K_c vs K_p

“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”

K_c uses molar concentrations; K_p uses partial pressures — they're related by K_p = K_c(RT)^Δn for gases.

strong acid vs weak acid

“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”

A strong acid ionises completely (HCl); a weak acid ionises only partly (CH₃COOH) and has an equilibrium.

adding heat to exothermic vs adding heat to endothermic

“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”

Heating an exothermic reaction shifts it backward (toward reactants); heating an endothermic one shifts it forward (toward products).

Myth: Equilibrium means equal concentrations

“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”

It means equal forward/reverse RATES, not equal amounts of reactants and products.

Oxidation and reduction — common mistake

“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”

A frequent error is mixing up which process loses and which gains electrons. In reality, oxidation is loss of electrons and reduction is gain of electrons (remember OIL RIG).

Memory trick: OIL RIG — Oxidation Is Loss, Reduction Is Gain.

Oxidation number

“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”

A bookkeeping charge assigned by rules to track electron transfer.

Memory trick: free elements always have oxidation number 0.

Oxidation number — common mistake

“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”

A frequent error is forgetting that oxidation number can be fractional or zero (e.g. in O₂, it's 0). In reality, a bookkeeping charge assigned by rules to track electron transfer.

Memory trick: free elements always have oxidation number 0.

Oxidising and reducing agents

“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”

The oxidising agent gets reduced (gains electrons); the reducing agent gets oxidised (loses electrons).

Memory trick: the agent does the opposite to itself — an oxidising agent is itself reduced.

Oxidising and reducing agents — common mistake

“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”

A frequent error is calling the substance that is oxidised the 'oxidising agent'. In reality, the oxidising agent gets reduced (gains electrons); the reducing agent gets oxidised (loses electrons).

Memory trick: the agent does the opposite to itself — an oxidising agent is itself reduced.

oxidation vs reduction

“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”

Oxidation is loss of electrons (oxidation number rises); reduction is gain of electrons (oxidation number falls).

oxidising agent vs reducing agent

“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”

An oxidising agent accepts electrons (and is reduced); a reducing agent donates electrons (and is oxidised).

Myth: Agent naming

“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”

The oxidising agent is itself reduced and the reducing agent is itself oxidised — name by what they DO to the other species.

Atomic radius trend — common mistake

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

A frequent error is reversing the across-a-period trend. In reality, atomic size decreases across a period (more nuclear pull) and increases down a group (new shells).

Memory trick: more protons but the same shell → electrons pulled in tighter across a period.

Ionization energy

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

The energy to remove the outermost electron; it rises across a period and falls down a group.

Memory trick: removing an electron is hardest from small, tightly-held atoms.

Ionization energy — common mistake

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

A frequent error is ignoring small dips (e.g. B<Be, O<N) caused by sub-shell stability. In reality, the energy to remove the outermost electron; it rises across a period and falls down a group.

Memory trick: removing an electron is hardest from small, tightly-held atoms.

Electronegativity

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

An atom's tendency to attract shared electrons; fluorine is the highest.

Memory trick: electronegativity rises toward the top-right (fluorine) of the table.

Electronegativity — common mistake

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

A frequent error is assuming metals have high electronegativity. In reality, an atom's tendency to attract shared electrons; fluorine is the highest.

Memory trick: electronegativity rises toward the top-right (fluorine) of the table.

Metallic character — common mistake

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

A frequent error is confusing metallic character with electronegativity (they're opposites). In reality, the tendency to lose electrons; it decreases across a period and increases down a group.

Memory trick: metals sit on the left and bottom; non-metals on the top-right.

atomic radius vs ionization energy

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

As atomic radius decreases across a period, ionization energy increases — smaller atoms hold their electrons more tightly.

metals vs non-metals

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

Metals lose electrons easily (low ionization energy, form cations); non-metals gain electrons (high electronegativity, form anions).

group vs period

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

A group is a vertical column (same valence electrons, similar properties); a period is a horizontal row (same number of shells).

Myth: Radius increases across a period

“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”

It DECREASES left-to-right as nuclear charge pulls electrons closer.

Order of reaction — common mistake

“Food lasts longer in a fridge because cold slows the reactions that spoil it.”

A frequent error is reading the order off the balanced equation's coefficients. In reality, the power to which a reactant's concentration is raised in the experimentally-determined rate law.

Memory trick: order comes from experiment, not stoichiometry (unless the step is elementary).

Rate constant

“Food lasts longer in a fridge because cold slows the reactions that spoil it.”

The proportionality constant k in the rate law; it rises sharply with temperature.

Memory trick: k depends on temperature and catalyst, not on how much reactant you have.