Physics - Chapter 4: Thermodynamics - Section 1

Thermodynamics and Kinetics

where a reaction wants to be ; How fast the reaction moves in that direction

Examples of path functions

work and heat

Entropy decreases when

you build larger molecules out of smaller constituents.

Entropy increases when..

you move from solid to liquid to gas

Conversion b/w Temperature Scales

°C = (°F - 32) x 0.5 °F = (°C x 2) + 32

ΔG under non-standard conditions

ΔG = ΔG° + RT ln Q

The relationship between Gibbs free energy and Keq equation...

ΔG° = -RT ln Keq *relates the spontaneity of the reaction under standard conditions to its equilibrium state

Measuring expansion in terms of linear expansion

ΔL/L = αLΔT

Measuring expansion in terms of volume expansion

ΔV/V = αVΔT

- W and + W

(+ positive) energy being added to a system (- negative) energy being taken away from a system

Thermodynamic product:

- More stable product (more negative ΔG)? - High activation energy - High Temperature

Kinetic product:

- product forms quickly - Low activation energy - Low Temperature

General conversions

-40°C = -40°F 0°C = 32°F 10°C = 50°F 20°C = 68°F 37°C = 98.6°F 40°C = 104°F 100°C = 212°F

Describe all subtypes of a thermodynamic system

1) A system that exchanges neither matter nor energy with its surroundings 2) A system that exchanges energy but not matter with its surroundings 3) A system that exchanges both energy and matter with its surroundings

In which of the following situations is it appropriate to work in degrees Celsius instead of Kelvin? Select all that apply.

1) When determining the amount of heat needed to change the temperature of a substance by a certain amount 2) When determining the change in length of a metal wire as it is heated up

Equilibrium State

> ΔG and Keq (equilibrium constant) > Keq = [products] / [reactants]

Kelvin

A Temperature scale: 0 K = -273.15°C = -459.67°F Kelvin and Celsius have the same increment scale K = °C + 273 and °C = K - 273

Fahrenheit

A Temperature scale: Freezing point of water = 32° F Boiling point of water = 212° F Normal Body Temperature = 98.6° F

Celsius

A Temperature scale: Melting point of water = 0° C Boiling point of water = 100° C Normal Body Temperature = 37° C

Thermal expansion

The tendency of matter to change its shape, area, volume, and density in response to a change in temperature.

Which law of thermodynamics is this example: Your arm and the table are at thermal equilibrium. Your clothes are at thermal equilibrium with your arm. Therefore, the table and your clothes are at thermal equilibrium.

Third Law states that crystals have no vibrational energy at absolute zero.

True or False: Catalysts do not affect the spontaneity of a reaction

True

True or False: Heat and temperature are different things

True

True or False: Spontaneity depends on Delta G

True

True or false: Kinetic energy can be converted into heat.

True: An application of energy conservation is the idea that kinetic energy can be converted into heat, especially when there are two objects in direct contact. If you think about gases on the molecular level, they are constantly hitting each other and transferring heat among themselves! Temperature is directly proportional to the average kinetic energy of the gas particles.

Isochoric system

Volume is held constant

ΔU = Q + W

Work is performed by the surroundings on the system

Second Law of thermodynamics also states..

You cannot reduce entropy spontaneously

Which law of thermodynamics is this example: At zero Kelvin, the molecules in a perfect crystal are perfectly still.

Zeroth Law

Q = Keq

at equilibrium; the ΔG of the reaction must be zero

Which law of thermodynamics is this example: A scientist fails to design a perpetual motion machine because it never seems to result in a net gain of energy.

first law of thermodynamics It states that energy and matter cannot be created or destroyed in an isolated system. This prevents the invention of a true perpetual motion machine since it will eventually lose energy to the environment over time due to the second law and cannot generate energy out of nothing due to the first law.

Catalysts

increase the rate of a reaction by lowering its activation energy. Consequently, this increases the rate of the reaction in both the forward and reverse directions.

How is temperature related to kinetic energy?

kinetic energy is directly proportional to temperature

Entropy

measure of disorder or randomness in a system

Adjusting the temperature of a reaction can be used to control which products are made and in what proportions

no definition

Equilibrium: Rate (forward) = Rate (reverse) k1[reactants] = k1[products] k[1]/k[-1] = [products]/[reactants]

no definition

If we know the amounts of reactants and products at a snapshot in time relative to where those concentrations want to be at equilibrium, we can predict the direction in which the reaction is likely to proceed spontaneously.

no definition

Kinetics does not care about the equilibrium state, only about the activation barrier

no definition

One way to frame how spontaneous or thermodynamically favorable a reaction is would be to express is would be to express its Gibbs free energy change, which is a measure of spontaneity, in terms of the equilibrium state of the reaction.

no definition

The equilibrium state of a reaction are focused on the thermodynamics characteristics of that reaction: How stabie the products are compared to the reactants.

no definition

Thermodynamics does not care about time. A reaction may be thermodynamically favorable but proceed very slowly, if its kinetically unfavorable due to a high activation barrier.

no definition

Thermodynamics has more to do with stability; How stable are the products relative to the reactants? kinetics has a lot to do with movement

no definition

spontaneity and speed of reaction are two different concepts!

no definition

the equilibrium constant keq is used only at equilibrium, Q, the reaction coefficient, may be used for reactant and product concentrations at any point in the reaction.

no definition

Adiabatic system

no heat or matter is transferred between a system and its surroundings - correspond to constant entropy values

Isobaric system

pressure is held constant in a system

Keq = 1

products and reactants are favored equally. ΔG° = 0 > This means that the forward and reverse reactions are equally favorable.

Keq > 1

products are favored over reactants = -ΔG° = spontaneous in the forward direction from reactants -> products (products are favored at equilibrium)

What is Temperature?

A property of the average kinetic energy of the particles that compose a substance

Activation barrier

Amount of activation energy that must be overcome for the reaction to go to completion. It represents the transition state - Unstable, high energy

Entropy can also be thought of..

As a measure of how much energy is available to do work. Energy and entropy are connected.

Third Law of Thermodynamics

At the lowest possible temperature (0 Kelvin) a pure crystal has zero entropy. - As we reduce the temperature of ice so that it approaches 0 Kelvin, those vibrations in the crystal structure will lessen and eventually stop completely. At which there is only 1 possible molecular configuration, and the entropy of the system is 0.

The kinetics of the reaction, how quickly it proceeds depend on the activation barrier between the starting point of the reaction and its destination...

Because, most of the time a reaction will proceed with the pathway that leads to the most stable product = the pathway with the most negative delta G. > However, when one reactant can form two or more different products, the concept of thermodynamic vs. kinetic control is applied.

Really cold and Really hot Temperatures (MCAT)

Below -40°C and Above 100°C

When calculating change in Temperature what scale do you use?

Celsius

What are path functions?

Describe how you get from one equilibrium to another - transitions

What are state functions?

Describe the state of the system w/o any reference to how the system got that way.

What is Heat?

Describes a type of energy transfer; Involves the transfer of energy through Temperature.

What is Conduction?

Direct transfer of heat between substances in contact with each other; mediated through the transfer of kinetic energy from the particles of one substance to those of the other; Can occur across all phases of matter. Good conductors = transmit heat well Good insulators = poor conductors of heat

What is Convection?

Direct transfer of kinetic energy; Heat transfer due to circulation of fluids (liquid or gas) - Can occur naturally due to thermally-related difference in the density of fluids.

What is Radiation?

Does not require direct contact between substances- Objects with heat emit EM waves, which transfer energy to objects they hit. Sunlight = thermal radiation. EM waves are generated by objects with a temperature greater than absolute zero.

What are open systems?

Exchange both matter and energy with their surroundings. Most of the systems we encounter in our daily lives.

What are closed systems?

Exchange only energy with their surroundings, but not matter.

Heat transfer

Heating a substance will cause it to expand; because increasing the kinetic energy of the molecules in a substance will mean that they start moving more and therefore separate to some extent.

Zeroth Law

If System A is in thermal equilibrium with Systems B and C, then systems B and C must be in thermal equilibrium with each other.

Second Law of Thermodynamics

If two objects are in thermal contact but not in thermal equilibrium, heat energy will spontaneously flow from the object with the higher temperature to the object with the lower temperature; The entropy of an isolated system will increase over time.

ΔU = Q - W

In closed systems = the total energy change of a system (ΔU) is equal to the transfer of energy into the system via heat minus the work performed by the system on its surroundings.

Thermodynamic conditions relevant for pressure/volume curves

Isobaric, Isothermic, Adiabatic, Isochoric

When solving a quantitative problem using T, what scale do you use?

Kelvin (don't have to worry about negative temperatures)

ΔL and ΔV are proportional to ΔT and proportional to..

L = length and V = volume

More degrees of freedom equal?

More entropy

What are isolated systems?

No exchange of energy or matter with their surroundings

A Thermodynamic system is at thermodynamic equilibrium internally. What is thermodynamic equilibrium?

No macroscopic transfers of energy or matter are taking place within a system

A consequence of the 2nd Law of Thermodynamics..

Perpetual motion/work machine are impossible. Also predicts that the entropy in the universe as a whole (the only truly isolated system) is increasing over time.

Work formula for a gas

Pressure x ∆V

Examples of state functions

Pressure, density, temperature, volume, enthalpy, internal energy, Gibbs free energy, and entropy

Which law of thermodynamics is this example: A hydroelectric dam converts only part of the potential energy of the stored water into usable electricity.

Second Law states that the entropy of the universe is always increasing, which precludes any and all perfectly efficient energy transfer (since that would imply a net entropy increase of zero for the process).

The atoms/molecules are more restricted in...

Solids, which are more restricted than those in liquids, which are more restricted than in gases

(+) ΔU, Q, W (-) ΔU, Q, W

System gaining energy, Heat flow into system, Work by system System losing energy, Heat flow out of system, Work on system

Standard Conditions

Temperature = 25°C Pressure = 1 atm Concentrations of all reactants and products: 1M

Describes an isochoric system

Temperature and pressure can change, but volume remains constant.

Which of the following describes an isobaric system?

Temperature and volume change, but pressure remains constant

In which of the following situations is it necessary to work in Kelvin?

The Gibbs free energy equation (∆G = ∆H − T∆S) must use absolute temperatures, not relative change in temperature. The ideal gas law equation (PV = nRT) must use absolute temperatures, not relative change in temperature.

Work

The area under the curve on a pressure-volume graph.

Describe what occurs as the temperature of a gas increases.

The average kinetic energy of the gas molecules increases. The frequency of collisions between molecules increases.

The change in length/volume that occurs with thermal expansion is proportional to..

The change in length/volume that occurs with thermal expansion is proportional to the original length or volume of the substance.

What underlying process causes a cold object and a hot object to reach thermal equilibrium when they are in contact with each other?

The heat from the hotter object transfers to the cold object through conduction. (Conduction requires two objects to be in contact with one another. Then, heat always travels from the warmer object to the cooler object.)

Keq < 1

reactants are favored over products = +ΔG° = non-spontaneous in the forward direction: it will want to proceed in the reverse direction toward the reactants (which are favored at equilibrium)

Isothermic system

temperature is constant

equlibrium

the end point of a reversible chemical reaction rates of the forward and reverse reactions are equal

How high the activation barrier is depends on..

the mechanisms and steps of the reaction and determines how quickly the reaction will proceed

Degrees of Freedom

the number of specific molecular configurations that an observable substance can have.

First Law of Thermodynamics

the principle of conservation of energy. The total energy in an isolated system is constant.

Q > Keq

the proportions of products to reactants is higher than it would like to be at equilibrium so the reaction will want to proceed in reverse to achieve equilibrium. (products -> reactants)

Q < Keq

the proportions of products to reactants is lower than it would like to be at equilibrium so the reaction will want to proceed in forward to achieve equilibrium. (reactants -> products)

Kinetics is concerned with...

the rate of the reaction. Kinetics tells us the speed of our vehicle.

Thermodynamics is concerned with..

the stability of the chemical players in a reaction; with the spontaneity (ΔG) of that reaction, and with where the reaction would like to be at equilibrium

Transition state

the state that the reactant molecules must transition through to become product molecules