chem unit 2

intermolecular forces (imfs)

Attractions between molecules. They affect properties such as boiling point, melting point and physical state.

What are the three main intermolecular forces you need to know?

Dispersion forces, dipole-dipole forces and hydrogen bonding.

What are dispersion forces?

Weak attractions caused by temporary/instantaneous dipoles formed when electrons are unevenly distributed.

Do all molecules have dispersion forces?

Yes. All atoms and molecules have dispersion forces.

What makes dispersion forces stronger?

More electrons and a larger/more polarisable electron cloud make dispersion forces stronger.

What are dipole-dipole forces?

Attractions between the permanent partial positive end of one polar molecule and the permanent partial negative end of another polar molecule.

Do non-polar molecules have dipole-dipole forces?

No. Dipole-dipole forces occur between polar molecules.

What is hydrogen bonding?

A particularly strong type of dipole-dipole attraction that occurs when H is covalently bonded to N, O or F and is attracted to N, O or F on another molecule.

Which is generally strongest: dispersion, dipole-dipole or hydrogen bonding?A: Dispersion < dipole-dipole < hydrogen bonding.

The three-dimensional arrangement of atoms in a molecule.

What determines molecular shape?

The arrangement of electron domains around the central atom.

What is an electron domain?

A region of electron density around the central atom, such as a bond or lone pair.

What is the shape of a molecule with 2 electron domains?

Linear — 180°

What is the shape with 3 bonding electron domains and no lone pairs?

Trigonal planar — 120°

What is the shape with 4 bonding electron domains and no lone pairs?

Tetrahedral — 109.5°

What is the shape with 3 bonding pairs and 1 lone pair?

Trigonal pyramidal — approximately 107°

What is the shape with 2 bonding pairs and 2 lone pairs?

Bent — approximately 104.5°

Why do lone pairs affect bond angles?

Lone pairs repel more strongly than bonding pairs, pushing bonds closer together.

What are the five shapes you need to recognise?

Linear, bent, trigonal planar, tetrahedral and trigonal pyramidal.

What is a polar molecule?

A molecule with an uneven distribution of charge, giving it a permanent dipole.

What is a non-polar molecule?

A molecule with no overall permanent dipole.

What two things determine whether a molecule is polar?

Bond polarity and molecular shape/symmetry.

Can a molecule contain polar bonds but still be non-polar?

Yes. If the bond dipoles cancel because of the molecule's symmetrical shape.

Example of a polar molecule?

H₂O — its bent shape means the bond dipoles do not cancel.

Example of a non-polar molecule?

CO₂ — its linear, symmetrical shape causes the bond dipoles to cancel.

Why is water polar?

O-H bonds are polar and the bent shape prevents the dipoles from cancelling.

What is a polyatomic ion?

A charged group of two or more covalently bonded atoms that behaves as one ion.

Give examples of polyatomic ions.

NO₃⁻, OH⁻, CO₃²⁻, SO₄²⁻ and NH₄⁺.

Is a polyatomic ion one atom?

No. It contains multiple atoms with an overall charge.

What is an acid according to the Arrhenius theory?

A substance that produces H⁺ ions in aqueous solution.

What is a base according to the Arrhenius theory?

A substance that produces OH⁻ ions in aqueous solution.

What is the pH scale?

A logarithmic scale used to describe the acidity or basicity of an aqueous solution.

Why is the pH scale called logarithmic?

Each change of 1 pH unit represents a tenfold change in hydrogen ion concentration.

How much more acidic is pH 3 than pH 4?

10 times more acidic in terms of H⁺ concentration.

How much more acidic is pH 3 than pH 5?

100 times more acidic in terms of H⁺ concentration.

What pH indicates a basic/alkaline solution?

Above 7.

What pH indicates an acidic solution?

Below 7.

What pH is neutral?

pH 7.

What does "strong acid" mean?

An acid that ionises essentially completely in water.

What does "weak acid" mean?

An acid that only partially ionises in water.

What does "strong base" mean?

A base that reacts/ionises extensively in water to produce OH⁻.

What does "weak base" mean?

A base that reacts/ionises only partially in water.

Does strong mean concentrated?

No! Strength and concentration are different concepts.

What is chromatography used for?

To separate and/or identify components of a mixture.

What is the basic principle behind chromatography?

Different substances have different attractions to the stationary phase and mobile phase, causing them to move at different rates.

What is paper/thin-layer chromatography used for?

Separating and analysing substances based on how far they travel with a solvent over a stationary surface.

What is gas chromatography (GC)?

A technique that separates volatile substances using a gas as the mobile phase.

What is HPLC?=

High-performance liquid chromatography, which separates substances using a liquid mobile phase under high pressure.

What does a solubility curve show?

How the solubility of a substance changes with temperature.

What does a point below the solubility curve represent?

An unsaturated solution.

What does a point on the curve represent?

A saturated solution.

What does a point above the curve represent?

A supersaturated solution.

Why is temperature important when reading a solubility curve?

Solubility can change as temperature changes.

What is a solute?

The substance that is dissolved.

What is a solvent?

The substance that dissolves the solute.

What is a solution?

A homogeneous mixture of a solute dissolved in a solvent.

What is a saturated solution?

A solution containing the maximum amount of dissolved solute at a particular temperature.

What is an unsaturated solution?

A solution that can still dissolve more solute at that temperature.

What is a supersaturated solution?

A solution containing more dissolved solute than normally possible at equilibrium under those conditions.

What is a precipitate?

An insoluble solid formed when substances react in solution.

What does (aq) mean?

Aqueous — dissolved in water.

What does (s) mean?

Solid.

What does concentrated mean?

A solution contains a large amount of solute per unit volume.

What does dilute mean?

A solution contains a small amount of solute per unit volume.

Can an acid be strong and dilute?

Yes.

Can an acid be weak and concentrated?

Yes.

What is the difference between strength and concentration?

Strength describes how extensively an acid/base ionises. Concentration describes how much solute is present per volume of solution.

what are the 7 types of molecular geometry

Linear

Trigonal planar

Tetrahedral

Trigonal pyramidal

Bent

Trigonal bipyramidal

Octahedral

What does atomic number tell you?

number or protons

How do you calculate neutrons?

Mass number − atomic number.

How many electrons does a neutral atom have?

The same number as protons

What does a positive ion mean?

It has lost electrons.

What does a negative ion mean?

It has gained electrons.

How do you find electrons in an ion?

Start with protons, then subtract electrons for + charge or add electrons for − charge.

What determines molecular shape?

The number of electron regions around the central atom.

What counts as ONE electron region?

Each bond or lone pair counts as one region.

2 electron regions → what shape?

Linear, 180°.

3 electron regions → what shape?

Trigonal planar, 120°.

4 electron regions with no lone pairs → what shape?

Tetrahedral, 109.5°.

4 regions + 1 lone pair → what shape?

Trigonal pyramidal.

4 regions + 2 lone pairs → what shape?

Bent.

5 electron regions → what shape?

Trigonal bipyramidal.

6 electron regions → what shape?

Octahedral.

What is electronegativity?

How strongly an atom attracts bonding electrons.

What is a polar bond?

A bond where electrons are shared unequally because atoms have different electronegativities.

What is a dipole?

A separation of partial positive and negative charge.

How can you tell if a molecule is polar?

Check whether it has polar bonds and whether the bond dipoles cancel.

What happens in a symmetrical molecule?

Bond dipoles can cancel, making the molecule non-polar.

What are dipole–dipole forces?

Attractions between permanent dipoles in polar molecules.

When does hydrogen bonding occur?

When H is directly bonded to N, O or F.

What is the stoichiometry pathway for a mass → mass question?

Mass → moles → mole ratio → moles → mass

Why must a chemical equation be balanced before stoichiometry?

The coefficients give the correct mole ratios.

What does a coefficient in a balanced equation represent?

The mole ratio between substances.

Rank these from strongest to weakest: ionic bonds, covalent bonds, hydrogen bonding, dipole–dipole forces and London dispersion forces.

Ionic = Covalent (roughly the same), >Hydrogen bonding, >Dipole-dipole, > London dispersion

Ionic bonding — VERY STRONG

Attraction between oppositely charged ions. e.g. NaCl

Covalent bonding — VERY STRONG

Atoms share electrons. e.g. H2O

Hydrogen bonding — strongest common intermolecular force

e.g. H2O usually when bonded which N,O and F

Dipole–dipole

Occurs between polar molecules. E.G. HCl

London dispersion forces — weakest generally

Caused by temporary/instantaneous dipoles. (all molecules have london dispersion forces)

Which interaction is strongest between molecules of H₂O?