Chem 120 Lecture 6 - Chapter 9: Nature of Energy

Energy

Capacity to do work/produce heat

Surroundings

Anything outside rxn

Also the universe outside of the system.

System

Studying part of the universe.

Will be considered as R and P

PV Work units

101.3 J = L x atm

Exothermic Reaction

Energy coming out of the system.

HOT!!!

Endothermic Reaction

Energy going into the system

COLD!!!

Potential Energy (PE)

Energy due to the position (mgh)

Kinetic Energy (KE)

Energy due to the objects motion (1/2 m v²)

PV Work

Work that results a change in V

Work also done on surroundings.

PV Work Formula

W = -P (change in V)

Internal Energy

The total energy of the system and the sum of PE + KE.

The change of the system = heat + work.

Internal Energy Formula

E = q + w

E = Change in the internal energy system

q = heat

w = work

What does q stand for?

The heat:

If -q = heat goes outside

If q = heat comes inside

Enthalpy

The measure of heat energy in a system at constant pressure. It includes the internal energy of a substance and the work done on or by the system.

Change in H Formula

H = (Qp = E + P x (V))

H = 5/2n x R x (change in T)

Change in Enthalpy Formula

H = n x Cp x T

H = change in heat

n = moles

Cp = Heat capacity with constant pressure

t = temperature (Use C)

Change in H in Endothermic Reaction

H + R —> P

Change in H in Exothermic Reaction

R —> P + H

Heat Flow of Ideal Gas at const V Formula

Qv = n x Cv x T = (Change in E)

Heat Flow of Ideal Gas at const P Formula

Qp = n x Cp x T = (Change in H)

State Functions

Property of system that depends on present state.

Examples of State Functions

Q and W are not state functions but E and H are state functions.

Molar Heat Capacity

Energy must change both translational energy to provide work gas does.

Substance defined as the energy needed to raise T by 1 K.

Molar Heat Capacity for constant v formula

Cv = 3/2 R

Molar Heat Capacity for constant p formula

Cp = Cv + R

Cp = 5/2 R

KE Average Formula

____= 3/2 R x T

Heat Flow Expression for constant v

Qv = nCv x T

T = Celcius NOT K

Heat Flow Expression for constant p

Qp = nCp x T

T = Celcius NOT K

How do you change the KE?

By changing the Temperature using the KE = 3/2 R x T formula

Average Transitional Energy

Energy of ideal gas by change of temp

Average Transitional Energy Formula

E = 3/2 R x T

Calorimetry

The measurement of heat transfer in a system. It helps determine the specific heat capacity or energy content of substances from chemical reactions.

If a system consists of 1 mol, what is it?

Molar Heat Capacity

Calorimetry Formula

Qrxn = -mCs x T

How does System and Surrounds affect Calorimetry?

System = Solid

Surrounds = Water

Calorimeter

Used to measure heat.

Standard Enthalpies of Formation: H Formula

Hrxn = Hf(products) - Hf(reactants)

Standard State in Condensed State

Pure Liquid/Solid

Standard State for Gas Pressure

1 atm

Substance in Solution of Standard States.

1 M at applied Pressure of 1 atm

Hess’s Law

The overall enthalpy change of a reaction.

Balancing formulas to find the change in H

Hess’s Laws Calulations

H must be reversed if rxn switches sides.

H must be multiplied to the same coefficients of rxn numbers.

From R → P, enthalpy = same either when rxn takes 1 or multisteps.

What is the best P and T where the gas behaves most ideally?

P = 0.5 atm T = 100 C