = ΔH -TΔS

What is an easy way to remembers the effect of ΔH, ΔS, and T on sponteneity?

• Universe favor +ΔS and -ΔH; the opposite is nonpontaneous

When both ΔS and ΔH are negative, how do you remember its spontaneity in relation to temperature?

• Only spontaneous at low temp.

When both ΔS and ΔH are positive, how do you remember its spontaneity in relation to temp?

• Only spontaneous at high temp.

Why would you need to know the effects of ΔS and ΔH on sponteneity?

• Not always given ΔH and ΔS values; use ΔH and ΔS changes

How do you know if ΔH increases or decreases in the system?

• Enthalpy (heat): endothermic or exothermic

How do you know if ΔS increases or decreases in the system?

• Entropy: reaction produces more or less mols of gas

consider other patterns as well

When calculating ΔG, what must you convert?

• Convert kJ to J

What units should ΔG be in?

• Joules

What do you do first when asked to find under what temperature conditions a reaction will be spontaneous?

• Set ΔG = 0 thus, 0 = ΔH - TΔS

If calculating for the temp at which a reaction is spontaneous, what wording MUST be included in answer?

• Spontaneous ABOVE ___K

What are the 4 patterns of sponteneity?

1. Physical states (s, l, g)

2. Molar mass

3. Molecular Complexity

4. Dissolution

What is the relationship of entropy (S) to molar mass?

• Larger molar mass; higher level of disorder

What is the relationship of entropy (S) to the amount of atoms (molecular complexity)?

• More atoms involved; higher level of disorder

What is more important molecular complexity or molar mass?

• Molecular complexity

What is the most important out of all four patterns of dissolution?

• Physical state, then molecular complexity 

What occurs to entropy with a larger amount of mols of gas?

• Entropy increases

What happens to the entropy (S) of a solid that has been dissolved?

• Entropy increases

What do you subtract for find ΔSrxn?

• (Mols of Products) - (Mols of Reactants)

What 3 ways can you use to calculate ΔG?

1. Using ΔGrxn=ΔHrxn - TΔSrxn

2. Using ΔGf values

3. Using stepwise equation

What is stepwise equation similar to?

• Hess’s Law

In stepwise equations, what must you do to the ΔGrxn when you reverse the equation?

• ΔGrxn value changes sign

In stepwise, equations what must you do to the ΔGrxn when multiplying the equation?

• ΔGrxn value is multiplied by same factor

What conditions are all of these equations and calculation under?

• Standard conditions only