Chemical Equations

These interactions are called chemical reactions. A substance that undergoes a chemical reaction is called a reactant. A substance that is formed in a chemical

reaction is called a product.

• The law of conservation of mass states that in any chemical reaction, the mass of the products is always equal to the mass of the reactants.

• Thus we must balance the reactions.

• Balance metals, followed by non-metals, and then polyatomic ions if they are located on both sides.

• Balance the hydrogens.• Finally balance the oxygens.• Verify all values.

Synthesis Reactions

• In a synthesis reaction, two or more elements or compounds combine to form a new substance.

A + B → C

Synthesis Reactions

• 2 H2(g) + O2(g) → 2 H2O(g)

• 2 Mg(s) + O2(g) → 2 MgO(s)

• 2 K(s) + Cl2(g) → 2 KCl(s)

• 4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)

Synthesis Reactions

• When a non-metallic oxide reacts with water, the product is an acid.

• SO3(g) + H2O(l) → H2SO4(aq)

• Conversely, when a metallic oxide reacts with water, the product is a metal hydroxide.

• CaO(s) + H2O(l) → Ca(OH)2(aq)

Decomposition Reactions

• In a decomposition reaction, a compound breaks down into elements or other compounds. Therefore, a decomposition reaction is the opposite of a synthesis reaction.

C → A + B

Decomposition Reactions

• 2 H2O → 2 H2 + O2

• NH4NO3(s) → N2O(g) + 2 H2O(g)

• Generally decompose into elemental form, but maybe some compounds.

Decomposition Reactions

• Carbonates decompose into carbon dioxide and a metal oxide:

• MgCO3(s) CO2(g) + MgO(s)

• Hydroxides decompose into water and a metal oxide:

• Ca(OH)2(s) H2O(l) + CaO(s)

Decomposition Reactions

• Chlorates decompose into oxygen and a metal halide:

• 2 KClO3(s) 3 O2(g) + 2 KCl(s)

Combustion Reactions• A combustion reaction occurs when a

compound reacts in the presence of oxygen to form oxides and energy.

• S8(s) + 8 O2(g) → 8 SO2(g)

• Complete combustion of a hydrocarbon occurs when the hydrocarbon reacts completely in the presence of sufficient oxygen. The complete combustion of a hydrocarbon produces only water vapour and carbon dioxide gas as products.

• C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)

• Incomplete combustion of a hydrocarbon occurs when there is not enough oxygen present for the hydrocarbon to react completely. The incomplete combustion of a hydrocarbon produces water, carbon dioxide, carbon monoxide, and solid carbon in varying amounts.

• 2 C3H8(g) + 7 O2(g) → 2 CO2(g) + 8 H2O(g) + 2 CO(g) + 2 C(s)