Vocabulary for Standard 9 Equilibrium





Acid ionization constant

(Ka) acid dissociation constant.


The equilibrium constant for the dissociation of an acid into a hydrogen ion and an anion. For example, the acid dissociation constant for acetic acid is the equilibrium constant for HC2H3O2(aq) doublearrowH+(aq) + C2H3O2-(aq), which is Ka = [H+][C2H3O2-]/[HC2H3O2].


Activated complex

transition state.

An intermediate structure formed in the conversion of reactants to products. The activated complex is the structure at the maximum energy point along the reaction path; the activation energy is the difference between the energies of the activated complex and the reactants.

Activation Barrier



Activation energy


The minimum energy required to convert reactants into products; the difference between the energies of the activated complex and the reactants.

Activity series


This is when you arrange elements in the order of how much they tend to react with water and acids.

Arrhenius effect



Buffered solution

pH buffer; buffer

A solution that can maintain its pH value with little change when acids or bases are added to it. Buffer solutions are usually prepared as mixtures of a weak acid with its own salt or mixtures of salts of weak acids. For example, a 50:50 mixture of 1 M acetic acid and 1 M sodium acetate buffers pH around 4.7.





catalyze; catalysis.

A substance that increases the rate of a chemical reaction, without being consumed or produced by the reaction. Catalysts speed both the forward and reverse reactions, without changing the position of equilibrium. Enzymes are catalysts for many biochemical reactions.

Chemical equilibrium



Chemical equilibrium expression



Chemical kinetics



Collision theory

collision model.

A theory that explains reaction rates in terms of collisions between reactant molecules.

Combustion reaction


A chemical reaction between a fuel and an oxidizing agent that produces heat (and usually, light). For example, the combustion of methane is represented as CH4(g) + 2 O2(g) = CO2(g) + 2 H2O( ell).

Common-ion effect


When the equilibrium position of a process is altered by adding another compound containing one of the same ions that's in the equilibrium.

Decomposition reaction

decompose; decomposable; decomposition

A reaction in which a compound is broken down into simpler compounds or elements. Compounds sometimes decompose if heated strongly or if subjected to a strong electric current (electrolysis).




Double displacement reaction

double displacement; double replacement; double replacement reaction; double exchange; exchange; metathesis.

A double displacement or metathesis is a reaction in which two reactants trade fragments:

AB + CD = AC + BD

Most commonly, the fragments are ions, e. g.

AgNO3(aq) + NaCl(aq) = AgCl(s) + NaNO3(aq)


Dynamic process



Dynamic equilibrium


Dynamic equilibrium is established when two opposing processes are occuring at precisely the same rate, so that there is no apparent change in the system over long periods of time.



The process of driving a redox reaction in the reverse direction by passage of an electric current through the reaction mixture.



A biological molecule that catalyzes reactions in living creatures.

Equilibrium   When the forward rate of a chemical reaction is the same as the reverse rate.  This only takes place in reversible reactions because these are the only type of reaction in which the forward and backward reactions can both take place.

Equilibrium constant

(K, Keq) equilibrium constant expression; law of mass action. Compare with reaction quotient *.

The product of the concentrations of the products, divided by the product of the concentrations of the reactants, for a chemical reaction at equilibrium. For example, the equilibrium constant for A + B = C + D is equal to [C][D] / ([A][B]), where the square brackets indicate equilibrium concentrations. Each concentration is raised to a power equal to its stoichiometric coefficient in the expression. The equilibrium constant for A + 2B = 3C is equal to [C]3/([A][B]2). For gas phase reactions, partial pressures can be used in the equilibrium constant expression in place of concentrations.



The oxidation or reduction part of a redox reaction.

Heterogeneous reactions





A catch-all term for any reaction in which the water molecule is split.


reactive intermediate; reaction intermediate.

A highly reactive substance that forms and then reacts further during the conversion of reactants to products in a chemical reaction. Intermediates never appear as products in the chemical equation for a net chemical reaction.

Le Châtelier’s Principle


Le Chatelier's principle predicts that when a stress is applied to an equilibrium mixture, the equilibrium will shift to relieve the stress. Stresses include temperature changes, pressure changes, and changes in the concentrations of species in the mixture. For example, increasing the concentration of a reactant drives the reaction forward; increasing the concentration of a product drives it backward.


oxidize; oxidizing; oxidized.

Oxidation is the loss of one or more electrons by an atom, molecule, or ion. Oxidation is accompanied by an increase in oxidation number on the atoms, molecules, or ions that lose electrons.

Oxidation-reduction reaction





When a substance loses electrons.

Oxidizing agent

oxidant; oxidizer.

A reactant that removing electrons from other reactants in a chemical reaction. Oxidizing agents cause other substances to be oxidized in chemical reactions while they themselves are reduced. For example, nitrate ion is an oxidizing agent in the following reaction:

Cu(s) + 4 H+(aq) + 2 NO3-(aq) rightarrowCu2+(aq) + 2 H2O( ell) + 2 NO2(g)

Copper gets oxidized (its oxidation number goes from 0 to +2) while the nitrogen gets reduced (from +5 in nitrate to +4 in nitrogen dioxide).


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An insoluble substance that has been formed from substances dissolved in a solution. For example, mixing silver nitrate and sodium chloride solutions produces a precipitate, insoluble silver chloride (along with soluble sodium nitrate.

Rate law


A rate law or rate equation relates reaction rate with the concentrations of reactants, catalysts, and inhibitors. For example, the rate law for the one-step reaction A + B rightarrowC is d[C]/dt = k[A][B].

Rate-determining step



Reaction mechanism


A list of all elementary reactions that occur in the course of an overall chemical reaction.

Reaction order



Reaction rate


A reaction rate is the speed at which reactants are converted into products in a chemical reaction. The reaction rate is given as the instantaneous rate of change for any reactant or product, and is usually written as a derivative (e. g. d[A]/dt) with units of concentration per unit time (e. g. mol L-1 s-1).

Reducing agent


A reducing agent is a substance that reduce another substance by supplying electrons to it. Reducing agents cause other substances to be reduced in chemical reactions while they themselves are oxidized. For example, tin(II) is a reducing agent in the following reaction:

Sn2+(aq) + 2 Fe3+(aq) rightarrowSn4+(aq) + 2 Fe2+(aq)



reduce; reduced; reducing.

Reduction is gain of one or more electrons by an atom, molecule, or ion. Reduction is accompanied by a decrease in oxidation number.

Reversible reactions


(A process or reaction that can be reversed by an infinitesimally small change in conditions. For example, ice and water coexist at 1 atm and 0°C; a very slight temperature increase causes the ice to melt; a tiny temperature decrease causes the water to freeze. Melting or freezing under these conditions can be considered reversible. Reversible processes are infinitesimally close to equilibrium. ) reversible process; reversible reaction. Compare with irreversible and irreversible process.



Single displacement reaction

single replacement reaction; single displacement reaction; single replacement.

A reaction of the form A + BC = B + AC. For example, zinc displaces hydrogen from hydrochloric acid in the following reaction: Zn(s) + 2 HCl(aq) = ZnCl2(aq) + H2(g).

Synthesis reaction

synthesize; synthetic reaction.

Formation of a complex product from simpler reactants. For example, water can be synthesized from oxygen and hydrogen gas: H2(g) + ½O2(g) rightarrowH2O( ell).