Princeton Ch 6 - Thermodynamics

Question 1

Thermodynamics.

Answer 1

The transformation of energy from one form to another.

Question 2

Zeroth law of thermodynamics.

Answer 2

If two systems are both in thermal eq with a 3rd system, then the two initial systems are in thermal eq with one another. Thermal eq = when temperatures are the same.

Question 3

First law of thermodynamics.

Answer 3

The total E of the universe is constant. E may be transformed from one form to another, but it cannot be created or destroyed. An isolated sys has constant E - no transformation of the energy is possible until the sys is in contact with something else. Work can be put into a sys to increase overall E.

Question 4

System versus surrounding.

Answer 4

The thing we are looking at: an ice cube, a solid dissolving into water, a beating heart - anything we want to study. Everything else: the table the ice cube is on, the surrounding air, the chest cavity holding the heart, is collectively known as the surrounding.

Question 5

Two basic statements regarding systems, surroundings and E flow.

Answer 5

When E flows into a sys from the surroundings, the E of the sys increases and the E of the surroundings decreases. When E flows out of a sys into the surroundings, the E of the sys decreases and the E of the surroundings increases.

Question 6

Enthalpy

Answer 6

The measure of the heat energy that is released or absrobed when bonds are broken and formed during a reaction that’s run at a constant pressure. Symbol = H.

Question 7

When a bond is formed, energy is released, so ΔH __ 0. Energy must be put into a bond in other to break it. ΔH _0.

Answer 7

When a bond is formed, energy is released, so ΔH < 0. Energy must be put into a bond in other to break it.
ΔH > 0 –> positive; we’re putting energy in.

Question 8

Equation for Enthalpy. Remember in a chemical rxn, reactant bonds are broken and then the atoms are rearranged to form products. As product forms, E is released.

Answer 8

ΔH = H(products) - H(reactants)

ΔH is also known as heat of reaction.

Question 9

If the product of a chemical rxn have __ bonds than the reactants, then more E is released in the making of product bonds than was put in to break the reactant bonds. In this case, energy is ___ overall from the sys, and the reaction is ___.

Answer 9

If the product of a chemical rxn have STRONGER bonds than the reactants, then more E is released in the making of product bonds than was put in to break the reactant bonds. In this case, energy is RELEASED overall from the sys, and the reaction is EXOTHERMIC. The products are lower in E and the ΔH is negative.

Question 10

If the products of a chemical rxn have weaker bonds than the reactants, then more E is put into breaking the reactant bonds. In this case energy is absorbed overall and the reaction is ____.

Answer 10

If the products of a chemical rxn have weaker bonds than the reactants, then more E is put into breaking the reactant bonds. In this case energy is absorbed overall and the reaction is ENDOTHERMIC. ΔH is positive, since heat had to be added to the system from surroundings.

Question 11

Standard conditions.

Answer 11

Under standard conditions, the temperature is 298K (25°C) and the pressure is 1 atm. All solids and liquids are assumed to be pure, and solutions are considered to be at a concentration of 1M. Values with “ ° “ superscript

Question 12

Standard temperate and pressure (STP).

Answer 12

0°C and 1 ATM.

Question 13

Standard heat of formation.

Answer 13

ΔH°f is the amount of energy required to make one mole of a compound from its constituent elements in their natural or standard state, which is the way element exists under standard conditions.

Question 14

What is the ΔH°f of O2? Cl2? H2?

Answer 14

ΔH°f is 0. The diatomic state is the NATURAL state for these elements at standard conditions. ΔH°f = 0 for O2, but for O, ΔH°f = 249kJ/mol at standard conditions, because it takes energy to break the O=O double bond.

Question 15

When ΔH°f of a compound is positive, then a ___ (input/output)of heat is required to make that compound from its constituent elements.

Answer 15

An input of heat is required. When ΔH°f is negative, making the compound from its elements give off energy.

Question 16

How do you calculate the heat of formation?

Answer 16

ΔH°f (reaction) =(∑n x ΔH°fproducts) - (∑n x ΔH°freactants)

n = moles

Question 17

Hess’s law of heat summation.

Answer 17

If a reaction occurs in several steps, then the sum of the energies absorbed or given off in all the steps will be the same as that for the overall reaction.

Question 18

If a reaction is reversed, the sign of ΔH is ___. If an equation is multiplied by a coefficient, then the ΔH must be multiplied by the same value.

Answer 18

If a reaction is reversed, the sign of ΔH is also reversed. If an equation is multiplied by a coefficient, then the ΔH must be multiplied by the same value.

Question 19

Are the following statements true:
Energy is needed to break a bond.
Energy is released in making a bond.

Answer 19

Yes. If it takes more energy to make a bond than break it, the molecule could immediately dissociate, as the bond is probably unstable.

Question 20

For bond energy calculates, how do we calculate enthalpy?

Answer 20

ΔHreaction = ∑(BDE bonds broken ) - ∑(BDE bonds formed).

Question 21

True or false. Using heat of formation and BDE to figure out enthalpy will always result in the same answer.

Answer 21

False. Bond energies are reported as the average of many examples of that type of bond, whereas heats of formation are determined for each individual chemical compound. The exact energy of a bond not only depends on the two atoms, but also the chemical encironment in which they reside.

Question 22

The second law of thermodynamics.

Answer 22

All processes tend to run in a direction that leads to maximum disorder.

Question 23

Equation for entropy.

Answer 23

Entropy is a masurement of disorder or randomness. IT is given by: ΔS = Sproducts - Sreactants

Question 24

True or false. Two moles of a substance have more entropy than one mole.

Answer 24

True.!

Question 25

When is entropy ΔS positive?

Answer 25

If randomness INCREASES, or order decreases, during a reaction, then ΔS is positive for the reaction. Ex. H2CO3 -> H2O + CO2 –> entropy is positive here because disorder is increased. It went from one molecule to two.

Question 26

Third law of thermodynamics.

Answer 26

The third law defines absolute zero to be a state of zero entropy. At absolute 0, thermal energy is absent and only the least energetic thermodynamic state is available to the system in question. S = 0. The t3rd law describes the least thermally energetic state, and therefore the lowest achievable temperature. 0 Kelvins!

Question 27

A gas is observed undergoing condensation. Are ΔH and ΔS positive or negative?

Answer 27

ΔH is negative and ΔS is also negative? Condensation is the phase change from gas to liquid, which RELEASES (since the reverse process, vaporization, requires an input of heat).

Question 28

Gibbs free energy ΔG.

Answer 28

The energy that available (free) to do useful work from a chemical reaction. Given by ΔG = ΔH - TΔS. The sign of ΔG will dictate whether or not a process is spontaneous.

Question 29

ΔG _ 0 –> spontaneous in the forward direction.
ΔG _ 0 –> reaction is at equilibrium.
ΔG _ 0 –> nonspontaneous in the forward direction.

Answer 29

ΔG < 0 –> spontaneous in the forward direction.
ΔG = 0 –> reaction is at equilibrium.
ΔG > 0 –> nonspontaneous in the forward direction.

Question 30

In general, the universe tends towards ____ disorder (ΔS) and stable bonds (___ ΔH).

Answer 30

In general, the universe tends towards INCREASING disorder (ΔS) and stable bonds (NEGATIVE ΔH).

Question 31

When ΔH and ΔS are both positive, and ΔG is negative at a high T, the reaction is ____.

Answer 31

Spontaneous.

Question 32

When ΔH and ΔS are both positive, and ΔG is positive at a sufficiently low T, then the reaction is.

Answer 32

NONspontaneous.

Question 33

Difference between thermodynamics and kinematics.

Answer 33

Thermodynamics predicts the spontaneity (and the equilibrium) of reactions, NOT their RATES. If you have a starting line, thermodynamics tells you how far you go and kinetics tells you how quickly you will get there.

Question 34

The ____ is the extra energy the reactants required to overcome the activation barrier and determines the kinetics of the reaction.

Answer 34

ACTIVATION ENERGY (Ea)

Question 35

True or false. The reverse reaction has the same magnitude for all thermodynamic values ( ΔH, ΔS, ΔG) but of the opposite sign, and the same reaction pathway, but in reverse.

Answer 35

True.