R-AECHE-10

How to write a good method

STAWA Set 4 Q4

Don’t use moles unless you’re doing calculations

Use:

Mass (g)

Volume (mL)

concentration (mol $L^{-1}$)

Example of mixture to separate:

Sand, NaCl and CuCl_2

Atomic Spectra

Bohr’s model helped describe the spectra of hydrogen.

It showed 4 lines of absorption.

What does spectra represent?

It represents different wavelengths which correlate to different colours.

NOTE: Electrons occupy fixed energy levels.

The lines are energy levels.

Lines inside absorption/emissions spectrum are called spectra.

Absorption spectrum: Black lines on coloured spectrum Emissions spectrum: Coloured lines on black spectrum

n = 1 → n = 2 does not exist in visible light. (infared)

Balmer series refers to the possible promotions of electrons that produce visible light.

Balmer goes from n = 2 to whatever the largest electron shell is.

Further Reading:

• Lyman series produces UV light
• Paschen series produces infared light

Particle Theory of Light

First proposed by Einstein, he called them photons.

Energy of photon related to frequency of light wave by the Planck constant

In essence, light acts as both a wave and a particle, the wave part being described by Schrodinger's equation when excited electrons return to their ground state.

https://www.youtube.com/watch?v=GhAn8x7Q-d8

Things that excite electrons:

• Light
• Electrical current
• Heat
• Bombarding it with electrons? (kinetic energy)

Each atom has a particular pattern for absorption and emissions

Some methods aren’t good, they don’t give enough energy or energy at the exact magnitude/wavelength to excite an electron.

Electrons can absorb energy, and become excited (promoted to a higher energy state) by:

• Heat
• Electrical discharge
• Electromagnetic radiation (photons)

Then they emit energy as electromagnetic radiation, and return to their ground state.

They will always emit the same amount of energy as they absorb.

Balmer series = promotion from n = 2 to other energy levels, which absorb/emit visible light.

Bohr’s Model of the Atom:

• Electrons exist in discrete energy levels - they cannot have intermediate energies.
• A photon can be emitted or absorbed when an electron moves between energy levels.
• The energy of the photon corresponds to the difference between energy levels ($\Delta E$)
• Frequency of photons found by $\Delta E = hv$

Continuous spectrum (white light) vs Emissions spectrum vs Absorption spectrum.

Continuous spectrum is what you see when you use a spectroscope on white light.

How to get continuous spectrum? Iridescent light (source of white light), which does through diffraction grating to create a continuous spectrum

Heating up gas creates an emission spectrum. - heating up gas will give electrons energy,

Lighting white light through cold gas will create an absorption spectrum.

Absorption spectra:

• Atom absorbs a photon, electron promoted to higher energy state
• Continuous spectrum of light has a specific frequencies of light missing - black lines on a continuous spectrum

Atomic absorption spectroscopy: - more detail

• Analyses absorption spectra
• Sample of substance is vaporised, light of specific frequencies shone through it (from a hollow cathode lamp of same element being tested for), atoms absorb photons as electrons are promoted, remaining light has frequencies missing.
• Specific frequencies missing identify the element. Intensity of absorptions gives concentration of metal in sample.
• Useful for identifying metals in water, biological tissue, mineral samples, etc.

Emission spectra:

• Atom becomes excited, electrons are already in higher energy levels.
• Electron drops down to lower energy level.
• Only specific frequencies of light are emitted - coloured lines on a black background.

The Bohr Model of the atom and Atomic Emission Spectra: Atomic Structure tutorial | Crash Chemistry

https://www.youtube.com/watch?v=apuWi_Fbtys

Plank discovered that Light energy is quantised.

Einstein discovered that light waves have the properties of a particle.

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