Thermodynamics Flashcard

State Functions
a property of a system that depends on the initial state and the final state of the system
Examples of State Functions
Enthalpy (H), Entropy (S), Free Energy (G), Temperature, Pressure, Volume
Calorie
the amount of energy needed to raise 1 gram of water 1°C
Calorie –> Joule Conversion Factor
1 calorie = 4.18 joule
Enthalpy (H)
the heat content of a substance or a system
Change in Enthalpy (?H)
the heat energy absorbed or released during a chemical or physical process
Enthalpy Unit
kJ/mol
Equation to Calculate ?H
?H = [sum of enthalpies of products]-[sum of enthalpies of reactants]
Exothermic Reaction
chemical potential energy is converted to heat energy and released to the surroundings
Examples of Exothermic Reactions
combustion; alkali or alkaline earth metal + H2O; Gas –> Liquid –> Solid
Graph of Exothermic Reaction
[image]
Sign on ?H if Exothermic
negative (-?H)
Endothermic Reaction
the system absorbs heat from surroundings and it’s converted to chemical potential energy
Examples of Endothermic Reactions
photosynthesis, any exothermic reaction performed in the reverse direction
Graph of Endothermic Reactions
[image]
Standard Reference Point for Enthalpy Values
Any free element in its most stable state at 25;C and 1atm has 0 enthalpy.
Three Characteristics of Enthalpy

1. It is an extensive porperty (mass dependent)

2.;;H for a reaction depends on the physical state of the reactants and products

3. If a reaction is endothermic in the forward direction, it is always exothermic in the reverse direction.

The Five Ways to Calculate;;H

1. From tables of enthalpy values

2. Stoichiometry

3. Hess’s Law

4. Calorimetry

5. Bond Energies

Enthalpies (heats) of Formation (Hf)
The enthalpy change associated with the formation of 1 mol of a compound from its constituent elements.
Thermite Reaction
Fe2O3 + Al –> Al2O3 + Fe
Hess’s Law
If a reaction is carried out in a series of steps,;;H for the reaction will be equal to the sum of the enthalpy changes for the individual steps.
When is Hess’s Law used?
Hess’s Law provides a useful means of calculating energy changes that are difficult to measure directly.
Rules for Hess’s Law

1. Look at the overall reaction for which you want the enthalpy change. Decide how to rearrange the given equations.

2. If an equation is multiplies by a coefficient, the ΔH value is multiplied too.

3. If a reaction is reversed, the sign on ΔH is also reversed.

4. Check to be sure that everything cancels except what you want.

Calorimetry
measures the heat flow accompaning chemical and physical processes.
Calorimeter
device used to measure heat flow (page 182)
Equation for Calculating Heat Released
Q = mcΔT
Secondary Equation About Heat Released (Involving ΔH)
Q = ΔH
Specific Heat Capacity (cp)
the amount of heat needed to raise the temperature of 1 gram of a substance by 1°C. It is intensive, so it is the same for any mass of an element. For water, it is 4.18 J/g.c or 1.00 cal/g.c
Heat Capacity (Unofficial HC)
the energy needed to raise the temperature of AN OBJECT by 1°C. Unit is J/C.
Molar Heat Capacity
the energy required to change the temperature of 1 mole of a substance by 1°C. The unit is J/mol.C
Bond Enthalpy (Energy)
the amount of energy required to break a given bond in 1 mol of a gasseous substance. Unit: kJ/mol
Equation Used for Bond Energy Enthalpy Change
ΔHrxn = [the sum of the enthalpies of bonds broken]-[the sum of the enthalpies of bonds formed].
What is the first step to solving an enthalpy problem using bond energies?
Draw the molecules involved and count the number of bonds.
Spontaneous
A process that occurs without outside intervention once the activation energy is provided.
Entropy (s)
the measure of the disorder in a substance or system.
Second Law of Thermodynamics
For any spontaneous process, the disorder of the universe must increase.
Equation for Finding ΔS
ΔS = [sum of the entropies of the products] – [sum of the entropies of the reactants].
What does a +ΔS mean?
Increased entropy, more disorder.
What does a -ΔS mean?
Decreased entropy, less disorder.
Isolated System:
A system that cannot exchange matter or energy with the surroundings (+ΔS)
Why does the universe naturally go to a state of increased disorder?
There is only one way to be ordered, billions of ways to be disordered. Probability.
What are the general trends in ΔS?

1. Solid –> Liquid –> Gas: +ΔS

2. Gas –> Liquid –> Solid: -ΔS

3. If temperature increases, +ΔS

4. When a gas expands, +ΔS

5. When a gas dissolves in liquid: -ΔS

6. When a solid dissolves in liquid: +ΔS

7. If the moles of gas in the reactants is less than the moles of gas in the product: +ΔS

Third Law of Thermodynamics
Any pure crystalline substance at absolute zero (0K = -273C) has an entropy of 0.
Formula for Calculating ΔSsurr
ΔSsurr = -(ΔHsys/Tsurr(K))
Formula for ΔSuniv
ΔSuniv = ΔSsys + ΔSsurr
Chart Comparing ΔHsys and ΔSsurr

ΔHsys         ΔSsurr

+            –

–            +

Chart Comparing ΔSsys, ΔSsurr, ΔSuniv, and spontenaeity
[image]
Formula to Calculate ΔG
ΔG = ΔH – TΔS
What does a -ΔG tell you?
it means the reaction is spontaneous. The process can do work. The magnitude of ΔG indicates the maximum amount of work that can be obtained from a process. (Think: a gallon of gas can only do so much work before it runs out)
What does a +ΔG tell you?
It means the process is nonspontaneous. The surroundings must do work on the system to make it go. The magnitude of ΔG indicates the theoretical minimum of work needed to drive the process.
Facts about ΔG

1. It is a state function.

2. The unit is kJ/mol

3. Any free element in its most stable state at 25°C and 1am has a free energy value of 0.

4. ΔG = [sum of the free energy of the products] – [sum of the free energy of the reactants].

Chart about ΔG and Temperature (*******)
[image]

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