Chem final 2025

1st law of thermodynamics is

Energy cannot be created or destroyed (energy is conserved)

2nd law of thermodynamics is

The entropy of the universe increases in a spontaneous process and remains unchanged if the system is at equilibrium

3rd law of thermodynamics

The entropy of a perfect crystalline substance is zero at absolute 0 (= 0 K)

Spontaneous change is

a change that occurs under specified conditions without a continuous input of energy from an outside system

No spontaneous change is

a change that only occurs if the surroundings continuously supply the system with an input of energy

How to predict relative S (standard) values, when does S (standard) increase?

S (standard) increases as T rises. (Bc Temperature increase # of microstates possible)

S (standard) increases as a more ordered phase changes to a less ordered phase. Ex (s) to (g)

In similar substances, increases in mass usually give increases in entropy. Ex (for solids) diamond or graphite have Very strong C-C bonds meaning they are Rigid, and have little motion compared to Si and Sn which have more mass & weaker bonds meaning they are Less rigid, more motion 

S (standard) of a dissolved solid or liquid is usually greater than the S (standard) of the pure solute.  The extent of increase depends on the nature of solute & solvent.

How to predict relative S (standard) values, when does S (standard) decrease?

S (standard) of gas decreases significantly when it dissolves in a liquid or solid. Ex O2 in H20 as there is less freedom of motion for the gas

What is the relationship between entropy and states of matter, and how does it apply to solutions?

Entropy, representing "disorder," increases across states of matter as follows: \text{Solids} < \text{Liquids} < \text{Gases}. This is because chemical entities in a solid have the least motion (fixed in a lattice), while gases have the most. The formation of solutions is favored when it leads to a greater "disorder"; for example, when fewer moles of crystalline solids produce more moles of gas or solvated ions, the increased entropy favors the solution over the pure solute and solvent.

Standard Molar Entropies and the 3rd Law

•For enthalpy, there is no zero point, thus we can only measure changes

•For entropy, there is a zero point, we can determine absolute values with 3rd law

•The 3rd law of thermodynamics: a perfect crystal at 0 K has 0 entropy.

Describe entropy (S) as a state function.

Entropy (S) is a state function, similar to internal energy (E) and enthalpy (H). This means its value depends only on the current state of the system and not on the path taken to reach that state. The change in entropy for a system is calculated as \Delta S{system} = S{final} - S_{initial}.

Which is more disordered, NaCl(s) or a solution of NaCl in water?

NaCl in water

Which is more disordered, A solution of NaCl at 10 °C or 50 °C?

at 50 C

Which is more disordered, Butane or cyclobutane?

Butane (Because butane can exist in many more arrangements than the constrained cyclobutane, it has a higher molar entropy, meaning it is more disordered)

Which is more disordered, CO(g) or CO2(g)?

CO2 (bc it is a larger molecule with more atoms, leading to more possible microstates (arrangements of energy) and vibrational and rotational motions. The greater number of atoms and bonds in CO2 provides more ways for the molecule to store and distribute energy, which increases its entropy

Which is more disordered, graphite or diamond?

Graphite has more entropy than diamond because its structure is more disordered. Diamond has a highly ordered, rigid, three-dimensional crystal structure, while graphite consists of flat, layered sheets of carbon atoms that can slide past one another, allowing for greater disorder and higher entropy

Which is more disordered, Propane or propene?

Propane is more disordered because is an alkane with only single bonds between its carbon atoms. These single bonds allow for free rotation around the C-C axis. This means the molecule can exist in many different conformations (arrangements in space) that constantly interconvert. The more possible conformations a molecule can adopt, the higher its entropy (disorder). In comparison Propene is an alkene with a double bond between two of its carbon atoms. The double bond is rigid and restricts rotation around that specific C=C axis. This limitation in conformational changes means propene has fewer possible arrangements compared to propane. 

Will delta S be positive or negative for the following proccesses?

1. Freezing of ethanol

2. Evaporation of water

3. Dissolving sugar

4. Cooling N2 gas from 80 °C to 20 °C

5. The vapor pressure of H2(g) at 25 °C is lowered from 5 atm to 1 atm.

6. N2(g) + 3F2(g) arrow 2NF3(g)

1. Freezing of ethanol Negative (lowering temp)

2. Evaporation of water Positive (Evaporation of water has a positive entropy because the gas state has far more disorder than the liquid state)

3. Dissolving sugar Positive (the sugar molecules transition from a highly ordered crystalline structure to a more random, dispersed state throughout the liquid. This dispersal increases the number of possible arrangements (microstates) for the molecules, which is the definition of increased disorder or entropy. 

4. Cooling N2 gas from 80 °C to 20 °C Negative (lowering temp)

5. The vapor pressure of H2(g) at 25 °C is lowered from 5 atm to 1 atm. Positive because when the pressure of a gas decreases at a constant temperature, the volume available to the gas molecules increases. This increased volume allows for greater freedom of movement and a larger number of possible microstates, leading to an increase in molecular disorder or randomness.

6. N2(g) + 3F2(g) arrow 2NF3(g) Negative (4 to 2)