Chemistry Module 2

How do you define relative atomic mass (Ar)?

The weighted mean mass of an atom or an element, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope

Takes into account the relative abundances of the different isotopes of an element

Ar = mean mass of an atom of an element / 1/12 x mean mass of C-12 isotope

How do you define relative isotopic mass?

The isotopic mass of an isotope relative to one twelfth of the mean mass of an atom of the carbon-12 isotope

How do you define relative molecular mass (Mr)?

The weighted mean mass of a molecule of a compound, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope

What is a binary compound?

A compound containing only two elements

What is a polyatomic ion?

An ion that contains more then one atom

What is Avogradro’s constant?

6.02 × 10²³

There are 6.02 × 10²³ particles in every mole of a substance

Number of particles = moles x 6.02 × 10²³

What is the empirical formula?

Simplest whole number ratio of atoms of each element present in a compound

Eg: C₂H₆ = CH₃ (in empirical formula)

What is the molecular formula?

The number and type of atoms of each element in a molecule

True number of each atom in the molecule

What is an anhydrous / hydrated substance?

Anhydrous: contains no water of crystallisation

Hydrated: contains water of crystallisation

What is the water of crystallisation?

Water that is part of the crystalline structure

The molecules are stoichiometrically bonded into the crystal structure

What is a standard solution?

A solution of known concentration

What is the limiting reagent?

The reactant that is not in excess

It will be used up first, causing the reaction to stop

In calculations, use the moles of the limiting reactant

What is the ideal gas equation?

UNDER STANDARD CONDITIONS:

pV = nRT

p = pressure (Pa)

V = volume (m³)

T = temperature (K)

n = moles (n) = m / Mr

m = mass (g)

R = ideal gas constant = 8.31^-1mol^-1

How do you calculate percentage yield?

(Experimental mass / Theoretical mass) x 100

Why is percentage yield usually below 100%?

Reaction being incomplete

Loss of products on equipment

Unwanted side reactions

What is the formula for atom economy?

(Mr of desired product / Mr of reactants) x 100

Why is it desirable to have a high atom economy?

Little / no waste product

Process is more economically viable

Helps preserve raw materials

How do you measure mass in an experiment?

Use a digital mass balance

Use a balance with a suitable resolution

How do you measure a volume of a solution in an experiment?

Use a measuring cylinder

If you are making up a standard solution, use a volumetric flask of the required volume

How do you measure the volume of gas produced in an experiment?

Use a gas syringe

OR

Measure the mass lost on a balance > use this value to calculate the moles of gas produced during the reaction

What is the definition of an acid?

A species that donates a proton (H⁺)

They neutralise bases

What is the definition of a base?

A species that accepts a proton (H⁺)

They neutralise acids

What is the difference between an alkali and a base?

They both neutralise acids

An alkali is a water soluble base - when dissolved, they release OH^- ions

What is the difference between a strong and weak acid?

A strong acid fully dissociates in an aqueous solution

A weak acid partially dissociates in an aqueous solution

What is neutralisation?

The reaction of an acid with a base
Ionic equation: H⁺_(aq) + OH^-_(aq) > H₂O _(l)

Acid + carbonate >

Salt + water + carbon dioxide

Acid + metal oxide >

Salt + water

Metal + acid >

Salt + hydrogen

In a titration, what are the steps to making up the standard solution?

1. Weigh the sample bottle containing the solid on a balance

2. Transfer solid to beaker and reweigh bottle

3. Record difference in mass (this is the mass of solid)

4. Add distilled water and stir with a glass rod until all the solid has dissolved

5. Transfer to a volumetric flask with washings

6. Make up to the 250cm³ mark with distilled water

7. Shake flask

In a titration, once you have the standard solution, what are the steps to completing the titration?

1. Fill the burette with the standard solution of known concentration, ensuring the jet space in the burette is filled and doesn’t contain air bubbles

2. Use a pipette filler and pipette to transfer 25cm³ of the solution with unknown concentration into a conical flask

3. Add 2/3 drops of indicator into the conical flask

4. Record the initial burette reading

5. Titrate the contents of the conical flask by adding the solution to it from the burette until the indicator undergoes a definite, permanent colour change (use a white tile)

6. Record the final burette reading (endpoint) and calculate the titre volume (find the difference)

7. Repeat until at least 2 concordant results are obtained (within 0.1cm³ of each other)

How can you reduce the uncertainty of a titration?

Increase the titre volume needed for the reaction

This can be done by increasing the volume and concentration of the substance in the conical flask (unknown solution) OR by decreasing the concentration of the substance in the burette (standard solution)

Titration equipment

Oxidation number rules

Element = 0

Neutral compound = add up to 0

Charged compound = total charge

Hydrogen = +1

Oxygen = -2

Halogens = -1

Group 1 = +1

Metal hydrides = hydrogen is -1

Peroxides = oxygen is -1

What is oxidation and reduction?

Oxidation is the loss of electrons > oxidation number becoming more positive

Reduction is the gain of electrons > oxidation number becoming more negative

Use the acronym OILRIG

What is a redox reaction?

When oxidation and reduction occur in the same reaction

What does an oxidising agent do?

Accepts electrons from the species that is being oxidised

Gains electrons and is reduced > reduction in oxidation number

What does an reducing agent do?

Donates electrons to the species being reduced

Loses electrons and is oxidised > increase in oxidation number

What is a disproportionation reaction?

Where a species is both oxidised and reduced in the same reaction

What is an electron shell?

Electrons orbiting the nucleus in different energy levels, which are an increasing distance from the nucleus

They are made up of atomic orbitals

What is an atomic orbital?

A region around the nucleus that can hold 1 electron, or 2 electrons with opposite spins

Shells are made up of atomic orbitals

What are the 4 types of orbitals?

s

p

d

f

What is the shape of an s orbital?

What is the shape of a p orbital?

What is the maximum number of electrons that can be held in each electron shell?

n: 1 = 2

n: 2 = 8

n:3 = 18

n:4 = 32

Formula = 2n²

How are orbitals arranged within shells?

Each shell from n = 1 contains one s orbital

Each shell from n = 2 contains three p orbitals

Each shell from n = 3 contains five d orbitals

Each shell from n = 4 contains seven f orbitals

How can you tell what orbital outer electrons are in?

What is the order of filling orbitals?

The energy of the orbitals increases from s to p to d to f > fill in that order (s > p > d > f)

ONE EXCEPTION: 4s has a lower energy than 3d > 3d is filled before 4s

How do you draw electron orbital diagrams?

Up arrow is always first

Only 2 electrons per box

Must complete all the up arrows first before going onto the down arrows

Must fill an orbital completely before moving on

What is ionic bonding?

Electrostatic attraction between positive (cation) and negative (anion) ions

Occurs between a metal and non metal

Electrons are transferred from the metal to the non metal to achieve full outer shells

When electrons are transferred, it creates charged particles called ions

How do you know the relative strength of ionic bonds?

Ions with a greater charge will have a greater attraction to the other ions resulting in stronger forces of attraction, and therefore stronger ionic bonding

Larger ions that have a greater ionic radius will have a weaker attraction to the oppositely charged ion as the attractive forces have to act over a greater distance

How can you represent an ionic bond using a dot and cross diagram?

Electron(s) transferred from the cation are seen on the outer shell of the anion

What is the melting / boiling point of ionic structures and why?

High > electrostatic forces holding the ionic lattice together are strong and require a lot of energy to overcome

When can ionic structures conduct / not conduct electricity and why?

When molten / aqueous (in solution) > ions separate and are no longer held in a lattice > ions are free to move and carry charge > can conduct electricity

When solid > ions are in fixed positions with no mobile charge carriers > cannot conduct electricity

Are ionic structures brittle?

Yes - when layers of alternating charges are distorted, like charges repel, breaking apart the lattice into fragments

What is a covalent bond?

The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms

Form between 2 non metals

Electrons are shared between the 2 outer shells to achieve a full outer shell

How can you represent covalent bonding using dot and cross diagrams?

How is the length of a covalent bond linked to its strength?

Shorter bonds tend to be stronger as the atoms are held closer together > forces of attraction are greater, requiring more energy to overcome

Double / triple bonds are shorter than single covalent bonds

What is a dative bond?

When both electrons from a covalent bond are supplied from a single atom

When calculating bond angles, how does a lone pair influence the bond angle?

Each lone pair reduces the bond angle by 2.5^o

What are the steps to determining the bond angle of a molecule?

1. Find the number of electron pairs

2. Determine how many of the lone pairs are bonding regions / lone pairs

3. Remember the table.

What is a linear shape?

2 bonding regions

0 lone pairs

Bonding angle = 180^o

What is a bent shape?

2 bonding regions

2 lone pairs

Bond angle = 104.5^o