01. Kinetic Molecular & Atomic Theory

Elements

Can’t be broken down by chemical means into simpler substances

Compound

2 or more atoms chemically bonded tgt

Mixtures

2 or more elements / compounds NOT chemically bonded tgt

Homogenous mixture

Same composition

Features of homogenous mixtures (2)

• Do not separate physically when stationary

• Can be separated via physical methods [e.g. fractional distillation]

Heterogenous mixture

Non-uniform composition

Features of heterogenous mixture (2)

• Separate into 2 separate layers when stationary

• Components are immiscible

Use of recrystallisation

Purify a solid product in an organic reaction

Recrystallisation steps (5)

1. Choose particular solvent

   • Solid should have low solubility at room temp and high solubility at high temp

2. Heat to boil / near boiling then add to solid

   • Add solvent & stir until all solid dissolves
     [impurities have now liberated]
     [dissolution]

3. Remove heat source and leave to cool at room temp

   • Place in ice bath

4. Crystallise [seed crystals can speed up process]

5. Filter out the crystals [Buchner funnel]

Vacuum filtration after recrystallisation (7)

1. Suction is created using aspirator

2. Pour remains in the beaker through Buchner funnel

3. If no crystals are visible, add activated carbon

4. Boil mixture

5. Use funnel system to transfer new mixture to a new beaker of boiling solvent

6. Remove excess carbon using funnel and filter paper

7. Cool mixture → Crystals should be present

Test purity of crystals

Perform “melting point determination”

ºC to K

+273

K to ºC

-273

Kelvin scale

• An absolute temperature scale

• Average KE of particles is proportional to the absolute temperature

Absolute zero (4)

• 0 Kelvin

• Motion of particles are minimal

• Substance at absolute zero has no transferable heat energy

• Ideal gas at absolute zero and at constant pressure would reach 0 volume

Deposition (2)

• Gas → Solid [directly]

• Releases energy

Melting (2)

• Solid → Liquid

• Absorbs energy

Evaporation / Boiling (2)

• Liquid → Gas

• Absorbs energy

Difference between boiling and evaporation (2-evaporation) (3-Boiling)

Evaporation

• Takes place at the surface of a liquid

• Can occur below the boiling point

Boiling

• Takes place throughout the liquid

• Occurs at specific temperature

• Bubbles of gas are formed within the liquid [not only at the surface]

Condensation (2)

• Gas → Liquid

• Releases energy

Freezing (2)

• Liquid → Solid

• Releases energy

Sublimation (2)

• Solid → Gas [directly]

• Absorbs energy

Atom

Smallest piece of element that can exist

Molecule

Formed when atoms join together by chemical reaction

Ions

Charged electrons

Position of protons

Nucleus

Charge of protons

+1

Relative mass of protons

1

Position of neutrons

Nucleus

Charge of neutrons

0

Relative mass of neutrons

1

Position of electrons

Shells around nucleus

Charge of electrons

-1

Relative mass of electrons

1/1836 or 0 or 0.0005

Atomic number (2)

• Number of protons / electrons in the nucleus

• Defines which element atom belongs to

Mass number

Number of protons and neutrons

Isotopes (4)

• 2 or more of the same element with same number of protons but different number of neutrons

• Same atomic numbers, Different mass number

• Different physical properties

• Identical chemical properties [same electronic configuration / no. of electrons in outer shell]

Radioisotope (2)

• Atom with an unstable nucleus which undergoes radioactive decay

• Emits gamma ray and / or an alpha particle or beta particle

Use of radioisotopes (4)

1. Nuclear medicine for diagnostics, treatment, & research

2. Medical tracers in biochemical & pharmaceutical research

3. Chemical clocks in geological & archaeological dating [radiocarbon dating]

4. PET scans give 3D images of tracer concentration in the body [can be used to detect cancer]

Relative atomic mass (Ar)

Average mass of the isotope of an element compared to an atom of Carbon-12

Relative atomic mass formula

(mass ÷ abundance) x 100

(mass x abundance %) ÷ 100

(mass₁ ÷ abundance₁) + (mass₂ ÷ abundance₂) x 100

(mass₁ x abundance₁) + (mass₂ x abundance₂) ÷ 100

Electromagnetic spectrum

Distribution of electromagnetic radiation according to their energies

Order of electromagnetic spectrum (7) [highest to lowest energy]

1. Gamma rays

2. X-rays

3. UV

4. Visible light

5. Infrared

6. Microwave

7. Radio waves

UV region (frequency, wavelength, energy)

1. High frequency

2. Short wavelength

3. High energy

Infrared region (frequency, wavelength, energy)

1. Low frequency

2. Long wavelength

3. Low energy

Explain the emission of colour during flame tests (5)

1. When burning, electrons in an atom gets excited

2. The electrons move up energy levels

3. Excited electrons are unstable and emits the same amt of energy it absorbed

4. Electrons drop back down to a lower energy level

5. Energy is emitted of a specific wavelength, causing a line on the spectrum

n=1 line spectrum

UV region

n=2 line spectrum

Visible region

n=3 or above line spectrum

IR region

Energy level and convergence

As energy levels increase, the lines in the spectrum start to converge

Why do spectrum lines converge

The convergence is due to the energy levels getting closer together

Orbital

Region of space where an electron is most likely to be found

Energy sub levels

s, p, d, f

Aufbau Principle

Electrons must be put in the orbitals of lowest energy first

Hund’s Rule [‘seats on the bus’ rule]

When filling sub-levels, each orbital must contain 1 electron before pairing up

Factors affecting size of first ionisation energy (4)

1. Distance

2. Shielding

3. Nuclear charge

4. Electron pairing

How does distance affect first ionisation energy (3)

1. Greater distance between nucleus and outer most electron

2. Less attractive force between them

3. Decrease in ionisation energy

How does shielding affect first ionisation energy (2)

1. Shielding by inner filled levels & sub-levels makes outer electrons easier to remove

2. Decrease ionisation energy

How does nuclear charge affect first ionisation energy

1. Nucleus is positively charged due to protons

2. More protons → greater size of charge

How does electron pairing affect first ionisation energy

Paired e^- is easier to remove than unpaired e^- due to repelling force

Effective nuclear charge

Ratio of protons and electrons

Explain large jumps in ionisation graphs

Focus on which energy level e^- is removed from

Graph typically moves from higher energy levels down to n=1

E.g. 1st jump from 2 to 3 in Mg is because 2nd electron is removed from n=3 while 3rd electron is removed from n=2 which is closer to the nucleus.

Determine number of occupied energy levels ionisation graph

Number of jumps in graph determines the number of occupied energy levels