A
absolute zero - the temperature at which all molecular motion should cease. Theoretically, absolute zero is the lowest temperature possible. It is -273.15°C or 0 K (zero kelvin).
accuracy - an indication of how close a measurement is to its accepted value.
acid - a substance that forms hydronium ions, H3O+, in water solution (Arrhenius definition); a proton, H+, donor (Brønsted-Lowry definition)
acid-base indicator - a dye that changes colours under different conditions of pH
acid-base titration - the procedure used to determine the concentration of an acid or base involving the gradual addition of either an acid or base.
activated complex - an unstable, short-lived particle formed as the result of a collision of particles in a chemical reaction. The activated complex is located at the top of a potential energy diagram. Bonds are in the process of both being formed and being broken.
activation energy (Ea) - the amount of energy required to initiate a chemical reaction. The difference in energy of the reactants and the energy of the activated complex.
activity series - a list of metals and hydrogen arranged in order of their chemical reactivity, such that any element in the series will displace ions of the elements below it from aqueous solutions of their salts.
amphiprotic - a substance that can function as an acid in some reactions and a base in others.
amphoteric - a term with the same meaning as amphiprotic.
anion - an ion carrying a negative charge. Anions result when atoms or groups of atoms (polyatomic ions) gain electrons.
anode - the site of oxidation (loss of electrons) in an electrochemical cell or electrolytic cell.
aqueous - made of, with, or by water.
aqueous solution - a homogeneous solution of particles dissolved in water.
Arrhenius theory - all acids produce hydrogen ions when they are dissolved in water, and all bases produce hydroxide ions; the hydrogen ions are responsible for the acidic properties of a solution and the hydroxide ions are responsible for the basic properties.
atom - the smallest particle of an element that can enter into chemical change. Atoms consist of a central nucleus, containing protons and usually neutrons, surrounded by electrons.
atomic mass - the exact mass of an atom in atomic mass units, amu or u.
atomic number - the number of protons in the nucleus of an atom.
Avogadro's hypothesis - equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules.
Avogadro's number - the number of particles in a mole; this number is approximately 6.02 × 1023
B
base - a substance that produces hydroxide, OH- ions in aqueous solution (Arrhenius definition; a proton, H+, acceptor `(Brønsted-Lowry definition)
binary acid - an acid whose molecules each consist of two elements; HCl for example.
bond enthalpy - the amount of energy stored in a single chemical bond between two atoms.
C
calorie - the quantity of heat required to raise the temperature of 1 gram of water by 1 degree Celsius.
calorimeter - an insulated device used to measure the amount of heat absorbed or released during a physical or chemical process.
catalyst - a substance that changes the speed of a chemical reaction without being permanently altered itself.
cathode - the electrode at which reduction (gain of electrons) occurs
cation - an ion carrying a positive charge due to the loss of electrons.
Celsius temperature scale - the temperature scale on which the boiling point of water is 100°;C and the freezing point, O°C.
chemical bond - the force holding atoms together in a combined state. This force may result from the attraction of opposite charges (ionic bond), the magnetic and electrical attraction of shared electrons (covalent bond), or a combination of these attractions.
chemical change - a change in the composition and properties of a substance, or substances, as the results of a chemical reaction.
chemical equation - a condensed statement that uses formulas to show the reactants and products in a chemical change.
chemical equilibrium - a condition in which two chemical changes exactly oppose one another. Equilibrium is a dynamic condition in which concentrations do not change and the rates of opposing reactions are equal.
chemical formula - a notation made with numbers and chemical symbols indicating the composition of a compound and the number of atoms of an element in one molecule.
chemical kinetics - the branch of chemistry concerned with the rates and mechanisms of chemical reactions.
chemistry - the study of matter, its structures, properties, and composition, and the changes that matter undergoes.
coefficients - a number preceding formula units (atoms, ions, molecules) in balanced chemical equations, indicating the relative number of units involved in the reaction.
combustion - a chemical reaction producing noticeable light and heat.
common-ion effect - the dissociation of a slightly soluble ionic compound is decreased by adding to the solution a readily soluble ionic compound that has an ion in common with the slightly soluble compound.
concentrated solution - one that contains a relatively large amount of solute.
concentration - the quantity of substance contained in a given volume of medium (solid, solution, etc.). It may be expressed as percentage, molarity, etc.
conjugate pair - an acid and a base that may be formed reversibly from one another. The conjugates differ from one another in composition by only a single hydrogen atom.
conversion factors - numbers that are used to change, or convert, from one unit to another.
corrosion - the deterioration and wearing away of metals.
covalent bond - a bond consisting of one pair of shared electrons. The force of attraction between elements sharing electrons.
D
Daniell cell - an electrochemical cell in which zinc metal reacts with aqueous copper ions
decomposition reaction - a chemical change in which a substance is broken down into two or more simpler substances.
density - mass per unit volume
diatomic - molecules consisting of two atoms. The seven diatomic molecules are H, N2, O2, F2, Cl2, Br2, I2
dilute solution - a solution that contains a relatively small concentration of solute.
dimension analysis - a problem solving technique that uses the units that are part of a measurement to help solve a problem. Also called factor analysis or unit analysis
dissociation - the separation of the ions of an ionic compound, usually brought about by dissolving an ionic compound in water.
double bond - the sharing of two electron pairs.
E
electrochemical cell - An apparatus that uses a spontaneous redox chemical reaction to produce electrical energy.
electrochemistry - the branch of chemistry that deals with the relation of the flow of electric current to chemical changes and with the conversion between chemical and electrical energy.
electrolysis - The process that uses electrical energy to bring about a nonspontaneous chemical reaction.
electrolytic cell- An apparatus that uses electrical energy to cause a nonspontaneous chemical reaction to occur.
electromotive force (emf) - the voltage difference between two electrodes in an electrochemical cell.
electron dot diagram - a diagram that uses dots to represent an element's valence electrons.
electronegativity - the attraction of an atom for electrons in a covalent bond.
electroplating - a process in which electrolysis is used as a means of coating an object with a layer (plate) of metal.
empirical formula - the formula showing the simplest ratio in which atoms combine to form a compound.
endothermic - A chemical reaction in which a greater amount of energy is required to break the existing bonds in the reactants than is released when the new bonds form in the product molecules. Energy appears as a reactant in the chemical equation.
end point - the point in a titration at which an indicator shows that neutralization has be reached, when [H+] = [OH–].
energy - The capacity to do work or produce heat. Energy exists as potential energy (stored energy) and kinetic energy (energy of motion).
enthalpy - The heat content of a system. The amount of energy stored within a substance.
entropy - A measure of the disorder or randomness of a system.
enzyme - an organic substance produced by living cells and capable of acting as a catalyst in biochemical reactions.
equilibrium constant - an expression of the ratio between the concentration of products in a reaction and the concentration of reactants. It expresses the extent to which equilibrium has proceeded in either direction. Its symbol is Keq
exothermic - A chemical reaction in which more energy is released than is required to break bonds in the initial reaction. Energy appears as a product in the chemical equation.
F
factor analysis - see dimension analysis.
FAQ - "Frequently Asked Question".
free energy - a measure of the tendency of a chemical change to occur spontaneously.
G
galvanic cell - an electrochemical cell.
Gibbs Free Energy - a measure of the tendency of a chemical change to occur spontaneously. Gibbs equation is given as ΔG = ΔH - TΔS where ΔH equals the change in enthalpy, T equals the Kelvin temperature, and ΔS equals the change in entropy.
gram atomic mass (gram molecular mass) - see also molar mass. The mass, in grams, of one mole of an element or molecule. Gram atomic mass is numerically equal to the atomic mass of the element which is expressed in atomic mass units (amu, or u).
H
Haber process - an industrial process for manufacturing ammonia from hydrogen and nitrogen.
half-cell - the anode or cathode and the chemicals that react there.
half-reaction - the reduction or oxidation portion of a redox reaction.
halogens - active nonmetals found in Group 17 of the periodic table
heat - The total amount of kinetic energy of the particles in a sample of matter; the form of energy produced by molecular motion. Heat energy always flows from a warmer object to a cooler object.
heat content - see enthalpy.
heat of formation - The amount of energy required or released when one mole of a compound is formed from its elements. Values are generally given for measurements made under standard conditions of 101.3 kPa and 25°C
Hess's law - The overall enthalpy change in a reaction is equal to the sum of the enthalpy changes of the individual steps of the process.
heterogeneous - describes a sample of matter that has parts with different compositions.
heterogeneous mixture - a mixture in which the ingredients are not uniformly dispersed.
heterogeneous reaction - a reaction that involves reactants in more than one phase.
homogeneous - describes a sample of matter that has uniform characteristics throughout.
homogeneous mixture - a uniform intermixture of particles. Samples from different parts of this mixture have the same composition.
homogeneous reaction - a reaction in which all the reactants are in the same phase.
hydrated ions - ions surrounded by water molecules.
hydrogen bond - the bond formed between a hydrogen atom in one molecule and a highly electronegative atom (N, O, F) in another molecule. The attraction is chiefly electrostatic.
hydrogen half-cell - see standard hydrogen half-cell.
hydronium ion - the H3O+ ion, which is formed by the combination of a proton with a water molecule. Its presence accounts for the properties of acids.
I
immiscible - liquids that are insoluble in one another. Oil and water are an example.
indicators - chemical substances that change colour at certain pH values. The colours and pH values vary with the indicator.
inhibitor - a substance that slows down the rate of a reaction.
insoluble - a substance that does not dissolve, or dissolves only slightly, in a solvent.
intensive property - a property that does not depend on the size of the sample. Melting points, boiling points, colour, conductivity, and density are examples.
intermolecular forces- attractive forces between separate covalent molecules. These forces may be very weak, as with van der Waals forces, or very strong, as with hydrogen bonding.
intramolecular bonding - the attractive forces within a molecule that holds the atoms together. Covalent bonding and ionic bonding, for example.
ion - an atom or group of atoms that has an electrical charge due to the gain or loss of electrons. Gaining electrons results in the particle having a negative charge (these are called anions). Losing electrons produces a positive charge (cations).
ionic bond - a chemical bond between ions resulting from the transfer of electrons from one of the bonding atoms to the other.
ionic compound - a compound, usually between a metal and a nonmetal, that is the result of ionic bonding.
ionization energy - the energy required to remove the most loosely held electron from an atom.
isoelectronic - atoms or ions that have the same electron configuration.
isotopes - atoms of the same element having different masses due to the different numbers of neutrons in their nuclei.
J
joule - the SI unit of heat and energy. One calorie equals 4.184 joules
K
Kelvin temperature scale - a temperature scale in which zero (0 K) is the lowest theoretical temperature. It is called absolute zero. All atomic motion stops at absolute zero. A change in temperature of 1 K is equal to a change in temperature of 1°C. 0 K = -273.15°C.
kinetic energy - Energy of motion; the energy a moving object has because of its motion.
kinetics - the study of reaction rates in chemical reactions.
L
law of conservation of energy - In any physical and chemical change, energy may change from one form to another but it cannot be created or destroyed. Also known as the First Law of Thermodynamics.
law of conservation of mass - the total mass of the reacting substances (the reactants) is equal to the total mass of the products of a chemical reaction. Matter cannot be created or destroyed during chemical reactions.
Le Châtelier's principle - when a system at equilibrium is subjected to a stress, the equilibrium will shift in the direction that tends to counteract or relieve the effect of the stress.
Lewis acid - an electron-pair acceptor.
Lewis dot diagram - see electron dot diagram
limiting reactant - the reactant that is used up first during a chemical reaction. Once that reactant is used up, the reaction will stop.
M
mass - a measure of the amount of material in a substance.
mass action expression - a mathematical relationship that compares the concentration of reactants and products in an equilibrium system. The concentration of each substance is raised to the power indicated by its coefficient in a balanced equation. Also referred to as the equilibrium constant expression, Keq.
miscible - two or more liquids that are soluble in one another.
molality - a unit of concentration defined as the number of moles of solute in one kilogram of solvent. Symbol m.
molarity - a unit of concentration defined as the number of moles of solute dissolved in one litre of solution. Symbol M (the SI symbol is c).
molar heat capacity - the heat energy required to raise the temperature of one mole of a substance by 1 Celsius (or 1 kelvin).
molar heat capacity = specific heat × molar mass
molar mass - the mass, in grams, of one mole of a substance. Molar mass is numerically equivalent to atomic (or molecular or formula) mass, except the unit is g·mol-1, not amu
N
net ionic equation - an equation that shows only ions that undergo change during a chemical reaction; spectator ions are omitted.
neutralization - a double displacement reaction between an acid and a base. The products are a salt and water.
O
open system - a system in which products are removed as they form and/or additional reactants are added.
oxidation - a half-reaction in which electrons are lost (released as products).
oxidation number - an integer assigned to an atom in a compound that helps track electrons during redox reactions. Sometimes called "apparent charge." During a redox reaction, an increase in oxidation number of an atom indicates a loss of electrons, or oxidation; a decrease in oxidation number indicates a gain of electrons, or reduction.
oxidizing agent - a substance that accepts/gains electrons and undergoes reduction. It "allows" the other substance to undergo oxidation.
P
polyatomic ion - an ion made up of two or more elements covalently-bonded.
polyprotic acid - an acid that can give up more than one hyrogoen atom. H2SO4 for example.
potential energy - Stored energy; the energy an object has because of its position.
product - the substance or substances that are produced in a chemical reaction.
R
rate-determining step - the slowest step in a reaction mechanism.
reactant - the starting substance in a chemical reaction.
reaction mechanism - the series of individual reactions that occur in an entire sequence of reactions.
reaction rate - the speed at which a reaction occurs; typically measured as the change in concentration of either reactants or products over time.
redox reaction - an oxidation-reduction reaction.
reducing agent - a substance that loses electrons and undergoes oxidation. It "allows" the other substance to undergo reduction.
reduction - a half-reaction in which electrons are gained.
reversible reaction - a reaction that can proceed in both the forward and reverse direction; products can react to re-form the initial reactants.
S
salt - an ionic compound consisting of a positive cation and a negative anion other than the hydroxide ion.
saturated solution - a solution that contains as much dissolved solute as it can under existing conditions.
significant figures - all certain digits in a measurement plus one uncertain digit.
single bond - the sharing of one electron pair.
solubility - the amount of solute that can dissolve in a given amount of solvent under a certain set of conditions.
solubility curve - a graph showing the relationship between solubility and temperature.
solubility product constant - the equilibrium constant for the solution of a slightly soluble ionic compound. Symbol Ksp. Ksp is equal to the product of the concentrations of ions raised to powers equatl to the subscripts in their chemical formulas.
solute - the dissolved substance in a solution.
solution - a homogeneous mixture of two or more substances.
solvent - the part of the solution that the solute is dissolved in; typically the substance present in larger amounts. In aqueous solutions the solvent is water.
specific heat - The amount of heat energy required to raise the temperature of one gram of a substance by one degree Celsius. The specific heat of water is 4.184 J/(g·°C)
spectator ion - an ion that is present during a reaction but does not undergo change; it appears on both sides of the reaction.
standard cell potential - see standard electrode potential.
standard electrode potential - the voltage obtained when a given half-cell is combined with a standard hydrogen half-cell, as measured under standard conditions (25°C, 1 M ion concentrations, 101.3 kPa pressure). Symbol E°.
standard hydrogen half-cell - the reference electrode that is used to determine the electrode potential of other half-cells. The hydrogen half-cell, assigned a voltage of 0.00 V under standard conditions (25°C, 1 M ion concentrations, 101.3 kPa pressure). The hydrogen half-cell reaction is:
2 H+(aq) + 2e- ↔ H2 (g)
standard solution - a solution with a known concentration.
standard temperature and pressure (STP) - the temperature and pressure at which many scientific measurements are made. Typically refers to a temperature of 0 °C (273 K) and 101.3 kPa (or 1 atm pressure).
stoichiometry - the area of mathematics that is concerned with numerical relationships of chemical formulas and chemical equations.
STP - see standard temperature and pressure.
strong acid - an acid that ionizes completely (or nearly so) in aqueous solution.
strong base - a base that dissociates completely into a metal ion and hydroxide ion (or nearly so) in aqueous solution.
structural formula - a chemical formula that shows how atoms are arranged within a moleucle or polyatomic ion. Each dash represents a single covalent bond.
supersaturated solution - a solution that is holding more dissolved solute than it normally can under a given set of conditions.
T
temperature - A measure of the average kinetic energy in a sample of matter.
thermodynamics - The study of energy changes during chemical reaction.
tincture - a solution in which the solvent is an alcohol.
titration - a procedure used to determine the concentration of a solution, involving reacting a solution with a known concentration with the solution with an unknown concentration. Often used with acids and bases.
triple bond - the sharing of three electron pairs.
U
unit analysis - see dimension analysis.
unsaturated - a solution which is holding less solute than it is able to hold under a given set of conditions.
V
valence electrons - electrons in the outermost energy level of an atom. Valence electrons are the electrons that participate in chemical bonding.
van der Waals forces - a weak attraction between molecules.
volt - the SI unit of voltage.
voltage - a measure of electric potential difference.
voltaic cell - an electrochemical cell.
W
weak acid - an acid that ionizes only slightly in aqueous solution.
weak base - a base that dissociates only slightly in aqueous solution.