MATSEC AM Chemistry flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Relative atomic mass definition

The weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom; isotopes are weighted by natural abundance.

Avogadro constant value

6.02 × 10²³ particles per mole. It links the macroscopic mole to countable entities such as atoms, molecules, ions and electrons in stoichiometric calculations.

Empirical vs molecular formula

The empirical formula gives the simplest whole-number atom ratio; the molecular formula gives the actual number of atoms per molecule and is a whole-number multiple of the empirical formula.

Limiting reagent identification

The reactant that is completely consumed first. Divide each reactant's moles by its stoichiometric coefficient; the smallest quotient identifies the limiting reagent and controls the maximum product yield.

Percentage yield formula

Percentage yield = (actual mass of product obtained ÷ theoretical mass predicted from stoichiometry) × 100. Losses arise from side reactions, incomplete reaction, transfer losses and purification steps.

Molar volume at s.t.p.

One mole of any gas occupies about 22.7 dm³ at 0 °C and 1 atm (22.4 dm³ using the older value). Moles of gas = volume in dm³ ÷ molar volume.

Ideal gas equation

pV = nRT, where p is in pascals, V in m³, n in moles, T in kelvin and R = 8.314 J K⁻¹ mol⁻¹. Real gases deviate most at high pressure and low temperature.

Titration calculation relationship

At the end point, moles of acid divided by its stoichiometric coefficient equal moles of base divided by its coefficient: c₁V₁/n₁ = c₂V₂/n₂, with volumes in the same units.

Brønsted–Lowry acid and base

An acid is a proton donor and a base is a proton acceptor. Conjugate pairs differ by one H⁺; the stronger the acid, the weaker its conjugate base.

pH and pOH relation

pH = −log₁₀[H⁺] and pOH = −log₁₀[OH⁻]. At 298 K, pH + pOH = 14 because K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol² dm⁻⁶.

Weak acid approximation

For a weak monoprotic acid of concentration c with K_a small, [H⁺] ≈ √(K_a·c). The approximation fails when the degree of dissociation exceeds roughly five per cent.

Buffer action explained

A buffer resists pH change on adding small amounts of acid or base. Added H⁺ is removed by the conjugate base; added OH⁻ is removed by the weak acid.

Henderson–Hasselbalch equation

pH = pK_a + log₁₀([conjugate base] ÷ [weak acid]). Maximum buffer capacity occurs when pH = pK_a, that is when the two concentrations are equal.

Standard enthalpy of formation

The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 1 bar; by definition it is zero for an element in its standard state.

Hess's law statement

The total enthalpy change of a reaction is independent of the route taken, provided initial and final states are identical. It allows ΔH to be calculated from formation or combustion data.

Mean bond enthalpy limitation

Mean bond enthalpies are averaged over many compounds, so calculated ΔH values differ from accurate values; they also ignore intermolecular forces and states of reactants and products.

Entropy change sign

ΔS is positive when disorder increases: solid to liquid, liquid to gas, or fewer moles of gas becoming more moles of gas. A reaction is feasible when ΔG = ΔH − TΔS is negative.

Standard electrode potential meaning

The e.m.f. of a cell with the half-cell as cathode and a standard hydrogen electrode as anode, under standard conditions. More positive E° means greater tendency to be reduced.

Electrochemical cell e.m.f.

E°cell = E°(cathode) − E°(anode), both being reduction potentials. A positive value indicates a feasible reaction; oxidation always occurs at the anode.

Faraday and electrolysis

One faraday, 96 500 C mol⁻¹, is the charge on one mole of electrons. Mass deposited = (Q × M) ÷ (nF), where n is the number of electrons per ion.

Rate equation meaning

Rate = k[A]ᵐ[B]ⁿ, where m and n are orders determined experimentally. The overall order is m + n; k is the rate constant and depends on temperature.

First-order half-life

For a first-order reaction, t½ = ln2 ÷ k ≈ 0.693 ÷ k. The half-life is constant and independent of the initial concentration of the reactant.

Arrhenius equation

k = A·e^(−E_a/RT). A plot of ln k against 1/T is a straight line of gradient −E_a/R and intercept ln A, allowing activation energy to be found.

Dynamic equilibrium condition

Forward and reverse reaction rates are equal, so concentrations remain constant while both reactions continue. Equilibrium can be reached from either direction at a given temperature.

Le Chatelier's principle

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the position of equilibrium shifts to oppose that change, provided temperature is the variable altered.

Catalyst effect on equilibrium

A catalyst lowers activation energy and speeds both forward and reverse reactions equally. Equilibrium is reached faster but the position and value of K remain unchanged.

K_p and K_c relation

K_p = K_c(RT)^Δn, where Δn is the change in moles of gas between products and reactants. Pure solids and liquids are omitted from both equilibrium expressions.

Homologous series definition

A family of organic compounds with the same general formula and functional group, showing similar chemical properties and a gradual trend in physical properties with increasing chain length.

Structural isomerism

Compounds with the same molecular formula but different structural formulae, including chain, position and functional-group isomers, which typically show different physical and chemical properties.

Stereoisomerism: optical

Non-superimposable mirror-image isomers arising from a chiral carbon bearing four different groups. They rotate plane-polarised light in opposite directions and have identical physical properties apart from this.

Electrophilic addition mechanism

An alkene's π electrons attack an electrophile such as HBr, forming a carbocation; the halide then attacks. Markovnikov's rule places the halogen on the more substituted carbon.

Nucleophilic substitution SN2

A one-step bimolecular mechanism in which the nucleophile attacks the carbon from the side opposite the leaving group, giving inversion of configuration and a rate depending on both reactants.