Ch. 8Writing chemical reactions

and looking for reaction patterns.

What is a Chemical Reaction?• Compounds break up and the

atoms get rearranged• The same atoms are there

before & after (conservation of mass)

• New substances are made• Evidence: gas released, heat

& energy change, precipitate forms (a solid, so solution gets cloudy)

• Overall, energy is conserved

How we show a reaction:

• WORDS: methane + oxygen burns to produce carbon dioxide and water

• SYMBOLS: CH4 + O2 CO2 + H2O

• But this isn’t balanced; it needs to have same number of atoms on each side, so we add coefficients to balance it.

Here it is, balanced:

• CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

• The (g) stands for “gas” • This means: 1 mole of methane

reacts with 2 moles of oxygen; making 1 mole of carbon dioxide and 2 moles of water.

Aluminum reacts with Bromine

How to Balance the Reaction

Three Steps to write and balance a reaction: • 1. Write down the correct formulas for

the reactants & the products• 2. Count up the types of atoms on the

left side and see if they match those on the right side

• 3. Add coefficients in front of the molecules until all atoms are balanced (both sides equal)

• Never Change Subscripts when adding coefficients!

A reminder about Diatomic Elements!Long live Mrs. HOFBrINCl !If it’s not one of these, then it is

shown a “monatomic” in a chemical reaction; for example, iron is just “Fe”

Five Types of Reactions to Recognize

Combustion (Burn!)

• Some Hydrocarbon burns (CxHy) in oxygen and produces carbon dioxide & water

• x and y represent the numbers of C and H atoms, and they vary a lot

• Example: Pentane, C5H12, burns. Write and balance the reaction.

• C5H12(g) + 8O2(g) 5CO2(g) + 6H2O(g)

Synthesis Reactions (Make!)

• Two elements react to make one product.

• Example: Magnesium burns in oxygen.• 2Mg(s) + O2(g) 2MgO(s) • Note: This is NOT a combustion

reaction. Why not?• Ans: Mg is not a hydrocarbon.

Decomposition Reactions (Break!)

• One reactant compound is heated and makes two or more products.

• Example: Water is broken down by electrolysis to make hydrogen and oxygen gas. Write and balance the reaction:

• 2H2O(l) 2H2(g) + O2(g)

Single Displacement (switch!)

• One element changes places with another element in a compound.

• compound + element compound + element

• Silver nitrate reacts with copper to yield copper (II) nitrate and silver.

• 2AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2Ag(s)

• Cu displaced Ag in AgNO3

• Which metals can displace others is seen in an “Activity Series” chart.

Double Displacement (switch!)

• Two compounds break up and switch partners.

• AB + CD AD + CB

• Calcium chloride and silver nitrate react to produce calcium nitrate and silver chloride.

• CaCl2(aq)+ 2AgNO3(aq) Ca(NO3)2(aq) + 2AgCl(s)

Acid + Base is a type of double displacement reaction.

• acid + base → water + salt

• Sulfuric acid, H2SO4 , and sodium hydroxide react. Write the reaction.

• H2SO4 + 2NaOH → 2HOH + Na2SO4

• Or:• H2SO4 + 2NaOH → 2H2O + Na2SO4

A helpful chart of the 5 reaction patterns:

A quick “Guide to the 5”If the reactant(s) are: Then it’s a ____ reaction And the products are

2 elements Synthesis One compound

1 compound heated Decomposition 2 or more chemicals

Element and a Compound Single Displacement An element and a compound

2 ionic Compounds Double Displacement 2 ionic compounds

A hydrocarbon burning Combustion CO2 and H2O

Learn these Patterns!

• These five patterns will help you write and balance almost all the types of reactions we come across in inorganic chemistry!