HESI A2 Chemistry flashcards

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

Density formula

Density equals mass divided by volume. Units are grams per cubic centimeter for solids, grams per milliliter for liquids, and grams per liter for gases. An object denser than water sinks.

Physical vs chemical change

A physical change alters state or form without creating new substances, such as melting or dissolving. A chemical change produces new substances, shown by color change, gas, precipitate, or heat release.

Significant figure multiplication rule

When multiplying or dividing, round the answer to the same number of significant figures as the measurement with the fewest significant figures. When adding or subtracting, round to the fewest decimal places instead.

Kelvin conversion

Kelvin equals degrees Celsius plus 273.15, or plus 273 for most exam calculations. Absolute zero is 0 K, equal to -273.15 degrees Celsius, the point where particle motion is minimal.

Atomic number and mass number

Atomic number Z counts protons and defines the element. Mass number A counts protons plus neutrons. Neutrons equal A minus Z, and a neutral atom has electrons equal to protons.

Isotopes

Isotopes are atoms of the same element with the same proton count but different neutron counts. They differ in mass number, and the weighted average of isotope masses gives the average atomic mass.

Periodic trend of atomic radius

Atomic radius decreases across a period because added protons pull electrons closer, and increases down a group because electrons occupy higher energy levels farther from the nucleus.

Aufbau electron filling order

Electrons fill orbitals from lowest to highest energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s. Each s holds two, p holds six, d holds ten, and f holds fourteen electrons.

Ion charge prediction

Group 1 metals form +1 ions, Group 2 metals form +2 ions, Group 17 halogens form -1 ions, and Group 16 elements form -2 ions, each reaching a noble-gas electron configuration.

Ionic bond formation

An ionic bond forms when a metal transfers one or more electrons to a nonmetal, creating oppositely charged ions held together by electrostatic attraction in a crystal lattice.

Crisscross formula method

Write the cation and anion with their charges, then use each charge as the subscript of the other ion. Reduce subscripts to the lowest whole-number ratio to keep the formula neutral.

Polyatomic ion examples

Common polyatomic ions: ammonium NH4+, nitrate NO3-, hydroxide OH-, sulfate SO4^2-, carbonate CO3^2-, and phosphate PO4^3-. Their charges must be memorized as units.

Covalent compound prefixes

Numeric prefixes for molecular compounds are mono- for one, di- for two, tri- for three, tetra- for four, penta- for five, and hexa- for six. The first element never uses mono-.

VSEPR tetrahedral shape

Four bonding pairs around a central atom give a tetrahedral shape with bond angles of about 109.5 degrees. Three bonds plus one lone pair give trigonal pyramidal; two bonds plus two lone pairs give bent.

Polar versus nonpolar molecule

A molecule is polar when it has polar bonds arranged asymmetrically so dipoles do not cancel. Symmetric arrangements, such as carbon dioxide, cancel dipoles and make the molecule nonpolar.

Balancing chemical equations

Adjust coefficients only, never subscripts, until each element has the same atom count on both sides. Coefficients represent mole ratios used in stoichiometry.

Limiting reactant

The limiting reactant is the reactant fully consumed first, setting the maximum product formed. Compare each reactant's moles divided by its coefficient; the smallest quotient is limiting.

Percent yield formula

Percent yield equals actual yield divided by theoretical yield, multiplied by 100. Theoretical yield is calculated from stoichiometry using the limiting reactant, while actual yield is measured experimentally.

Molarity formula

Molarity equals moles of solute divided by liters of solution. It expresses concentration and changes with temperature because solution volume expands when heated.

Dilution equation

M1 times V1 equals M2 times V2. The moles of solute stay constant when solvent is added, so concentration falls as volume rises.

Combustion reaction products

Burning a hydrocarbon with sufficient oxygen always produces carbon dioxide and water. Balance carbon first, then hydrogen, then oxygen last.

Solubility rules summary

Most Group 1 and ammonium salts dissolve, as do nitrates and acetates. Most chlorides dissolve except silver, lead, and mercury chlorides. Most carbonates, phosphates, and hydroxides are insoluble.

pH formula

pH equals the negative base-10 logarithm of hydrogen ion concentration. At 25 degrees Celsius, pH plus pOH equals 14, and a pH below 7 indicates an acidic solution.

Ideal gas law

Pressure times volume equals moles times the gas constant times kelvin temperature. Use R equal to 0.0821 liter-atmospheres per mole-kelvin, and always convert temperature to kelvin.

Specific heat equation

Heat equals mass times specific heat times temperature change. Water's specific heat of 4.184 joules per gram-degree means it absorbs considerable heat with little temperature change.

Exothermic versus endothermic

Exothermic reactions release heat and have negative enthalpy change. Endothermic reactions absorb heat and have positive enthalpy change. Bond formation releases energy; bond breaking absorbs it.

Le Chatelier's principle

A system at equilibrium shifts to counteract a disturbance. Adding reactant shifts toward products, adding product shifts toward reactants, and raising pressure shifts toward fewer gas moles.

Percent by mass

Percent by mass equals the mass of solute divided by the total mass of solution, multiplied by 100. It is temperature independent, unlike molarity.

Electrolyte definition

An electrolyte dissolves in water to produce ions and conducts electricity, such as salts and strong acids. Nonelectrolytes dissolve as intact molecules and do not conduct.

Precision versus accuracy

Precision describes how closely repeated measurements agree with each other. Accuracy describes how close a measurement is to the true accepted value. Random error harms precision; systematic error harms accuracy.

Empirical formula determination

Assume 100 grams, convert each element's mass to moles, then divide all mole values by the smallest one. Round to whole numbers to get the lowest whole-number ratio.

Alpha and beta decay

Alpha decay emits a helium nucleus, lowering mass number by four and atomic number by two. Beta decay raises atomic number by one with mass number unchanged.

Half-life calculation

After n half-lives, the remaining fraction of a sample is one-half raised to the n power. After three half-lives, one-eighth of the original amount remains.

Net ionic equation

A net ionic equation shows only the ions that actually react. Spectator ions, which appear unchanged on both sides, are canceled from the complete ionic equation.

Oxidation and reduction

Oxidation is loss of electrons and raises oxidation number. Reduction is gain of electrons and lowers oxidation number. Both always occur together in a redox reaction.

Percent error formula

Percent error equals the absolute difference between measured and accepted values, divided by the accepted value, multiplied by 100. A large value signals a systematic problem.

Titration equivalence point

At the equivalence point, moles of hydrogen ions equal moles of hydroxide ions. Use molarity times volume times the number of ionizable protons or hydroxides to compare reactants.

Buffer solution

A buffer resists pH change when small amounts of acid or base are added. It contains a weak acid with its conjugate base, or a weak base with its conjugate acid.

Solution stoichiometry steps

Convert volume and molarity to moles, apply the balanced-equation mole ratio, then convert the result to the requested unit of mass, volume, or concentration.

Hydrogen bonding

Hydrogen bonds form when hydrogen bonded to nitrogen, oxygen, or fluorine is attracted to a lone pair on another such atom. They raise boiling point and surface tension.

Collision theory

Reactions occur only when particles collide with enough energy and correct orientation. Raising temperature, concentration, or surface area increases the frequency or energy of effective collisions.

Calorimetry relationship

In an insulated calorimeter, heat released by the reaction equals heat absorbed by the water, so the reaction's heat is the negative of mass times specific heat times temperature change.

Hydrate naming

A hydrate contains water molecules in its crystal structure. Name it with a prefix and the word hydrate, as in copper(II) sulfate pentahydrate for CuSO4 with five waters.

Chromatography separation

Chromatography separates mixture components by how strongly they adhere to a stationary phase versus dissolve in a moving mobile phase. Different attractions cause different travel distances.

Metallic bonding

Metallic bonding involves a lattice of positive ions surrounded by delocalized electrons. Moving electrons explain electrical and thermal conductivity, malleability, ductility, and metallic luster.