Chem Unit 2
\Q: What is an intermolecular force?
A: An attractive force between molecules.
Q: What is the difference between an intramolecular and intermolecular force?
A: Intramolecular forces hold atoms together within a molecule; intermolecular forces act between molecules.
Q: What are the three main intermolecular forces?
A: London dispersion forces, dipole–dipole attractions and hydrogen bonding.
London dispersion forces
Q: What are London dispersion forces?
A: Weak attractions caused by temporary fluctuations in electron distribution that create temporary dipoles.
Q: Do all molecules experience London dispersion forces?
A: Yes.
Q: When do London dispersion forces become stronger?
A: As the number of electrons increases and as molecular surface area increases.
Q: Why do larger molecules generally have stronger dispersion forces?
A: They have more electrons, making their electron clouds more easily distorted.
Q: How does molecular shape affect dispersion forces?
A: Molecules with greater surface contact generally have stronger dispersion forces.
Q: Which has stronger dispersion forces: a long-chain molecule or a highly branched molecule with the same formula?
A: The long-chain molecule, because it has greater surface contact.
Dipole–dipole
Q: What is a permanent dipole?
A: An unequal distribution of charge caused by differences in electronegativity in a polar molecule.
Q: What are dipole–dipole attractions?
A: Attractions between the oppositely charged ends of neighbouring polar molecules.
Q: Which molecules experience dipole–dipole attractions?
A: Polar molecules.
Hydrogen bonding
Q: What is hydrogen bonding?
A: A strong intermolecular attraction between H bonded to N, O or F and a lone pair on N, O or F in another molecule.
Q: What must be present for hydrogen bonding to occur?
A: H covalently bonded to N, O or F, plus a lone pair on N, O or F of another molecule.
Q: Why is hydrogen bonding stronger than ordinary dipole–dipole attraction?
A: N, O and F are highly electronegative and small, producing particularly strong partial charges.
Comparing IMFs
Q: What is the general strength order of intermolecular forces?
A: London dispersion < dipole–dipole < hydrogen bonding.
Q: What happens to boiling point when intermolecular forces become stronger?
A: Boiling point generally increases.
Q: Why does stronger IMF increase boiling point?
A: More energy is required to overcome the attractions between molecules.
Q: What happens to vapour pressure when intermolecular forces become stronger?
A: Vapour pressure decreases.
Q: Why does stronger IMF decrease vapour pressure?
A: Fewer molecules have enough energy to escape into the gas phase.
Q: What happens to volatility when intermolecular forces become stronger?
A: Volatility decreases.
Q: What is volatility?
A: How readily a substance changes from liquid to gas.
Q: How does IMF strength affect melting point?
A: Stronger intermolecular attractions generally increase melting point, although molecular structure also affects it.
Q: How does IMF affect solubility?
A: Substances are more likely to dissolve when the attractions formed between solute and solvent particles are similar in strength to those being broken.
2. MOLECULAR GEOMETRY
Lewis structures
Q: What is a Lewis structure?
A: A diagram showing valence electrons, bonding pairs and lone pairs in a molecule or ion.
Q: What is a bonding pair?
A: A pair of electrons shared between two atoms in a covalent bond.
Q: What is a lone pair?
A: A pair of valence electrons that is not involved in bonding.
VSEPR
Q: What does VSEPR stand for?
A: Valence Shell Electron Pair Repulsion.
Q: What is the main idea of VSEPR theory?
A: Electron pairs around a central atom repel each other and arrange themselves as far apart as possible.
Q: Which repels more strongly: bonding pairs or lone pairs?
A: Lone pairs.
Q: What is the repulsion order?
A: Lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.
Q: How do lone pairs affect bond angles?
A: They repel bonding pairs more strongly, causing bond angles to decrease.
Shapes
Q: What is the shape of a molecule with 2 electron regions and no lone pairs?
A: Linear, 180°.
Q: What is the shape with 3 bonding regions and no lone pairs?
A: Trigonal planar, 120°.
Q: What is the shape with 4 bonding regions and no lone pairs?
A: Tetrahedral, 109.5°.
Q: What is the shape with 3 bonding pairs and 1 lone pair?
A: Trigonal pyramidal, approximately 107°.
Q: What is the shape with 2 bonding pairs and 2 lone pairs?
A: Bent, approximately 104.5°.
Q: Why isn't NH₃ trigonal planar?
A: Nitrogen has three bonding pairs and one lone pair, giving four electron regions and a trigonal pyramidal molecular shape.
Polarity
Q: What is electronegativity?
A: The ability of an atom to attract shared electrons towards itself.
Q: What is a polar bond?
A: A covalent bond in which electrons are shared unequally because of a difference in electronegativity.
Q: What is a non-polar bond?
A: A covalent bond in which electrons are shared equally or nearly equally.
Q: What determines whether a molecule is polar?
A: Both the polarity of its bonds and its molecular geometry.
Q: Can a molecule have polar bonds but be non-polar?
A: Yes. If the molecule is symmetrical, the bond dipoles can cancel.
Q: Why is CO₂ non-polar?
A: It is linear and symmetrical, so the two C=O bond dipoles cancel.
Q: Why is H₂O polar?
A: Its bent shape means the O–H bond dipoles do not cancel.
3. GASES
Kinetic theory
Q: What does the kinetic molecular theory describe?
A: The behaviour of gases in terms of the motion and collisions of their particles.
Q: What assumptions are made for an ideal gas?
A:
Particles have negligible volume.
There are negligible intermolecular forces.
Particles move randomly.
Collisions are elastic.
Average kinetic energy depends on absolute temperature.
Q: What does gas temperature represent at the particle level?
A: The average kinetic energy of the particles.
Q: What happens to particle kinetic energy when temperature increases?
A: Average kinetic energy increases.
Pressure
Q: What causes gas pressure?
A: Collisions of gas particles with the walls of the container.
Q: Why does pressure increase when temperature increases at constant volume?
A: Particles move faster, causing more frequent and more energetic collisions with the container walls.
Q: Why does pressure increase when volume decreases at constant temperature?
A: Particles collide with the container walls more frequently because they have less space.
Gas relationships
Q: What is the ideal gas equation?
A:
PV=nRT
Q: What does each symbol in PV=nRT represent?
A:
P = pressure
V = volume
n = amount of gas
R = ideal gas constant
T = temperature in kelvin
Q: What temperature scale must be used in the ideal gas equation?
A: Kelvin.
Q: How do you convert °C to K?
A:
T(K)=T(∘C)+273.15
Q: What units are commonly used for P and V in PV=nRT?
A: Pa and m³ when using R=8.31 J mol−1K−1.
4. ACIDS
Definitions
Q: What is an Arrhenius acid?
A: A substance that produces H⁺ ions in aqueous solution.
Q: What is an Arrhenius base?
A: A substance that produces OH⁻ ions in aqueous solution.
Q: What is a strong acid?
A: An acid that ionises essentially completely in water.
Q: What is a weak acid?
A: An acid that only partially ionises in water.
Q: What is the difference between acid strength and acid concentration?
A: Strength describes the extent of ionisation; concentration describes the amount of acid per unit volume.
Q: Can a weak acid be concentrated?
A: Yes.
Q: Can a strong acid be dilute?
A: Yes.
pH
Q: What is pH?
A: A measure of the hydrogen ion concentration of an aqueous solution.
Q: What is the equation for pH?
pH=−log10[H+]
Q: What does a decrease of 1 pH unit mean?
A: The hydrogen ion concentration increases by a factor of 10.
Q: What does an increase of 1 pH unit mean?
A: The hydrogen ion concentration decreases by a factor of 10.
Q: What is the relationship between acidity and pH?
A: Lower pH means greater acidity.
Acid reactions
Q: What happens when an acid reacts with a base?
A: Neutralisation occurs, producing a salt and water.
Q: What is the net ionic equation for neutralisation?
H++OH−→H2O
Q: What happens when an acid reacts with a metal?
A: A salt and hydrogen gas are produced.
Q: What happens when an acid reacts with a carbonate?
A: A salt, water and carbon dioxide are produced.
Q: How can CO₂ be identified?
A: It turns limewater milky/cloudy.
5. CONCENTRATION CALCULATIONS
Core equation
Q: What is the equation for molar concentration?
c=Vn
Q: What are the units of concentration?
A: mol L⁻¹ or mol dm⁻³.
Q: What is the rearranged equation for amount of substance?
n=cV
Q: What is the most important unit conversion for c=n/V?
A: Volume must be in litres if concentration is in mol L⁻¹.
Q: How do you convert mL to L?
A: Divide by 1000.
Mass → concentration
Q: How do you calculate moles from mass?
n=Mm
Q: How do you calculate concentration when given mass, molar mass and volume?
A:
c=MVm
Q: What is molar mass?
A: The mass of one mole of a substance, in g mol⁻¹.
Dilution
Q: What happens to the number of moles of solute during dilution?
A: It remains constant.
Q: What happens to concentration during dilution?
A: It decreases because the same amount of solute is spread through a larger volume.
Q: What equation can be used for dilution?
c1V1=c2V2
Stoichiometry
Q: What is the basic sequence for a concentration stoichiometry calculation?
A:
volume → moles → mole ratio → concentration/other quantity
Q: Why must an equation be balanced before using mole ratios?
A: The coefficients give the correct mole ratio between reactants and products.
Q: What does a coefficient of 2 mean in a balanced equation?
A: Two moles of that substance react/are produced for every one mole represented by a coefficient of 1.
6. SOLUBILITY
Q: What is solubility?
A: The maximum amount of a solute that can dissolve in a given amount of solvent at a specified temperature.
Q: What is a solute?
A: The substance being dissolved.
Q: What is a solvent?
A: The substance that dissolves the solute.
Q: What is a solution?
A: A homogeneous mixture of solute and solvent.
Saturation
Q: What is an unsaturated solution?
A: A solution containing less solute than the maximum amount that can dissolve at that temperature.
Q: What is a saturated solution?
A: A solution containing the maximum amount of dissolved solute possible at that temperature.
Q: What is a supersaturated solution?
A: A solution containing more dissolved solute than normally possible at that temperature; it is unstable.
Solubility and temperature
Q: How does temperature usually affect the solubility of a solid in water?
A: Solubility generally increases as temperature increases.
Q: How does temperature usually affect the solubility of a gas in water?
A: Gas solubility generally decreases as temperature increases.
Q: How does pressure affect the solubility of gases?
A: Increasing pressure generally increases gas solubility.
Like dissolves like
Q: What does "like dissolves like" mean?
A: Polar substances tend to dissolve in polar solvents, while non-polar substances tend to dissolve in non-polar solvents.
Q: Why does water dissolve many ionic substances?
A: Water is polar, so its partial charges can attract and stabilise ions.
Q: Why don't non-polar substances generally dissolve well in water?
A: They cannot form sufficiently strong favourable interactions with polar water molecules.
7. CHROMATOGRAPHY
Since this is only one question, these are the cards I'd prioritise.
Q: What is chromatography used for?
A: Separating and identifying components of a mixture.
Q: What is the stationary phase?
A: The phase that remains fixed.
Q: What is the mobile phase?
A: The phase that moves through the stationary phase.
Q: Why do substances separate during chromatography?
A: Different substances have different attractions to the stationary and mobile phases, so they travel at different rates.
Q: What is Rf?
A: The ratio of the distance travelled by a substance to the distance travelled by the solvent front.
Rf=distance travelled by solvent frontdistance travelled by substance
Q: What units does Rf have?
A: No units.
Q: What range can Rf have?
A: Between 0 and 1.
Q: What does a larger Rf generally mean?
A: The substance travelled further with the mobile phase and had a greater relative attraction to it than to the stationary phase.
Q: How can chromatography help identify an unknown substance?
A: Compare its Rf value with those of known substances under the same conditions.
Q: Why must Rf values be compared under the same conditions?
A: Rf depends on the stationary phase, mobile phase and experimental conditions.
Bond dipoles
Q: What is a bond dipole?
A: A separation of partial positive and partial negative charge across a polar covalent bond caused by a difference in electronegativity.
Q: How is a bond dipole represented?
A: The more electronegative atom has δ−, the less electronegative atom has δ+, with the dipole pointing towards the more electronegative atom.
Q: What is a net dipole?
A: The overall dipole of a molecule after all individual bond dipoles are considered together.
Molecular symmetry
Q: What does it mean for a molecule to be symmetrical?
A: Its atoms and bond dipoles are arranged so that the effects of the bond dipoles cancel.
Q: What happens to the net dipole in a symmetrical molecule?
A: The bond dipoles cancel, giving no net dipole.
Q: Is a symmetrical molecule necessarily non-polar?
A: If its bond dipoles cancel completely, yes, the molecule is non-polar.
Q: What does it mean for a molecule to be non-symmetrical?
A: Its bond dipoles are arranged unevenly, so they do not completely cancel.
Q: What happens to the net dipole in a non-symmetrical molecule with polar bonds?
A: A net dipole remains, so the molecule is polar.