ORELA Chemistry (NT321) flashcards

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

Significant figures in division

When multiplying or dividing measured values, the answer keeps the same number of significant figures as the factor with the fewest. Example: 4.56 × 1.2 gives 5.5, not 5.472.

Average atomic mass

The weighted average of the masses of an element's naturally occurring isotopes, each weighted by its fractional abundance. It appears on the periodic table and is not the mass of any single atom.

Quantum number l

The angular momentum (subshell) quantum number. It ranges from 0 to n−1 and labels subshells s, p, d, f. It determines orbital shape and limits how many orbitals a shell contains.

Homogeneous versus heterogeneous mixture

A homogeneous mixture has uniform composition throughout, such as salt water or brass. A heterogeneous mixture has visibly distinct regions, such as sand with iron filings or oil and water.

Density and specific gravity

Density is mass divided by volume, commonly in g/mL or g/cm³. Specific gravity is the dimensionless ratio of a substance's density to water's density at 4 °C, about 1.00 g/mL.

Kelvin and Celsius conversion

K = °C + 273.15. A temperature difference of one degree Celsius equals one kelvin, but converting an absolute temperature requires adding or subtracting 273.15, not multiplying.

Ion charge from protons and electrons

Charge equals the number of protons minus the number of electrons. A neutral atom has equal numbers; losing electrons gives a positive cation, and gaining electrons gives a negative anion.

Periodic trend in ionization energy

Ionization energy generally increases left to right across a period and decreases down a group. Successive ionizations of one atom cost increasingly more energy, with large jumps after a shell empties.

Electronegativity and bond type

The electronegativity difference guides bond classification: below about 0.4 is nonpolar covalent, roughly 0.4 to 1.7 is polar covalent, and above about 1.7 is largely ionic.

VSEPR steric number

The steric number is bonding domains plus lone pairs on the central atom. It predicts electron geometry: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral.

Hydrogen bonding

A strong dipole attraction occurring when hydrogen is covalently bonded to N, O, or F and interacts with a lone pair on a nearby N, O, or F. It explains water's high boiling point.

Molecular polarity

A molecule is polar only when polar bonds are arranged asymmetrically so bond dipoles do not cancel. CO₂ is nonpolar with polar bonds; H₂O is polar because its bent shape leaves a net dipole.

Formal charge

Formal charge = valence electrons − nonbonding electrons − half of bonding electrons. The preferred resonance structure minimizes formal charges and places negative charge on the more electronegative atom.

Octet rule exceptions

Some stable molecules violate the octet rule: BeCl₂ and BF₃ are electron deficient, NO has an odd electron, and period 3+ elements such as S and P can expand their octets.

London dispersion forces

Weak attractions from temporary, fluctuating dipoles in all molecules. They strengthen with more electrons and greater polarizability, so they grow with molar mass and matter most in nonpolar substances.

Mole conversions

One mole contains 6.022×10²³ particles and has a mass in grams equal to the molar mass. Moles equal mass divided by molar mass; one mole of ideal gas occupies 22.4 L at STP.

Limiting reactant test

Divide each reactant's available moles by its balanced-equation coefficient. The smallest quotient identifies the limiting reactant, which sets the maximum theoretical product yield.

Molarity and dilution

Molarity is moles of solute per liter of solution. Dilution conserves moles of solute, so M₁V₁ = M₂V₂; the volume is the total solution volume, not just the solvent added.

Net ionic equation

An equation showing only the species that actually change. Soluble strong electrolytes are written as separated ions, and spectator ions appearing unchanged on both sides are omitted.

Equilibrium constant expression

For aA + bB ⇌ cC + dD, K = [C]^c[D]^d divided by [A]^a[B]^b. Pure solids and liquids are excluded. K greater than 1 means products are favored at equilibrium.

Le Châtelier's principle

When a stress is applied to a system at equilibrium, the system shifts to relieve it. Adding reactant or removing product shifts forward; compressing gas shifts toward fewer gas moles.

pH and pOH

pH = −log[H₃O⁺] and pOH = −log[OH⁻]. At 25 °C, pH + pOH = 14 and K_w = 1.0×10⁻¹⁴, so a solution with pH 3 has pOH 11.

Calorimetry equation

q = mcΔT gives heat absorbed or released by a substance, using mass, specific heat, and temperature change. In a bomb calorimeter, heat of reaction equals the negative of C_calΔT.

Hess's law

The enthalpy change of an overall reaction equals the sum of the enthalpy changes of its steps. It allows ΔH to be calculated from formation enthalpies or from any path of known reactions.

Gibbs free energy criterion

ΔG = ΔH − TΔS determines spontaneity: negative ΔG is spontaneous, positive is nonspontaneous, and zero means equilibrium. ΔG° = −RT ln K and ΔG° = −nFE°_cell.

First-order half-life

For a first-order reaction, t₁/₂ = 0.693/k, independent of starting concentration. After n half-lives, the fraction of reactant remaining is (1/2)ⁿ.

Arrhenius equation

k = Ae^(−E_a/RT) shows that the rate constant rises with temperature and falls with activation energy. A catalyst speeds a reaction by providing a path with lower E_a.

Electrochemical cell conventions

Oxidation occurs at the anode and reduction at the cathode. E°_cell = E°_cathode − E°_anode; a positive cell potential means a spontaneous galvanic reaction with negative ΔG°.

Half-life in nuclear decay

The time for half of a radioactive sample to decay. After n half-lives the fraction remaining is (1/2)ⁿ, so activity follows A = A₀(1/2)^(t/t₁/₂).

Alpha and beta decay

Alpha decay emits a helium-4 nucleus, lowering mass number by 4 and atomic number by 2. Beta-minus decay converts a neutron to a proton, raising atomic number by 1 with mass number unchanged.

Esterification

A carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester plus water. The reaction is reversible, and the ester usually has a pleasant fruity odor.

Chirality and enantiomers

A chiral carbon is bonded to four different groups. Its two nonsuperimposable mirror-image forms are enantiomers, which rotate plane-polarized light in opposite directions and have identical physical properties.