Ch. 8 Writing 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: CH 4 + O 2  CO 2 + H 2 O 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: CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (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 (C x H y ) 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, C 5 H 12, burns. Write and balance the reaction. C 5 H 12(g) + 8O 2(g)  5CO 2(g) + 6H 2 O (g)

Synthesis Reactions (Make!) Two elements react to make one product. Example: Magnesium burns in oxygen. 2Mg (s) + O 2(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: 2H 2 O (l)  2H 2(g) + O 2(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. 2AgNO 3(aq) + Cu (s)  Cu(NO 3 ) 2(aq) + 2Ag (s) Cu displaced Ag in AgNO 3 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. CaCl 2(aq) + 2AgNO 3(aq)  Ca(NO 3 ) 2(aq) + 2AgCl (s)

Acid + Base is a type of double displacement reaction. acid + base → water + salt Sulfuric acid, H 2 SO 4, and sodium hydroxide react. Write the reaction. H 2 SO 4 + 2NaOH → 2HOH + Na 2 SO 4 Or: H 2 SO 4 + 2NaOH → 2H 2 O + Na 2 SO 4

A helpful chart of the 5 reaction patterns:

A quick “Guide to the 5” If the reactant(s) are:Then it’s a ____ reactionAnd the products are 2 elementsSynthesisOne compound 1 compound heatedDecomposition2 or more chemicals Element and a CompoundSingle DisplacementAn element and a compound 2 ionic CompoundsDouble Displacement2 ionic compounds A hydrocarbon burningCombustionCO 2 and H 2 O

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