Classifying chemical reactions Advanced Chemistry

Driving Force 3 – Formation of a Gas Double replacement reactions may also form a gaseous product Physically leaves and prevents reformation of reactants Very strong driving force

Compounds that Tend to Form Gases If you see the following substances in a reaction, realize that they will break up into gases and leave These compounds are formed from acids with carbonates, sulfites, sulfides, nitrites, and bases with ammonium compounds Watch For… It Turns Into… H2CO3 → CO2 (g) + H2O H2SO3 → SO2 (g) + H2O H2S → H2S (g) [rotten egg smell] NH4OH → NH3 + H2O 2HNO2 → NO (g) + NO2 (g) + H2O

Driving Force 4 – Transfer of Electrons Occurs between a metal and a non-metal Forms an ionic compound Metal transfers electrons to the non-metal Metal becomes a cation and non-metal becomes an anion Called an oxidation-reduction reaction (REDOX) Oxidation Loss of electrons Reduction Gain of electrons LEO GER OIL RIG

Oxidation Losing Electrons is Oxidation (LEO) Al → Al3+ + 3e- Mg → Mg2+ + 2e-

Reduction Gaining Electrons is Reduction (GER) Fe3+ + 3e- → Fe Cl + e- → Cl-

Oxidation-Reduction Mneumonic

Summary of Oxidation-Reduction Reaction

Practice, Practice, Practice 2Cs(s) + F2(g) → 2CsF(s) Ions present in CsF? Which element is oxidized? Which element is reduced? 2Na(s) + Br2(l) → 2NaBr(s) Ions Present: Oxidation: Reduction: 2Ca(s) + O2(g) → 2CaO(s)

Transfer of Electrons Visual

Driving Forces of Chemical Reactions Remember: Chemical changes are a result of chemical reactions Chemical reactions involve a change in energy Two compounds will react if there is at least one driving force present: Formation of a solid (precipitate) Formation of water Transfer of electrons Formation of a gas But there’s so many compounds and so many reactions! How can I possibly know them all??

Why Classify Chemical Reactions? There are MILLIONS of chemical reactions that occur in the world By classifying the type of chemical reaction, reactions become easier to understand One popular classification scheme breaks up the types of chemical reactions into six different categories: Double Replacement Acid-Base Neutralization Synthesis Decomposition Combustion Single Replacement

Double Displacement Reactions Anions and cations of two different molecules switch places Form entirely different compounds Driving forces observed: Formation of a solid Formation of a gas General form: AB + CD→AD + CB

Double Replacement Reactions Assume each reactant is aqueous Need to use solubility rules to determine if a solid forms If a solid does form, reaction can be written as a net ionic equation eliminating the spectator ions that remain unchanged during the reaction Example: Reaction of lead (II) nitrate with potassium iodide to form lead (II) iodide and potassium nitrate 𝑃𝑏 𝑁 𝑂 3 2 +2 𝐾𝐼→ 𝑃𝑏𝐼 2 +2𝐾𝑁 𝑂 3

Acid-Base Neutralization Reactions Special kind of double displacement reaction Takes place when a strong acid and strong base react with each other Driving forces observed: Formation of water H+ ion in the acid reacts with the OH- ion in the base Products formed are water and some ionic salt General form: HA + BOH → BA + HOH (H2O) Example: Reaction of hydrobromic acid with sodium hydroxide 𝐻𝐵𝑟+𝑁𝑎𝑂𝐻→𝑁𝑎𝐵𝑟+ 𝐻 2 𝑂

Reaction of iron and sulfur to form iron (II) sulfide Synthesis Reactions Two or more simple compounds combine to form a more complicated one Driving forces observed: Transfer of electrons General form A + B→AB or AB + C → ABC Example: Reaction of iron and sulfur to form iron (II) sulfide 8 𝐹𝑒+ 𝑆 8 →8 𝐹𝑒𝑆

Decomposition Reactions Opposite of a synthesis reaction A complex molecule breaks down to make simpler ones Driving forces observed: Transfer of electrons Formation of a gas General form: AB → A + B or ABC → AB + C Example: Electrolysis of water to make oxygen and hydrogen gas (Electricity is applied to break up the water into its components) 2 𝐻 2 𝑂→ 2𝐻 2 + 𝑂 2

Some Decomposition Rules Compound Decomposition Products Carbonates (HCO3-, CO3 2-) CO2 + Metal Oxide [Metal oxide can be H2O] Hydroxides (OH-) H2O and Metal Oxide Chlorates (ClO4-, ClO3-, ClO2-, ClO-) O2 and Metal Chloride Oxides O2 and Metal

Combustion Reactions Oxygen combines with a hydrocarbon (compound that contains only carbon and hydrogen) to form water and carbon dioxide Reactions produce heat Exothermic Driving forces observed: Formation of a gas Formation of water General form: CxHy + O2 → CO2 (g) + H2O (g) Example Burning of napthalene 𝐶 10 𝐻 8 +12 𝑂 2 →10 𝐶𝑂 2 +4 𝐻 2 𝑂

Single Displacement Reactions One element trades places with another element in a compound Driving forces observed: Transfer of electrons General form: A + BC → AC + B or A + BC → BA + C Example Magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas 𝑀𝑔+2 𝐻 2 𝑂→𝑀𝑔( 𝑂𝐻) 2 + 𝐻 2

Metals and Non-metals Reactivity Not all elements are equal in their ability to replace other elements Metals, such as lithium, sodium, potassium, strontium and calcium are referred to as “very active” metals due to the fact that they give up electrons very easily Other metals are “less active” Non-metals also vary in their reactivities In order to predict products of single replacement reactions or to predict if they will occur at all, an activity series is used New pink sheet!

A Handy Checklist for Classifying Type of Reactions Follow this series of questions When you answer “yes” to a question, stop! Does your reaction have oxygen as one of its reactants and carbon dioxide and water as products? If yes, then it’s a combustion reaction! Does your reaction have two (or more) chemicals combining to form one chemical? If yes, then it’s a synthesis reaction! Does your reaction have one large molecule falling apart to make several small ones? If yes, then it’s a decomposition reaction! Does your reaction have any molecules that contain only one element? If yes, then it’s a single displacement reaction Does your reaction have water as one of the products? If yes, then it’s an acid-base reaction! If you haven’t answered “yes” to any of the questions above, then you’ve got a double displacement reaction!