SYNTHESIS AND DECOMPOSITION REACTIONS

ELEMENTS AND THEIR OXIDES

Chapter 4.1 & 5.3

Another good one:

http://www.funnyjunk.com/channel/Chemistry+Jokes+are+fun/cpmqGpc

Predicting Chemical Reactions

Using the periodic table, we can figure out the properties of the elements

Ex. H, N, O, F, Cl, Br, I elements are all diatomic

Since the properties of the elements are predictable, the chemical reactions they undergo are also predictable

Ex. Na and K are both alkali metals that react with Cl to form very stable compounds

Synthesis Reactions

Two reactants combine to make a larger, more complex product

Synthesis reaction between sodium and chlorine:http://www.youtube.com/watch?v=Mx5JJWI2aaw

Synthesis of Ionic Compounds

Metal + Non-Metal Ionic Compound

Ex. 2 K (s) + Cl2 (g) 2 KCl (s)

REMEMBER: Always apply the criss cross rule when making ionic compounds!

Synthesis of Molecular Compounds

Non-metal + Non-metal Molecular compound

Can involve:

HYDROGEN: Easy to predict because H usually forms molecular compounds and follows general pattern of synthesis

Ex. H2 (g) + F2 (g) HF (g)

NO HYDROGEN: difficult to predict because the reaction depends on reaction conditions. Ex. C and O can make CO or CO2 depending on the amount of oxygen available to react

Oxides– A Special Type of Synthesis

Oxide: a compound made up of any element and oxygen

Basic oxides

Mg + O2 MgO

MgO + H2O Mg(OH)2

Acidic oxides

CO2 (g) + H2O (l) H2CO3 (aq)

SO3 (g) + H2O (l) H2SO4 (aq)

NO2 (g) + H2O (l) HNO3 (aq) + HNO2 (aq)

Recall:

pH < 7 = acidic solution, pH= 7 neutral solution, pH > 7= basic solution

Acids produce H+ ions in solution Bases produce OH- ions in solution

Decomposition Reactions

A compound breaks down to form two or more simpler products

Generally need energy (heat, catalyst, electricity) to get started

Decomposition of Ionic Compounds

Ionic compound Element + Element Ex. 2 KCl (l) 2 K (s) + Cl2 (g)

This reaction involves passing electricity through KCl to break it down into its elements K and Cl

Decomposition Involving Compounds

Compounds with polyatomic ions or molecular compounds are difficult to predict

Use chemical tests to detect what the products are (often testing for presence of gas)

Ex. 2 KClO3 2 KCl + 3 O2

Ex. CaCO3 CaO + CO2

Note: Both reactions require heat

Examples

Identify the reaction type:

1) 2 HCl H2 + Cl2

2) Cl2O (g) + H2O (l) 2 HClO (aq)

3) 2 AlCl3 (s) 2 Al (s) + 3 Cl2 (g)

4) KO (g) + H2O (l) 2 KOH (aq)

Some Helpful Links

Synthesis and decomposition reactions explained:http://www.youtube.com/watch?v=yS8noHTIJ_E

Basic and acidic oxides:http://www.youtube.com/watch?v=MtL64hRoTzQ

Homework

pg. 161 # 1, 2, 4pg. 204 #3, 4, 9