Section 5

1. Polyatomic ions 2. Balancing chemical equations 3. Types of chemical reactions

Ammonium ion

NH₄+

Hydroxide

OH^-

Nitrate

NO_3^-

Carbonate

CO_3^2-

Sulfate

SO_4^2-

Phosphate

PO₄^3-

What is a chemical reaction?

A chemical reaction is a process in which one set of substances, the reactants is converted into another set of substances, the products

What two forms can chemical equations be written in?

• a word equation in which the chemical names of all reactants and products are displayed

• a symbol equation in which the names of the substances are replaced by their elemental symbols.

Balance the equation of the combustion of ethanol:

ethanol + oxygen → carbon dioxide + water

  C₂H₅OH + O₂ → CO₂ + H₂O

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Calcium hydroxide will react with hydrochloric acid to produce calcium chloride and water. The equation for this reaction is:

Ca(OH)₂ + 2HCl → CaCl₂ + H₂O

How many water molecules do you need to make this equation balanced?

2 water molecules

Synthesis/formation

• combining atoms or elements to form a new compound

• usually multiple reactants to get 1 product

ex: hydrogen gas + oxygen gas → water

2H_{2 (g)} + O_{2 (g)} → 2H₂O _(l)

Single displacement

• when a single element is replaced by another element that is part of a compound

AB + C → A + BC

ex : a non-metal hydrogen is displaced by a metal zinc

Zn + 2HCl → ZnCl₂ + H₂ .

Double displacement

when parts of two ionic compounds are exchanged, making two new compounds

A⁺B^- + C⁺D^- → A⁺D^- + C⁺B^-

Ex : BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

Decomposition

• breaking apart a compound

ex: liquid water could be electrolysed into hydrogen and oxygen gas

2H₂O _(l) → 2H_{2 (g)} + O_{2 (g)}

Combustion

• when something burn in oxygen

ex: When methane gas burns in oxygen to produce carbon dioxide and water

CH_{4 (g)} + O_{2 (g)} → CO_{2 (g)} + H₂O _(l)