NCEA L2 Chemistry flashcards

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

Mole formula

n = m/M, where m is mass in grams and M is molar mass in g mol⁻¹. This converts a measured mass of a substance into an amount in moles for stoichiometric calculations.

Concentration formula

c = n/V, with n in moles and V in litres, giving concentration in mol L⁻¹. Rearranged, n = cV gives the moles of solute present in a measured volume of solution.

Limiting reagent

The reactant that is completely consumed first and so determines the maximum product formed. Find it by dividing each reactant's moles by its stoichiometric coefficient and taking the smallest value.

Percentage yield

Actual mass of product divided by theoretical mass predicted from the limiting reagent, multiplied by 100. A yield below 100 percent can result from side reactions, losses in transfer or incomplete reaction.

Empirical formula

The simplest whole-number ratio of atoms in a compound, found by dividing percentage composition by atomic mass and then by the smallest ratio. The molecular formula is a whole-number multiple of it.

Molar gas volume

At standard temperature and pressure, one mole of any ideal gas occupies 22.4 L, so n = V/22.4. This allows volumes of gases to be converted directly into moles in stoichiometric calculations.

Dilution relationship

c₁V₁ = c₂V₂, because diluting a solution adds solvent but does not change the number of moles of solute. Use it to find the concentration after dilution or the volume of water added.

Ionic bonding

Electrostatic attraction between oppositely charged ions formed by electron transfer from a metal to a non-metal. The resulting lattice has high melting points and conducts only when molten or dissolved.

Metallic bonding

A lattice of positive metal ions surrounded by delocalised valence electrons. The mobile electrons explain electrical and thermal conductivity, while layers sliding over one another explain malleability and ductility.

Hydrogen bonding

A strong intermolecular attraction between a hydrogen atom covalently bonded to N, O or F and a lone pair on a nearby N, O or F atom. It raises melting and boiling points.

Dispersion forces

Weak, temporary attractions caused by instantaneous dipoles in electron clouds. They increase with the number of electrons, so larger molecules have stronger dispersion forces and higher boiling points.

Polar versus non-polar molecules

Polar bonds may cancel by symmetry, making a molecule non-polar, as in CO₂ and CCl₄. If bond dipoles do not cancel, the molecule has a net dipole and is polar.

VSEPR shapes

Electron regions repel to maximise separation. Two regions give linear 180°, three give trigonal planar 120°, four give tetrahedral 109.5°, with lone pairs reducing angles to about 107° or 104.5°.

Lewis structure construction

Count valence electrons, join atoms with single bonds, complete outer octets, then place leftover electrons as lone pairs on the central atom. Check that the total electron count is preserved.

pH definition

pH = −log₁₀[H₃O⁺], where [H₃O⁺] is the hydronium ion concentration in mol L⁻¹. A change of one pH unit corresponds to a tenfold change in hydrogen ion concentration.

Strong versus weak acids

Strong acids ionise completely in water, so their hydrogen ion concentration equals the acid concentration. Weak acids ionise only partially, producing fewer hydrogen ions at the same concentration.

Titration calculation

Use n = cV for the solution of known concentration, apply the balanced equation's mole ratio, then divide by the other solution's volume to find its concentration.

Oxidation and reduction

Oxidation is loss of electrons and reduction is gain of electrons. The oxidant gains electrons and is reduced; the reductant loses electrons and is oxidised in the same reaction.

Oxidation number rules

Oxygen is usually −2, hydrogen +1, free elements 0, and monatomic ions equal their charge. The sum of oxidation numbers equals the overall charge of the species.

Balancing redox half-equations

Balance atoms, add water to balance oxygen, add hydrogen ions to balance hydrogen, then add electrons to balance charge. Multiply each half-equation so electrons cancel before adding them.

Enthalpy change sign

A negative ΔH means the reaction releases energy to the surroundings and is exothermic. A positive ΔH means energy is absorbed and the reaction is endothermic.

Bond enthalpy calculation

ΔH equals the sum of bond energies broken minus the sum of bond energies formed. Bond breaking absorbs energy and bond making releases it, so the sign follows this difference.

Calorimetry formula

q = mcΔT gives the heat energy transferred, with m in grams, c = 4.18 J g⁻¹ °C⁻¹ for water, and ΔT in °C. Divide by moles to obtain enthalpy per mole.

Activation energy

The minimum collision energy needed for a reaction to occur. A catalyst lowers it by providing an alternative pathway, increasing the proportion of successful collisions.

Collision theory and temperature

Raising temperature increases both the frequency of collisions and the fraction of molecules with energy above the activation energy, so reaction rate increases markedly.

Dynamic equilibrium

Forward and reverse reactions proceed at equal rates in a closed system, so concentrations stay constant even though both reactions continue at the molecular level.

Equilibrium constant expression

K_c is the product of product concentrations raised to their coefficients divided by the same for reactants. Pure solids and liquids are omitted because their concentration is constant.

Le Chatelier's principle

When a constraint such as concentration, pressure or temperature changes, the equilibrium shifts in the direction that opposes the change until a new position of equilibrium is established.

Alkane general formula

Alkanes have the formula CₙH₂ₙ₊₂ and contain only single carbon–carbon bonds. They are saturated, undergo substitution with halogens, and burn in oxygen.

Alkene addition reactions

Alkenes contain a C=C double bond and undergo addition: hydrogen gives alkanes, halogens give dihaloalkanes, hydrogen halides give haloalkanes, and water with acid catalyst gives alcohols.

IUPAC naming rules

Select the longest carbon chain, number from the end nearest the functional group, and name substituents with locants in alphabetical order, for example 2-methylbutan-1-ol.

Functional group identification tests

Bromine water is decolourised by alkenes; acidified dichromate turns orange to green with primary or secondary alcohols; limewater turns milky with carbon dioxide from carboxylic acids and carbonates.