Glossary
Acid-Base Reactions
Reactions that involve the transfer of protons (H⁺ ions) from an acid to a base, often resulting in the formation of a salt and water.
Example:
The reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) to form sodium chloride and water is a common acid-base reaction.
Acids (Brønsted-Lowry)
According to the Brønsted-Lowry definition, a substance that donates a proton (H⁺ ion) in a chemical reaction.
Example:
In the reaction HCl + H₂O → H₃O⁺ + Cl⁻, HCl acts as a Brønsted-Lowry acid by donating a proton to water.
Amphiprotic
A substance that can act as both a Brønsted-Lowry acid (by donating a proton) and a Brønsted-Lowry base (by accepting a proton).
Example:
Water (H₂O) is an amphiprotic substance because it can donate a proton to become OH⁻ or accept a proton to become H₃O⁺.
Balanced Chemical Equation
A chemical equation where the number of atoms for each element and the total charge are the same on both the reactant and product sides.
Example:
The equation 2H₂ + O₂ → 2H₂O is a balanced chemical equation because there are four hydrogen atoms and two oxygen atoms on both sides.
Bases (Brønsted-Lowry)
According to the Brønsted-Lowry definition, a substance that accepts a proton (H⁺ ion) in a chemical reaction.
Example:
In the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, NH₃ acts as a Brønsted-Lowry base by accepting a proton from water.
Chemical Changes
Changes that result in the formation of new substances with different chemical properties due to the breaking and forming of chemical bonds.
Example:
The browning of an apple after being cut is a chemical change as new compounds are formed through oxidation.
Chemical Reaction
A process that involves the rearrangement of atoms and molecules to form new substances with different chemical properties.
Example:
When baking soda and vinegar mix, they undergo a chemical reaction to produce carbon dioxide gas, causing fizzing.
Combustion Reactions
A rapid reaction, typically with oxygen, that produces heat and light, often involving a hydrocarbon reacting to form carbon dioxide and water.
Example:
Burning methane gas (CH₄) in a Bunsen burner flame is a combustion reaction that releases significant energy.
Complete Ionic Equation
A chemical equation that shows all soluble ionic compounds as dissociated ions in solution, while insoluble compounds, gases, and pure liquids remain in their molecular form.
Example:
For the reaction of AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), the complete ionic equation would show Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq).
Conjugate Acid-Base Pairs
Two species that differ from each other by the presence or absence of a single proton (H⁺ ion). When an acid loses a proton, it forms its conjugate base; when a base gains a proton, it forms its conjugate acid.
Example:
In the reaction of acetic acid (CH₃COOH) with water, CH₃COOH is the acid and CH₃COO⁻ is its conjugate base.
Decomposition Reactions
A type of chemical reaction where a complex substance breaks down into two or more simpler substances.
Example:
Heating calcium carbonate (CaCO₃) to produce calcium oxide (CaO) and carbon dioxide (CO₂) is a decomposition reaction.
Dissociation
The process by which ionic compounds or acids separate into their constituent ions when dissolved in a solvent, typically water.
Example:
When solid NaCl is added to water, it undergoes dissociation, breaking apart into Na⁺(aq) and Cl⁻(aq) ions.
Double Replacement Reactions
A type of chemical reaction where the ions of two different compounds exchange places, forming two new compounds.
Example:
Mixing solutions of silver nitrate and sodium chloride results in a double replacement reaction, forming a white precipitate of silver chloride.
Endpoint
The point in a titration where a visible change, often a color change due to an indicator, signals that the reaction is complete.
Example:
During an acid-base titration, the solution turns pink when the endpoint is reached, indicating neutralization.
Equivalence Point
The theoretical point in a titration where the moles of titrant added are stoichiometrically equal to the moles of analyte initially present.
Example:
The equivalence point in an acid-base titration occurs when the moles of H⁺ ions from the acid exactly equal the moles of OH⁻ ions from the base.
Excess Reactant
The reactant that is left over after the limiting reactant has been completely consumed in a chemical reaction.
Example:
In the grilled cheese example, the cheese would be the excess reactant since some slices would remain after all the bread is used.
Insoluble
A term describing a substance that does not dissolve significantly in a particular solvent, often forming a precipitate in aqueous solutions.
Example:
Silver chloride (AgCl) is an insoluble compound in water, which is why it forms a solid precipitate when silver ions and chloride ions are mixed.
Intermolecular Forces
Attractive forces that exist between molecules, influencing physical properties like boiling point and melting point.
Example:
The hydrogen bonds between different water molecules are intermolecular forces that cause water to have a relatively high boiling point.
Intramolecular Bonds
Chemical bonds that exist within a molecule, holding atoms together to form the molecule.
Example:
The covalent bonds between hydrogen and oxygen atoms within a single water molecule (H-O-H) are intramolecular bonds.
Law of Conservation of Mass
A fundamental principle stating that matter cannot be created or destroyed in a chemical reaction; the total mass of reactants must equal the total mass of products.
Example:
If you burn a log, the mass of the ash, smoke, and gases produced will exactly equal the original mass of the log and the oxygen consumed, illustrating the Law of Conservation of Mass.
Limiting Reactant
The reactant that is completely consumed first in a chemical reaction, thereby determining the maximum amount of product that can be formed.
Example:
If you have 5 slices of bread and 10 slices of cheese, but only need 2 slices of bread per sandwich, the bread is the limiting reactant for making grilled cheese sandwiches.
Mole Ratios
Ratios derived from the coefficients in a balanced chemical equation, used to convert between moles of different substances in a reaction.
Example:
In the reaction 2H₂ + O₂ → 2H₂O, the mole ratio of H₂ to O₂ is 2:1, meaning two moles of hydrogen react with one mole of oxygen.
Net Ionic Equation
A chemical equation that shows only the species (ions, atoms, molecules) that are directly involved in the chemical reaction, excluding spectator ions.
Example:
For the reaction of AgNO₃ and NaCl, the net ionic equation is Ag⁺(aq) + Cl⁻(aq) → AgCl(s), focusing only on the formation of the precipitate.
Neutralization
A specific type of acid-base reaction where an acid and a base react to form a salt and water, typically resulting in a solution with a pH close to 7.
Example:
Adding antacid (a base) to stomach acid (an acid) to relieve heartburn is an example of a neutralization reaction.
Oxidation
The process in a redox reaction where an atom, ion, or molecule loses electrons, resulting in an increase in its oxidation number.
Example:
When elemental iron (Fe) turns into Fe³⁺ in rust, it undergoes oxidation by losing three electrons.
Oxidation Number
A number assigned to an atom in a molecule or ion that indicates the degree of oxidation (loss of electrons) or reduction (gain of electrons) of that atom.
Example:
In H₂O, the oxidation number of oxygen is -2, and for each hydrogen atom, it is +1.
Oxidation-Reduction (Redox) Reactions
Chemical reactions that involve the transfer of electrons between reactants, leading to changes in their oxidation states.
Example:
The rusting of iron, where iron loses electrons and oxygen gains them, is a classic example of an oxidation-reduction (redox) reaction.
Physical Changes
Changes in the form or appearance of a substance that do not alter its chemical composition.
Example:
Melting an ice cube into liquid water is a physical change because the substance is still H₂O, just in a different state.
Precipitation Reactions
Reactions that occur in aqueous solutions where two soluble ionic compounds combine to form an insoluble solid product, called a precipitate.
Example:
When lead(II) nitrate solution is mixed with potassium iodide solution, a yellow solid of lead(II) iodide forms, indicating a precipitation reaction.
Reduction
The process in a redox reaction where an atom, ion, or molecule gains electrons, resulting in a decrease in its oxidation number.
Example:
In the formation of rust, oxygen gas (O₂) undergoes reduction as it gains electrons to form oxide ions (O²⁻).
Single Replacement Reactions
A type of chemical reaction where one element replaces another element in a compound.
Example:
When a piece of zinc metal is placed in a copper(II) sulfate solution, the zinc replaces the copper, forming zinc sulfate and solid copper in a single replacement reaction.
Spectator Ions
Ions present in a solution that do not participate in the chemical reaction and remain unchanged on both sides of the equation.
Example:
In the reaction between sodium chloride and silver nitrate, the Na⁺ and NO₃⁻ ions are spectator ions because they remain dissolved in solution.
Stoichiometry
The quantitative study of reactants and products in chemical reactions, based on the relationships derived from balanced chemical equations.
Example:
Using stoichiometry, a chemist can calculate exactly how much oxygen is needed to completely burn a specific amount of propane.
Synthesis Reactions
A type of chemical reaction where two or more simpler substances combine to form a more complex product.
Example:
The formation of water from hydrogen and oxygen gas (2H₂ + O₂ → 2H₂O) is a classic synthesis reaction.
Titration
An experimental technique used to determine the unknown concentration of a solution (analyte) by reacting it with a solution of known concentration (titrant).
Example:
A lab technician performs a titration to find the exact concentration of acetic acid in a sample of vinegar.