Thermodynamics Flashcards
(17 cards)
Spontaneous reactions
-reactions that occur without external intervention
Ex: ball spontaneously rolls down a slope, NaCl spontaneously, dissolves in water
-spontaneous reactions can be fast or slow. Thermodynamics tells us the direction in which a reaction will occur, but not the rate of the process.
Entropy
-Entropy (S) thermodynamic quantity that is often described as a measure of randomness in a system
-it’s about the number of possible microscopic arrangements called micro states that correspond to a system macroscopic state (what we observe: temperature, pressure)
Ex:
-if all the gas molecules are in one corner of a box that’s low entropy because there are a few ways to arrange molecules to get that result
-If the gas molecule is spread out evenly that’s high entropy because there are many ways for the molecules to be arranged and still look evenly spread out overall
-Entropy increases as the number of possible microscopic energy arrangements increase. Nature favours high entropy states because there’s just more ways for them to happen.
W
-the number of micro states- number of ways the components of a system like atoms, or molecules can be arranged without changing the observable state (macro state)
-4 possible macro states: within each macro state, the three particles can arrange themselves in the container in multiple ways, each arrangement is called a micro state
-if W is large, the larger the entropy (S) because there are lots of possible arrangements, and energy is more spread out or dispersed
-so entropy increases because the system is more random more flexible more unpredictable
Ex: liquid water has a larger entropy with energy, dispersed in many more ways with many more micro states than ice
Ice: molecules are fixed in a structured crystal. They can vibrate a little, but that’s it. Few micro states (low W so low entropy)
Liquid water: molecules are free to move, rotate vibrate, and slide pass each other. Way more micro states (high W) so higher entropy
-the system moves from ordered (ice) to disordered (water)
-systems naturally tend to move toward higher entropy because there are more ways for that to happen
What causes the increase of W and S
-the number of available energy states and entropy increase when
- Temperature is raised.
- Volume of system increases.
- When more particles are produced (solids, disassociate into ions)
- When products are in a more random state (solid-liquid-gas)
-Solid becomes a liquid
-liquid becomes a gas
Calculating entropy changes deltaS^0
-change in entropy for a process in which all reactants and products are in their standard states
Products minus reactants
S^0 = standard molar enthalpy (J/molk)
ni = stoichiometric coefficient of reactants/products
Absolute entropy (S)
-absolute entropy is a total amount of energy dispersed in a substance- reflects how much randomness exists in the arrangement and motion of those particles
-At 0K everything is frozen there’s no thermal motion so there’s only one way to arrange things
S^0 standard molar entropy (system)
-unit: J/mol x K
- State (most important)
-gas, liquid, solid - Larger the molar mass larger the entropy.
- Allotropes (same element different forms) looser structure = higher entropy
- Molecular complexity:
more complex molecules = higher entropy, even in same state
-more atoms, more bonds more ways to rotate vibrate flex - Dissolution
-Dissolving a solid usually increases entropy, when dissolved the energy that was concentrated within the crystal becomes dispersed throughout the entire solution
From second law thermodynamics
-when determining spontaneity, we look at the value of Delta S universe not Delta S system
-spontaneous reactions Delta S universe>0 (larger than zero) -the total entropy of the universe must be positive
Changes in entropy of the surroundings
-at low temperatures, the -Delta System is overcome by the large + delta Surroundin thus delta S universe is positive
-water freezes spontaneously below 0°C because the heat released on freezing increases the entropy of the surroundings enough to make delta S positive
-above 0°C the increase in entropy of the surroundings is insufficient to make delta S positive
Units of Delta S surroundings =
J/mol x K
Gibbs free energy
-DeltarG<0 for spontaneous reaction (less than zero)
-DeltarG> 0 for a non-spontaneous reaction (more than zero)
-deltarG = 0 at equilibrium
Effect of delta r H, delta R s, and tea on spontaneity
-always spontaneous: if Delta H is negative and Delta S is positive
-Always non-spontaneous: if Delta H is positive and Delta S is negative
-Spontaneous at low temperatures: if Delta S and Delta H are negative
-spontaneous at high temperatures if Delta H and Delta S are both positive
Delta H = KJ/mol
Delta S = J/mol
Calculating Gibbs free energy
delta r G
- Delta rG = delta rH-Tx Delta rS
Must be both in kJ or J
- Delta rG can be calculated from a series of reactions with no one, Gibbs energy
-the sum of the G values of the individual reactions is the G value of the total reaction
-If a reaction is reversed the sign of its G value reverses
-if the amount of materials is multiplied by a factor, the value of the G is multiplied by the same factor
What is Gibb’s energy?
-change in Gibbs energy of a chemical reaction represents the maximum amount of energy available to do work on the surroundings
-other than losing energy to change delta S, in a real reaction some of the free energy is lost as heat
Delta rG vs delta rG (0)
Delta rG = delta rG (0) only when the reactants and products are in their standard states
Under non-standard conditions : delta G = delta G 0 +RTInQ
Temperature dependent of K
-a plot of In K vs 1/T yield a straight line
Slope = -delta rH0/R
Y intercept: = delta rS0/R
DeltarH =-slopexR (KJ/MOL)
DeltarS = y interceptxR (J/molK)
-if DeltarH is known, you can calculate K at another temperature
