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Chemical equations

A chemical equation lists the initial and final chemicals involved in a chemical reaction.

In a chemical reaction, the starting materials are called reactants. The materials created are the products of the reaction.

A chemical equation lists the reactants followed by an arrow that points towards the products made in the reaction.

The equation below shows iron metal $$(\ce{Fe})$$ reacting with copper sulfate solution $$(\ce{CuSO4})$$ to form copper $$(\ce{Cu})$$ and iron sulfate solution $$(\ce{FeSO4})$$.

$$$\ce{Fe{(s)} + CuSO4{(aq)} -> Cu{(s)} +FeSO4{(aq)}}$$$

Aqueous ionic compounds are sometimes listed as individual ions in chemical equations.

The process of dissolving table salt can be written as:

$$$\ce{NaCl{(s)} + H2O{(l)} →Na^+ {(aq)} + Cl^- {(aq)} +H2O{(l)}}$$$

In a balanced equation, the number of atoms of different chemical elements is the same before and after the reaction.

Atoms are only re-arranged and not created or destroyed in a chemical reaction.

In order to balance chemical equations, coefficients are placed to the left of certain molecules. These coefficients indicate the relative number of molecules that react.

If a molecule does not have a coefficient, then the assumed coefficient is 1.

$$$\ce{CH4 {(g)} + 2O2 {(g)} → CO2 {(g)} + 2H2O {(g)}}$$$

This equation is balanced because there are the same number of atoms of each type on both sides of the equation: one carbon atom, four hydrogen atoms and four oxygen atoms.

A non-balanced chemical equation of the reaction between methane and oxygen.
A non-balanced chemical equation of the reaction between methane and oxygen.
The following equation is not balanced because there are more oxygen atoms in the products than in the reactants.

$$$\ce{CH4 {(g)} + O2 {(g)} -> CO2 {(g)} + 2H2O {(g)}}$$$

Non-balanced equations cannot occur.

Chemical equations can be balanced easily using a few guidelines.

If the reactants and products are known but the equation is not balanced, it is usually best to balance the more massive atoms first. Less massive atoms should be balanced later.

If one of the reactants or products consists entirely of one element, then this element should be balanced last because its coefficient does not affect the other elements.

In the combustion of propane, $$$\ce{C3H8 {(g)} + 5O2 {(g)} -> 3CO2 {(g)} +4H2O {(g)}}$$$ carbon is the most massive atom and is balanced first. Hydrogen is balanced second. Oxygen is balanced last because one of the reactants is pure oxygen. The $$\ce{O2}$$ coefficient is added last.

Balanced equations tend to be written so that the reactants and products have coefficients in the simplest possible whole number ratios.

If the coefficients have a greatest common factor greater than 1, they are all divided by this factor to reduce the equation.

In the equation $$$\ce{4C2H6 + 14O2 ->8CO2 + 12H2O}$$$ the coefficients are 4, 14, 8 and 12. The greatest common factor of the coefficients is 2.

The equation can therefore be simplified to $$$\ce{2C2H6 + 7O2->4CO2 + 6H2O}.$$$

The equation cannot be reduced any further because the greatest common factor of the coefficients is now 1.

Spectator ions are neither produced nor consumed in an aqueous chemical reaction.

These ions are present on both sides of the chemical equation.

In the reaction of magnesium metal $$(\ce{Mg})$$ with copper (II) sulfate $$(\ce{CuSO4})$$, sulfate ions $$(\ce{SO4^2-})$$ are spectators.

$$$ \begin{align*} &\ce{Mg {(s)} + Cu^2+ {(aq)} + SO4^2- {(aq)}} \\ &\ce{-> Cu {(s)} + Mg^2+ {(aq)} + SO4^2- {(aq)}} \end{align*}$$$

Chemical equations can be simplified by leaving out spectator ions because they are not produced or consumed. These simplified equations are called net ionic equations.

The net ionic equation for the previous magnesium metal reaction is $$$\ce{Mg {(s)} + Cu^2+ {(aq)} -> Cu {(s)} + Mg^2+ {(aq)}}$$$

Equations can also be simplified by removing molecules (such as water) that appear on both sides.

$$$\ce{NaCl {(s)} + H2O {(l)} →Na^+ {(aq)} + Cl^- {(aq)} +H2O{(l)}}$$$can be simplified to:

$$$\ce{NaCl {(s)} →Na^+ {(aq)} + Cl^- {(aq)}}$$$