Chemical Equation · aiwiki.page

A chemical equation is a symbolic representation of a chemical reaction, using chemical formulas to identify the substances involved and coefficients to express their relative quantities. Reactants appear on the left and products on the right. A balanced equation gives the reaction’s stoichiometry: the quantitative relationships between substances consumed and formed. It describes a specified chemical transformation rather than necessarily showing the individual molecular steps through which that transformation occurs. (goldbook.iupac.org)

Symbols and notation

A general reaction can be written as:

a\mathrm{A}+b\mathrm{B}\rightarrow c\mathrm{C}+d\mathrm{D}.

The letters represent chemical species, while the coefficients $a,b,c,d$ specify their proportions. Plus signs separate species on the same side. A forward arrow indicates a net forward reaction; opposing half-arrows, $\rightleftharpoons$ , indicate chemical equilibrium. IUPAC also distinguishes an equals sign expressing a stoichiometric relationship from arrows describing reaction direction. (goldbook.iupac.org)

Subscripts belong to a substance’s formula, whereas coefficients multiply the entire formula. Thus, $2\mathrm{H_2O}$ represents two water molecules, containing four hydrogen atoms and two oxygen atoms. Changing a subscript changes the substance’s identity; changing a coefficient changes its represented quantity. Physical-state labels commonly include $(s)$ for solid, $(l)$ for liquid, $(g)$ for gas, and $(aq)$ for an aqueous solution. Conditions such as temperature or a catalyst associated with catalysis may be indicated near the arrow. (openstax.org)

Conservation and balancing

Balancing requires equal numbers of atoms of each chemical element on both sides. This expresses conservation of mass in ordinary chemical reactions. For example, the complete combustion of methane is:

\mathrm{CH_4}+2\mathrm{O_2}\rightarrow \mathrm{CO_2}+2\mathrm{H_2O}.

Both sides contain one carbon atom, four hydrogen atoms, and four oxygen atoms. The equation identifies oxygen as a reactant and carbon dioxide as a product. An equation showing the correct species but lacking balanced coefficients is often called a skeleton equation. Balancing changes coefficients, not formulas. Coefficients are conventionally reduced to the smallest whole-number ratio, and a coefficient of one is omitted. (openstax.org)

For equations involving charged species, total electric charge must also balance. In oxidation–reduction reactions, oxidation and reduction can be represented separately as half-reactions involving electrons. When these are combined into the overall equation, the numbers of electrons lost and gained must match, allowing the electrons to cancel. (openstax.org)

Quantitative interpretation

Coefficients give ratios of particles and of amounts of substance, measured in moles. For molecular substances, they specify relative numbers of molecules; for ionic substances, the interpretation may involve formula units or ions. They do not directly give mass ratios. Converting between mass and chemical amount requires molar mass. (openstax.org)

For the formation of ammonia, the stoichiometric equation is:

\mathrm{N_2}+3\mathrm{H_2}\rightarrow2\mathrm{NH_3}.

One mole of nitrogen reacts with three moles of hydrogen to form two moles of ammonia according to the represented transformation. The coefficient ratio therefore gives the conversion factor:

n(\mathrm{NH_3})=\frac{2}{3}n(\mathrm{H_2}),

where $n(\mathrm{H_2})$ denotes the hydrogen consumed in this reaction. A balanced equation supplies these proportional relationships; it does not specify how much material was initially present or establish that all reactants have actually reacted. (openstax.org)

Molecular and ionic equations

Reactions in aqueous solution can be represented at different levels. A molecular equation writes substances using their full formulas, even when dissolved ionic compounds are present as separated ions. A complete ionic equation explicitly displays dissolved strong electrolytes as ions. A net ionic equation removes spectator ions that appear unchanged on both sides. Solids, liquids, gases, and weak electrolytes are not indiscriminately split into ions. (openstax.org)

For example, mixing aqueous silver nitrate and sodium chloride can be represented by:

\mathrm{AgNO_3(aq)+NaCl(aq)\rightarrow AgCl(s)+NaNO_3(aq)}.

Its complete ionic form is:

\mathrm{Ag^+(aq)+NO_3^-(aq)+Na^+(aq)+Cl^-(aq)} \rightarrow \mathrm{AgCl(s)+Na^+(aq)+NO_3^-(aq)}.

Removing sodium and nitrate ions gives:

\mathrm{Ag^+(aq)+Cl^-(aq)\rightarrow AgCl(s)}.

The net ionic form emphasizes formation of solid silver chloride rather than the unchanged dissolved ions. All three representations preserve elemental and charge balance. (openstax.org)

Thermochemical equations and limitations

A thermochemical equation adds an enthalpy change, $\Delta H$ , to the balanced reaction. This value refers to the quantities represented by the coefficients and the specified physical states and conditions. Negative $\Delta H$ denotes an exothermic process; positive $\Delta H$ denotes an endothermic process. Reversing the equation reverses the sign, while multiplying all coefficients by a factor multiplies the associated enthalpy change by that factor. Fractional coefficients are therefore useful when normalizing a reaction to one mole of a substance. (openstax.org)

An overall chemical equation is not a reaction mechanism. In chemical kinetics, the experimentally determined rate law generally cannot be inferred simply by treating the overall stoichiometric coefficients as concentration exponents. Equations also have limits as descriptions of complex transformations: minor products may be omitted, and accumulating intermediates can make the relationship between reactant and product amounts vary during the process. A single overall equation may consequently describe only an approximation or a specified stage of a reaction. (openstax.org)