SACE Stage 2 Chemistry: Semester 2 Exam

What is an acidic oxide, and what does it form in acid-base reactions?

An acidic oxide is the oxide of a non-metallic element, which reacts with water to form acids, or with bases to form salts and water.

What is the difference between a weak and a strong acid/base?

The strength of acids and bases are determined by their degree of ionisation in water. A strong acid completely ionises in water so that 1 mole yields 1 mole of hydronium, whilst a weak acid partially ionises in water so that 1 mole yields <1 mole of hydronium.

How do neutralisation reactions occur between strong acids and bases?

One mole of a strong acid reacts with one mole of a strong base. Hydrogen ions from the acid combine with hydroxide ions from the base to form water.

What is the enthalpy of a neutralisation reaction?

Neutralisation reactions are exothermic (negative enthalpy)

How do you calculate the concentration of hydronium or hydroxide in a solution?

1 × 10^-14 = [OH][H₃O] can be rearranged to find the concentration of hydronium or hydroxide if given the opposite concentration. If you are given the pH, you can calculate the concentration of hydronium with 10^-pH = [H₃O]

What is the ionic product (K_w) of pure water?

K_w = [OH^-][H₃O⁺]

K_w is the product of the concentration of hydroxide and hydronium in a volume of pure water. It is equal to 1 × 10^-14 at 25°C.

Explain the equilibrium between hydronium and hydroxide in pure water

Water has a weak electrical conductivity, meaning it contains dissociated ions. 1/10¹⁴ H₂O molecules ionise to form H₃O and OH; because these are strong acids/bases, they immediately react to form H₂O again, creating the following equilibrium: 2H₂O_(l) ⇌ H₃O⁺_(aq) + OH^-_(aq)

How do you calculate the pH of a solution?

pH = -log[H₃O⁺]

You must know the concentration of H₃O⁺ in molL^-1.

What is the product of reactions formed by acids and metallic oxides?

Salt and water

What is the product of reactions formed by acids and metallic hydroxides?

Salt and water (not all metallic hydroxides are soluble)

What is the product of reactions formed by acids and carbonates?

Salt, water, and carbon dioxide

What is the product of reactions formed by acids and metallic hydrogencarbonates?

Salt, water and carbon dioxide

What are acids and bases?

Acids are substances that donate protons and increase the hydronium (H₃O⁺) concentration in water. Bases are substances that accept protons, and increase the concentration of hydroxide (OH^-) when added to water.

What is the name for a substance which can both accept and donate protons in a chemical reaction?

Amphiprotic substance e.g. water

What is the name for an oxide which has both acid and base characteristics?

Amphoteric oxides (e.g. zinc and aluminium oxide)

What are conjugate acid/bases?

Conjugate acids are formed when a base accepts protons, and acts as an acid in reverse reaction by donating the proton back.

Conjugate bases are formed when an acid loses a proton, and acts as a base in reverse reactions by accepting protons.

Acid + Base ⇌ Conjugate Base + Conjugate Acid

What is hydronium?

Hydronium (interchangable with hydrogen in base chemistry) is formed when a single proton with high electronegativity accepts an electron pair from an oxygen atom in water. Protons cannot exist freely in solution, so hydrogen immediately binds to water to form hydronium.

H₂O + H⁺ → H₃O⁺

Why is rain acidic?

Carbon dioxide in the atmosphere dissolves in water to create carbonic acid (H₂CO₃), which partially ionises in rain to produce hydronium, lowering the pH.

Rain usually has a pH of 5.6 - anything lower is acid rain.

How does acid rain from from sulphur?

Sulphur is found in fossil fuels, carbon and metallic ores, which are burned and released into the atmosphere.

S₂ + O₂ → suphur dioxide (SO₂)

SO₂ + H₂O → sulfurous acid (H₂SO₃)

H₂SO₃ + O₂ → sulfuric trioxide (SO₃)

SO3 + H₂O → sulfuric acid (H₂SO₄) → acid rain

How does acid rain form from nitrogen?

Nitrogen is released into the atmosphere through processes that use high temperature, like lightning discharges, bushfires, industrial process and jet engines.

N₂ + O₂ → nitric oxide (NO)

NO + O₂ → nitrogen dioxide (NO₂)

NO₂ + H₂O → nitrous acid (HNO₂) and nitric acid (HNO₃). Nitrous acid does not ionise completely in water. but nitric acid does, forming acid rain.

What are the universal indicators for acids and bases?

Acids: Red, orange, yellow

Neutral: Green

Bases: Blue, violet, purple

What are the bromothymol blue indicators for acids and bases?

Acids: Yellow

Neutral: Green

Bases: Blue

What are the red litmus indicators for acids and bases?

Acids: Red

Neutral: Red

Bases: Blue

What are the blue litmus indicators for acids and bases?

Acids: Red

Neutral: Blue

Bases: Blue

What are the phenolphthalein indicators for acids and bases?

Acids: Colourless

Neutral: Colourless

Bases: Pink, red

What are concordant titres?

Concordant titres are titre measurements within 0.1mL of eachother.

How do you convert from molL^-1 to gL^-1?

Multiply by molar mass.

How do you convert from gL^-1 to w/v%?

Divide by 10

How do you convert between gL^-1, ppm, and ppb?

Multiply by 10³

How do you calculate molar concentration?

Molarity = moles of solute (n) ÷ litres of solution (V)

Multiply by the dilution factor if applicable.

What is the shape of sulfuric acid?

Tetrahedral (H₂SO₄)

What is the shape of sulfuric trioxide?

Trigonal planar (SO₃)

Does the acidity of elements increase or decrease as you move across the periodic table?

Increases left to right (increasing electronegativity)

What is a miscible solution?

A miscible solution is a homogenous mixture formed from a solute dissolved in a solvent.

Why are some solvents polar?

Polar solvents have significant dipole movements arising from polar bonds formed between atoms of different electronegativities.

How do non-polar bonds form?

Non-polar bonds form via covalent bonds between atoms of similar electronegativities.

How does size affect solubility?

The intermolecular forces between solute molecules increase with size, making it more difficult for solvent particles to separate and surround the solute. Therefore, miscibility decreases as the size and mass of a molecule increases.

What is the weakest intermolecular force, and how does it occur?

The weakest intermolecular forces are london dispersion forces, which occur between polar and non-polar molecules. Because of the constant movement of electrons, atoms and molecules form instantaneous dipoles when electrons are unsymmetrically distributed around the nucleus. This dipole can distort another atom or molecule, repelling its electrons and forming an instantaneous dipole.

How do dispersion forces increase with size?

In larger molecules, the valence are electrons are further from the nucleus and less tightly bound, making it easier for them to move around to form temporary dipoles. Thus, larger and heavier molecules exhibit stronger dispersion forces than smaller and lighter ones.

What intermolecular force acts between hydroxyl groups?

Hydrogen bonds

Why is detergent amphiphilic?

Detergent has a hydrophilic ‘head’ and a hydrophobic ‘tail.’ The ‘head’ is a polar ionic sulfate or sulfanate that is miscible in water, whilst the ‘tail is a larger non-polar hydrocarbon that does not mix with water. The polar head is attracted to water and the tail is attracted to oil, creating an emulsion.

What is a micelle?

A micelle is a spherical structure in which the non-polar tail of the detergent face the centre of the molecule. The heads are negatively charged, preventing the aggregation of multiple micelles together, so the oil remains dispersed through the water.

What is a ‘hydration shell?’

A hydration shell is a protective layer of water molecules which surround a molecule to prevent it from recrystallising.

How do ion-dipole interactions occur in water?

Water molecules are electric dipoles, containing partial positive and negative poles. The anions and cations within the solid salt lattice of a solute carry a full magnitude of electric charge; the hydrogen (H⁺) atoms in water (positive pole) attract the anions of the salt (negative pole) and the oxygen (O₂^-) atoms in water attract the cations of the salt (positive pole). If the force of attraction between the ions and water is greater in magnitude than the ionic bonds within the lattice, then the ions dissociate from the lattice and are surrounded by water.

How does precipitation form in a solution?

Precipitation forms when two ionic reactants dissociate and bond with the other reactant. The ions replace eachother based on their charges as a cation or anion.

What type of reaction is the formation of precipitation?

Precipitation is normally the result of a single/double displacement reaction, when two water-soluble solutions react to form an insoluble product.

What is the difference between precipitation and crystallisation?

Crystallisation occurs slowly, giving a regular solid lattice time to form and produce crystal. Precipitation occurs when a solid is formed quickly; there is no time for the large regular lattice to form, so many small particles form instead.

How do you calculate mass concentration?

p = m(g)/V

Concentration (gL^-1) = mass of solute (g) ÷ volume of solution (V)

How do you calculate percentage weight per volume?

% w/V = m(g)/V × 100

% w/V (g100mL^-1) = (mass of solute (g) ÷ volume of solution (L)) × 100

Represents the mass of the solute (g) dissolved in 100mL of solution

How do you calculate parts per million?

ppm = m(mg)/V

Parts per million (mgL^-1) = mass of solute (mg) ÷ volume of solution (V)

How do you calculate parts per billion?

ppb = m(μg)/V

Parts per billion (μgL^-1) = mass of solute (μg) ÷ volume of solution (V)

What is a standard solution?

A standard solution is a chemical solution with an exact known concentration of solute.

How do you create a standard solution?

1. Decide the desired concentration and volume

2. Calculate moles (n = c × V) and mass (m = n × M) of primary standard needed

3. Weigh and transfer the standard into a volumetric flask with distilled water. Swirl to ensure the solute has dissolved.

4. Fill the flask so that the bottom of the meniscus is on the calibration mark.

5. Stopper and invert the flask to mix the contents and create a homogenous solution.

How many times should you invert a standard solution?

Once - inverting the flask multiple times dissolves trapped air into the solution, giving an innaccurate reading of the volume.

How do you calculation the dilution of a solution?

c₁v₁ = c₂v₂

Initial concentration × initial volume = final concentration × final volume

What is enthalpy?

Enthalpy is a thermodynamic property which represents the heat content of a system.

What does a change in enthalpy represent?

Change in enthalpy = lattice energy - hydration energy

A negative enthalpy means the hydration energy is greater than the lattice energy (exothermic reaction)

A positive enthalpy means that the lattice energy is greater than the hydration energy (endothermic reaction)

What is the specific heat capacity of water?

4.184 J/g°C

How do you calculate the amount of heat energy absorbed or released by a substance during a chemical reaction?

Q = mcΔT

Heat energy (J) = mass (g) × specific heat capacity (J/g°C) × temperature change (°C)

How do you calculate change in enthalpy?

ΔH = Q/1000n

Enthalpy (kJ/mol) = heat energy (J) ÷ (1000 × number of moles)

What is a redox reaction?

A redox reaction is a reaction where electrons move from one reactant to another.

What are some examples of redox reactions?

Combustion, rusting, photosynthesis, batteries, respiration and decomposition

What is the oxidation number of an atom in its elemental state?

0

What is the oxidation number of a monoatomic ion?

Equal to the ion’s charge

What is the oxidation number of a compound?

The sum of oxidation numbers within a compound is equal to the compound’s charge.

What is the oxidation number of flourine?

-1

What is the oxidation number of Group I metals?

+1

What is the oxidation number of Group II metals?

+2

What is the oxidation number of hydrogen?

Normally +1, unless bonded to group I or II metals (-1)

What is the oxidation number of oxygen?

Normally -2, unless in a peroxide group (-1)

What is the oxidation number of halogen anions?

-1, except when bonded with fluorine or oxygen.

What is the reducing agent in a redox reaction?

The reactant that loses electrons (it reduces another reactant)

What is the oxidising agent in a redox reaction?

The reactant that gains electrons (it oxidises another reactant)

Does oxidation occur at the anode or cathode?

Anode

Does reduction occur at the anode or cathode?

Cathode

What does an increase in oxidation number represent?

Loss of electrons