AP Chem Unit 3 Test

Endothermic

• Energy gained

• Energy as reactant

• ΔH positive

• Bonds break

• Reactants will have less PE than products

Exothermic

• Energy lost

• Energy as product

• ΔH negative

• Bonds form

• Reactants will have more PE than products

temperature

How hot something is; reflects the average translational KE of molecules

Heat

thermal energy transfer—flows thermodynamically favorably from hot to cold places

On a heating curve, PE changes:

During the phase change (plateau)

On a heating curve, KE changes:

During temperature changes

Specific heat capacity

The energy required to raise the temperature of one gram of substance one degree Celsius

Units of specific heat

J/g˚C

Heat of formation (ΔH_f)

The heat gained or lost when ONE mole of compound is formed from its elements under STANDARD conditions

*when balancing chemical equation, need to make sure products only have ONE mole of product

What is the ΔH_f and ΔG_f of elements in their standard state?

0

Hess’ Law

The total energy change in a chemical reaction will be the same if the reaction happens in one step or several steps

ΔH

change in enthalpy

ΔH units

kJ/mol, AKA Q_sys/mol_sys

q_sys = ?

-q_surr

H

enthalpy: the total heat content of a system

S

Entropy: The measure of the dispersal of matter/energy

The more possible microstates (arrangements), the _____ the entropy

higher

What factors affect entropy (in order of importance)?

1. Phase

2. Moles

3. Temperature

4. Molecular complexity

more moles = ____ entropy

higher

higher temp = ___ entropy

higher

larger molecules = ___ entropy

higher

Positive ΔS

increase in entropy, increase in dispersal/randomness

Negative ΔS

decrease in entropy, decrease in dispersal/randomness

Second law of thermodynamics

The entropy of an isolated system does not decrease—”the entropy of the universe must increase”

Third law of thermodynamics

The entropy of a perfect crystal at absolute zero (0 K) is exactly equal to zero. 

Equation for ΔS_surr

ΔS_surr = -ΔH/T

Spontaneity

A spontaneous reaction is thermodynamically favorable and proceeds to completion without any outside help.

Is +ΔH favorable?

No, since energy needs to be put into the system

Is -ΔH favorable?

Yes

Is +ΔS favorable?

Yes, 2nd law of thermodynamics

Is -ΔS favorable?

No

Gibb’s Free Energy

The maximum amount of useful work that can be obtained from a process at standard conditions

+ΔG

Reaction/process is NOT thermodynamically favorable/spontaneous

-ΔG

Reaction/process IS thermodynamically favorable/spontaneous

Relationship between ΔH, ΔS and ΔG

ΔG˚ = ΔH˚ - TΔS˚

what units of temperature are used in big mama?

KELVIN

Relationship between ΔG and equilibrium

At equilibrium, ΔG = 0, which means equilibrium is the point where a reaction will become thermodynamically favorable or not favorable

Calculate the temperature at which a reaction becomes spontaneous

T = ΔH/ΔS (from big mama equation)

Kinetic Energy (KE)

The energy an object has due to its movement

Bond Dissociation Energy (BDE)

The energy gained/lost to break/form a specific chemical bond in a molecule

How to calculate ΔH_rxn using BDE

ΔH_rxn = ∑bonds broken in reactants - ∑bonds formed in products