MTTC Chemistry (018) flashcards

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

Accuracy versus precision

Accuracy is closeness of a measurement to the true value; precision is closeness of repeated measurements to one another. A set can be precise yet inaccurate when a systematic error is present.

Percent error formula

Percent error equals the absolute value of measured minus accepted, divided by the accepted value, then multiplied by 100. It is reported as a positive percentage.

Density as conversion factor

Density equals mass divided by volume. It converts between grams and milliliters for a given material, and it changes with temperature, especially for gases.

Independent variable

The independent variable is the factor deliberately changed by the experimenter. The dependent variable is measured in response, and controlled variables are held constant so comparisons remain valid.

Quantum numbers n and l

The principal quantum number n gives the shell and average energy. The value l ranges from zero to n minus one and sets subshell shape: s, p, d, and f.

Hund's rule

Within a subshell of equal-energy orbitals, electrons occupy orbitals singly with parallel spins before any pairing occurs, minimizing electron-electron repulsion and giving the lowest-energy configuration.

Atomic radius trend

Atomic radius decreases left to right across a period as effective nuclear charge rises, and increases down a group as electrons fill higher shells with greater shielding.

Photoelectric equation

Einstein's relation E = hν, or E = hc/λ, gives the energy of one photon. Electrons are ejected only when photon energy exceeds the metal's work function.

VSEPR tetrahedral geometry

Four electron domains around a central atom give a tetrahedral arrangement with ideal angles of 109.5°. Each lone pair replaces a bond and compresses the angles to about 107° or 104.5°.

Hydrogen bonding

Hydrogen bonding is a strong dipole attraction between hydrogen bonded to nitrogen, oxygen, or fluorine and a lone pair on another such atom. It explains water's unusually high boiling point.

Bond order and bond length

Higher bond order means more shared electron pairs, a shorter bond, and greater bond dissociation energy. A triple bond is therefore shorter and stronger than a double or single bond.

Formal charge

Formal charge equals valence electrons minus the sum of nonbonding electrons and half the bonding electrons. Preferred structures keep formal charges near zero and negative charge on the more electronegative atom.

Limiting reactant

The limiting reactant is consumed completely first and fixes the maximum amount of product. It is identified by converting each reactant to product moles and choosing the smallest result.

Percent yield

Percent yield equals actual yield divided by theoretical yield, multiplied by 100. The theoretical yield comes from stoichiometry using the limiting reactant and the balanced equation.

Empirical formula

The empirical formula gives the simplest whole-number ratio of atoms in a compound. The molecular formula is a whole-number multiple of it, determined from the molar mass.

Net ionic equation

A net ionic equation shows only the species that actually change. Spectator ions appear identically on both sides and are dropped after soluble strong electrolytes are written as separated ions.

Hess's law

The enthalpy change of an overall reaction equals the sum of the enthalpy changes of its individual steps. Reversing a step changes the sign of ΔH; multiplying a step multiplies ΔH.

Arrhenius equation

The expression k = Ae to the power minus Ea over RT shows that rate constants rise with temperature and fall with activation energy. Plotting ln k against 1/T gives slope minus Ea over R.

Le Châtelier's principle

An equilibrium system shifts to partially offset a stress such as a concentration change, pressure change, or temperature change. Only temperature changes actually alter the value of the equilibrium constant.

Gibbs free energy criterion

The relation ΔG = ΔH minus TΔS predicts spontaneity at constant temperature and pressure: a negative ΔG means the process is spontaneous, and ΔG equal to zero defines equilibrium.

Molality

Molality is moles of solute per kilogram of solvent. Unlike molarity, it does not change with temperature, so it is the concentration unit used in colligative property calculations.

Freezing point depression

The equation ΔTf = i·Kf·m gives the freezing point lowering, where i is the van't Hoff factor for dissolved particles, Kf is a solvent constant, and m is molality.

Strong versus weak electrolytes

Strong electrolytes ionize essentially completely and conduct electricity well. Weak electrolytes ionize only partially and conduct poorly, while nonelectrolytes dissolve without producing any ions.

Dilution equation

The relation M₁V₁ = M₂V₂ holds because the moles of solute are conserved during dilution. It is valid only when both volumes are expressed in the same unit.

Brønsted-Lowry acid

A Brønsted-Lowry acid is a proton donor and a Brønsted-Lowry base is a proton acceptor. Every acid has a conjugate base formed by removing one proton.

pH and pOH relation

pH equals negative log of hydronium concentration and pOH equals negative log of hydroxide concentration. At 25 °C, pH plus pOH equals 14 because Kw is 1.0 × 10⁻¹⁴.

Buffer capacity

A buffer resists pH change when it contains comparable amounts of a weak acid and its conjugate base. Buffering is strongest at pH equal to the pKa of that acid.

Equivalence point versus endpoint

The equivalence point is where acid and base have reacted in exact stoichiometric amounts. The endpoint is where the indicator changes color and should be chosen to match the equivalence pH.

Esterification

A carboxylic acid reacts with an alcohol under acid catalysis to form an ester plus water. The process is a reversible condensation that removes a water molecule.

Peptide bond

A peptide bond forms by condensation between the carboxyl group of one amino acid and the amino group of another, releasing water and linking the residues in a protein chain.

Addition versus substitution

Alkenes and alkynes typically undergo addition across the multiple bond. Alkanes and aromatic rings instead undergo substitution, as when benzene is halogenated with a catalyst.

Structural isomers

Structural isomers share a molecular formula but differ in how atoms are connected. Geometric isomers differ in arrangement about a double bond, while optical isomers are mirror images.