AP chem unit 2

Davis North allegheny 2025-26

light is described in terms of waves and particles. when talking about spectrophotometry what do we view them as?

it is most convenient to view them as a stream of discrete particles called photons (when they ENTER the molecule

• the spectrophotometer measures the intensity of light by wavelength

energies relationship with frequency

proportional ( E = hv , h is plancks constant/ v is frequency)

the three possible situations to occur when light strikes and object

1. reflected

2. absorbed

3. transmitted through the object

how does color occur?

absorption of visible light

absorption

the process by which chemical species selectively remove certain frequencies of electromagnetic radiation

how does a photon affect an atom

a photon converts the atoms/molecule to a more energetic or excited state → energy is lost as heat

what is the effect of the absorption of specific wavelengths

Species which absorb visible light will appear to the eye as the complementary color of the region of the spectrum which has been observed

what happens if no visible light is absorbed

object appears colorless

How do wavelengths help identify materials

the specific wavelengths that are absorbed are unique to each atom/molecule → useful in identifying an unknown (qualitative analysis)

quantitative analysis (in the experiment)

how much material is present in an unknown sample (more concentrated solution absorbs more/transmits less light → appears more intensely colored)

calibration curve (experiment)

the slope/line of known concentrations —> used to find unknown

visible colorimetry

using a series of known concentrations to approximate the concentration of an unknown (by visual comparison)

• not used bc not very accurate

how does spectrophotometry work

1. LED + tungsten bulb used to transmit light thru the sample

2. transmitted light passes thru a diffractor

3. light is sorted/collected by a detector

Why do we calibrate the spectrophotometer w/ a blank

to compensate for…

1. small reflective losses for the cuvette

   any absorption by the container/ solvent in which the sample is dissolved

What is transmittance

ratio of the intensity of light transmitted through the sample to that transmitted through the blank

%T = blue/pink

How is absorbance related to transmittance

logarithmically

• absorbance = log(100/ %T)

How do we choose the wavelength to test the absorbance at

wavelength of maximum absorbance is used because

1. gives the highest sensitivity

   • largest absorbance change per concentration change

2. most reliable measurements

   • small errors in wavelength near the maximum have little effect on absorbance.

3. Measurements are also typically kept within the 0.2-0.7 absorbance range

   • ensure accumulation, since Beer’s Law is most linear there

what is the wavelength of maximum absorbance called

working wavelength (found from absorbance/wavelength graph)

What is Beers Law!

equation for the relationship between the intensity and the concentration of a thing (directly proportional relationship)

Equation of beers law (and its components)

A = εbc

A: Absorbance (unitless)

ε: proportionality constant known as molar absorptivity (unique to each thing & wavelength)

• probability of a photon being absorbed + causing an electronic transition ( L / mol*cm)

b: amt of solution light passes thru

c: concentration of solution (mol/L)

How does an air bubble affect the results

the light is reflected off the bubble, making the reader detect a high absorbance than present (light is moved away from the detector so not all is accounted for)

subatomic particles

Subatomic particles are particles smaller than an atom. They include protons, neutrons, and electrons which make up atoms.

cathode rays

streams of negatively charged particles

alpha particles

Helium nucleus (2 protons, 2 neutrons)

law of conservation of mass

made by antoine Lavoisier, reactants must equal the total mass of the products

Law of multiple proportions

John Dalton, given compound should always contain the same combo of atoms

electrons

subatomic particle w/ negative electric charge

protons

positively charged subatomic particles

neutrons

subatomic particles with no charge

nucleus

Center of an atom (protons and neutrons)

average atomic mass

the weighted average of the atomic masses of the naturally occurring isotopes of an element

atomic mass unit

a unit of mass that describes the mass of an atom or molecule

angstrom

A unit of length use to measure wavelength (nm + m also used)

atomic number

the number of protons in an atom

mass number

the sum of the number of neutrons and protons in an atomic nucleus

isotopes

Atoms of the same element that have different numbers of neutrons

What does isotopic notation show?

The chemical symbol, atomic number, and mass number for an isotope of an element.

What is included in isotopic notation?

Chemical symbol, atomic number, and mass number.

percent abundance

the naturally occurring amount of a certain isotope of an element

percent composition

the percent by mass of each element in a compound

mass spectrometry

a technique that separates particles according to their mass (used to find isotopes)

fragments

smaller, charged pieces of a molecule produced during ionization in a mass spectrometer

UV-Vis spectroscopy

analytical technique that measures the absorption of ultraviolet (UV) and visible (Vis) light by a sample

functional groups

A specific configuration of atoms involved in chemical reactions.

(each interact w/ light in a specific way)

conjugated pi system

alternating single and double bonds (aka degree of conjugation)

How to know if the thing is an UV observer or Vis

if <4 degree --> Uv

if >4 degree --> visible

>4 --> visible

IR spectroscopy

IR light causes bonds to vibrate at different frequencies, shows which functional groups are present

What is x-ray crystallography?

A technique used to study the three-dimensional structure of a molecule.

What does x-ray crystallography depend on?

The diffraction of an X-ray beam by the individual atoms of a crystallized molecule.

electromagnetic radiation

a form of energy that exhibits wavelike behavior as it travels through space

waves

waves are oscillating electric and magnetic fields that move through space

Emission Spectrum

a spectrum of the electromagnetic radiation emitted by a source

wavelength

The distance between two corresponding parts of a wave

Frequency

The number of complete waves that pass a given point in a certain amount of time

speed of light

3.00 x 10^8 m/s

quantum/quanta

the minimum quantity of energy that can be lost or gained by an atom

blackbody radiation

The electromagnetic radiation emitted from a heated solid (planck realized that its behavior couldn't be explained by wave behavior)

Planck's constant

6.626 x 10^-34

what is planck's constant used for

to calculate energy of a photon

photoelectric effect

The emission of electrons from a metal when light shines on the metal

1. no electron is emitted until the light reaches threshold frequency

2. the more intesnse the light beam, the more e^- emitted

3. emitted e^- move faster if light applied has higher frequency

photons

particles of light

orbits

electrons travel in fixed circular paths around the nucleus (disproved)

ground state

The lowest energy state of an atom (most stable)

excited state

at least one electron has jumped to a higher principal energy level or orbital (unstable)

matter waves

wave characteristics of material particles

Heisenberg uncertainty principle

it is impossible to know exactly both the velocity and the position of an electron at the same time

Schrodinger wave equation

describes electrons as waves and predicts the regions (orbitals) where electrons are most likely to be found

quantum numbers

specify the properties of atomic orbitals and the properties of electrons in orbitals (level, sublevel, orbital, spin)

percent composition

mass of element/total mass of element

What is A

# of protons and neutrons in nucleus

What is Z

Atomic number/protons

What is X

chemical symbol for element

formula for average atomic mass

(mass₁ * abundance/percentage₁) + (mass₂ * abundance/percentage₂)

5 part of mass spectrometry

1. Vaporization

2. dissociation/ionization (becomes +1 charge)

3. Acceleration

4. Deflection

5. Detection

equation when frequency given

E=hv

E= energy of a photon

h= 6.636 × 10^-34

v= frequency

what is plancks constant

h= 6.636 × 10^-34

how to find energy when wavelength given

E= hc/lambda

E= energy of a photon

h= 6.636 × 10^-34

c= 3 × 10⁸

lambda= wavelength

relationship between amplitude and energy

porportional

Convert FM to Hz

FM = MHz

96.1 —> 9.61 × 10⁹

wavelength of red light

700 nm

wavelength of green light

550 nm

wavelength of violet light

400 nm

what equation do you use when given wavelength and speed of light

c = wavelength x frequency

what happens when two waves combine on peaks

they combine (constructive)

what happens if they meet at a peak and a trough

they cancel out (destructive)

ground state

lowest energy level of an electron occurs in the orbit closest to the nucleus

excited state

specific amt of energy(quanta) needed to excite electron to higher level

visible spectroscopy

most effective on transition metal ions

debroglie matter waves

matter waves support the bohr idea of orbits with only certain allowable energies and no e^- spiraling into the nucleus

bohr model and emisson spectrum, how do they connect

the model talks about quantized energy levels, the emission spectrum comes from the differences in energy levels of an atom (spectral lines)

axis of mass spectrum

x = mass numbers

y = abundance (relative or percent)

axis of UV-Vis graphs

x = wavelength (nm)

y = absorbance

axis of photoelectron spectroscopy

x = energy

y = relative # of electrons

how does gold foil experiment work

rutherford shot alpha particles at a thin peice of gold

• particles deflected off the gold nucleus (most of the mass was empty space)

• gold chosen bc malleable

• shot w/ higher energy = more angled deflection/ less overall e^- deflected & vice versa