For the reaction
(g) + 2 NO(g)
(g) + 2 H
O(g) the observed rate equation (or rate
As for many reactions, the experimental rate equation does not simply reflect
order in H
and second order in NO, even though the stoichiometric coefficients of
In chemical kinetics, the overall reaction rate is often explained using a
mechanism consisting of a number of elementary steps. Not all of these steps affect
the rate of reaction; normally the slowest elementary step controls the reaction rate.
For this example, a possible mechanism is:
O + H
Reactions 1 and 3 are very rapid compared to the second, so the slow reaction 2
is the rate determining step. This is a bimolecular elementary reaction whose rate is
given by the second order equation:
is the rate constant for the second step.
is an unstable intermediate whose concentration is determined
by the fact that the first step is in equilibrium, so that [N
] = K
, where K
equation leads to a rate equation expressed in terms of the original reactants
This agrees with the form of the observed rate equation if it is assumed that k =
. In practice the rate equation is used to suggest possible mechanisms which
The second molecule of H
does not appear in the rate equation because it
does not affect the overall reaction rate.
Main article: Arrhenius equation
Each reaction rate coefficient k has a temperature dependency, which is usually
given by the Arrhenius equation:
is the activation energy and R is the gas constant. Since at temperature T the
number of collisions with energy greater than E
to be proportional to e
. A is the
pre-exponential factor or frequency factor.
The values for A and E
are dependent on the reaction. There are also more
constants that do not follow this pattern.
A chemical reaction takes place only when the reacting particles collide.
However, not all collisions are effective in causing the reaction. Products are formed
only when the colliding particles possess a certain minimum energy called threshold
energy. As a rule of thumb, reaction rates for many reactions double for every 10
degrees Celsius increase in temperature,
constant at a higher temperature to its rate constant at a lower temperature is known
as its temperature coefficient (Q).Q
is commonly used as the ratio of rate constants
The pressure dependence of the rate constant for condensed-phase reactions
(i.e., when reactants and products are solids or liquid) is usually sufficiently weak in
the range of pressures normally encountered in industry that it is neglected in
The pressure dependence of the rate constant is associated with the activation
volume. For the reaction proceeding through an activation-state complex:
A + B
indicates the activation-state complex.
For the above reaction, one can expect the change of the reaction rate constant
(based either on mole-fraction or on molar-concentration) with pressure at constant
temperature to be:
In practice, the matter can be complicated because the partial molar volumes
and the activation volume can themselves be a function of pressure.
Reactions can increase or decrease their rates with pressure, depending on the
. As an example of the possible magnitude of the pressure effect, some
increased from atmospheric (0.1 MPa) to 50 MPa (which gives
A chemical equation
the form of symbols and formulae, wherein the reactant entities are given on the left-
hand side and the product entities on the right-hand side.
the symbols and formulae of entities are the absolute values of the stoichiometric
numbers. The first chemical equation was diagrammed by Jean Beguin in 1615.
A chemical equation consists of the chemical formulas of the reactants (the
starting substances) and the chemical formula of the products (substances formed in
the chemical reaction). The two are separated by an arrow symbol (
, usually read
as "yields") and each individual substance's chemical formula is separated from
others by a plus sign.
As an example, the equation for the reaction of hydrochloric acid with sodium
can be denoted:
2 HCl + 2 Na
→ 2 NaCl + H
This equation would be read as "two HCl plus two Na yields two NaCl and H
and its subscript, the chemical formulas are read using IUPAC nomenclature. Using
IUPAC nomenclature, this equation would be read as "hydrochloric acid plus sodium
yields sodium chloride andhydrogen gas."
This equation indicates that sodium and HCl react to form NaCl and H
. It also
molecules and the reaction will form two sodium chloride molecules and one
diatomic molecule of hydrogen gas molecule for every two hydrochloric acid and two
sodium molecules that react. Thestoichiometric coefficients (the numbers in front of
the chemical formulas) result from the law of conservation of mass and the law of
conservation of charge (see "Balancing Chemical Equation" section below for more
Symbols are used to differentiate between different types of reactions. To
denote the type of reaction:
"=" symbol is used to denote a stoichiometric relation.
" symbol is used to denote a reaction in both directions.
The physical state of chemicals is also very commonly stated in parentheses
after the chemical symbol, especially for ionic reactions. When stating physical state,
(s) denotes a solid, (l) denotes a liquid, (g) denotes a gas and (aq) denotes an aqueous
If the reaction requires energy, it is indicated above the arrow. A capital Greek
letter delta ( ) is put on the reaction arrow to show that energy in the form of heat is
added to the reaction.
is used if the energy is added in the form of light. Other
symbols are used for other specific types of energy or radiation.
Balancing chemical equations
As seen from the equation CH
2O, a coefficient of 2 must be placed before the oxygen gas on the reactants
side and before the water on the products side in order for, as per the law of
conservation of mass, the quantity of each element does not change during the
This chemical equation is being balanced by first multiplying H
match the number of P atoms, and then multiplying H
O by six to match the numbers
The law of conservation of mass dictates that the quantity of each element does
not change in a chemical reaction. Thus, each side of the chemical equation must
represent the same quantity of any particular element. Likewise, the charge is
conserved in a chemical reaction. Therefore, the same charge must be present on both
sides of the balanced equation.
One balances a chemical equation by changing the scalar number for each
chemical formula. Simple chemical equations can be balanced by inspection, that is,
by trial and error. Another technique involves solving a system of linear equations.
Balanced equations are written with smallest whole-number coefficients. If
there is no coefficient before a chemical formula, the coefficient 1 is understood.
The method of inspection can be outlined as putting a coefficient of 1 in front
of the most complex chemical formula and putting the other coefficients before
everything else such that both sides of the arrows have the same number of each
atom. If any fractional coefficient exists, multiply every coefficient with the smallest
number required to make them whole, typically the denominator of the fractional
coefficient for a reaction with a single fractional coefficient.
As an example, seen in the above image, the burning of methane would be
balanced by putting a coefficient of 1 before the CH
Since there is one carbon on each side of the arrow, the first atom (carbon) is
Looking at the next atom (hydrogen), the right-hand side has two atoms, while
the left-hand side has four. To balance the hydrogens, 2 goes in front of the H
+ 2 H
Inspection of the last atom to be balanced (oxygen) shows that the right-hand
2 before O
, giving the balanced equation:
+ 2 O
This equation does not have any coefficients in front of CH
, since a
coefficient of 1 is dropped.
An ionic equation is a chemical equation in which electrolytes are written as
dissociated ions. Ionic equations are used for single and double displacement
reactions that occur inaqueous solutions. For example, in the following precipitation
the full ionic equation is:
In this reaction, the Ca
and the NO
ions remain in solution and are not part
of the chemical equation. Because such ions do not participate in the reaction, they
are called spectator ions. A net ionic equation is the full ionic equation from which
the spectator ions have been removed. The net ionic equation of the proceeding
or, in reduced balanced form,
In a neutralization or acid/base reaction, the net ionic equation will usually be:
(aq) + OH
water molecule shown above. An example is the reaction of barium hydroxide with
phosphoric acid, which produces not only water but also the insoluble salt barium
phosphate. In this reaction, there are no spectator ions, so the net ionic equation is the
same as the full ionic equation.
Double displacement reactions that feature a carbonate reacting with an acid
If every ion is a "spectator ion" then there was no reaction, and the net ionic
An acid-base reaction is a chemical reaction that occurs between an acid and a
mechanisms involved and their application in solving related problems. Despite
several similarities in definitions, their importance becomes apparent as different
methods of analysis when applied to acid-base reactions for gaseous or liquid species,
or when acid or base character may be somewhat less apparent. Historically, the first
of these scientific concepts of acids and bases was provided by the French
chemistAntoine Lavoisier, circa 1776.
Since Lavoisier's knowledge of strong acids was mainly restricted to oxyacids,
which tend to contain central atoms in high oxidation states surrounded by oxygen,
such as HNO
hydrohalic acids, HCl, HBr, and HI, he defined acids in terms of their containing
The Lavoisier definition was held as absolute truth for over 30 years, until the 1810
article and subsequent lectures by Sir Humphry Davy in which he proved the lack of
oxygen in H
Te, and the hydrohalic acids.
This definition was proposed by Justus von Liebig circa 1838,
based on his
extensive works on the chemical composition of organic acids. This finished the
doctrinal shift from oxygen-based acids to hydrogen-based acids, started by Davy.
According to Liebig, an acid is a hydrogen-containing substance in which the
hydrogen could be replaced by a metal.
Liebig's definition, while completely
The Arrhenius definition of acid-base reactions is a more simplified acid-base
of bases that followed from his work with Friedrich Wilhelm Ostwald in establishing
the presence of ions in aqueous solution in 1884, and led to Arrhenius receiving the
Nobel prize in chemistry in 1903 for "recognition of the extraordinary services ...
As defined at the time of discovery, acid-base reactions are characterized by
Arrhenius acids, which dissociate in aqueous solution form hydrogen or the later-
termed oxonium (H
be used in favor of the older accepted term "oxonium"
reaction mechanisms such as those defined in the Brønsted-Lowry and solvent system
definitions more clearly, with the Arrhenius definition serving as a simple general
outline of acid-base character
The universal aqueous acid-base definition of the Arrhenius concept is
described as the formation of water from hydrogen and hydroxide ions, or hydronium
ions and hydroxide ions produced from the dissociation of an acid and base in
aqueous solution (2 H
Arrhenius acid-base reactions, a salt and water is formed from the reaction between
an acid and a base --
called a Neutralization reaction.
→ salt + water
For example, two moles of the basesodium hydroxide (NaOH) can combine with one
mole of sulfuric acid (H
) to form two moles of water and one mole of sodium
2NaOH + H
→ 2 H
O + Na
The Brønsted-Lowry definition, formulated independently by its two
proponents Johannes Nicolaus Brønsted andMartin Lowry in 1923 is based upon the
idea of protonation of bases through the de-protonation of acids -- more commonly
referred to as the ability of acids to "donate" hydrogen ions (H
) or protons to bases,
definition refers to the products of an acid-base reaction as conjugate acids and bases
to refer to the relation of one proton, and to indicate that there has been a reaction
between the two quantities, rather than a "formation" of salt and water, as explained
in the Arrhenius definition.
It defines that in reactions, there is the donation and reception of a proton,
which essentially refers to the removal of a hydrogen ion bonded within a compound
and its reaction with another compound, and not the removal of a proton from the
nucleus of an atom, which would require inordinate amounts of energy not attainable
through the simple dissociation of acids. In differentiation from the Arrhenius
definition, the Brønsted-Lowry definition postulates that for each acid, there is a
conjugate acid and base or "conjugate acid-base pair" that is formed through a
complete reaction, which also includes water, which is amphoteric
AH + B
General formula for representing Brønsted-Lowry reactions.
HCl (aq) + H
Acetic acid reacts incompletely with ammonia, no hydronium ions being