CHAPTER 11 Chemical Reactions

Parts of an Equation Iron + Oxygen → Iron (III) Oxide Chemical reactions are written with two parts: REACTANTS: Things you start with. They are at the beginning of the arrow and usually written first (though they don't have to be). PRODUCTS: The things you make. The arrow points to these. These are the things you end with.

Parts of an Equation THE ARROW ALWAYS POINTS TO THE PRODUCTS NO MATTER WHAT! Sometimes, words or symbols will be written on top of the arrow. That's fine. It's called a CATALYST. A catalyst is anything that makes chemical reactions happen faster – like sunlight with plants.

Take 2-3 minutes to label the reactions on your note sheet. - Highlight the reactants blue or green - Circle the products - If there is a catalyst, label it.

Parts of an Equation Its a good idea to indicate which PHASE OF MATTER each substance is in your written chemical reaction. Use these. • (s) – for solids • (l) – for liquids • (g) – if its a gas • (aq) – This one is new for you guys. This means AQUEOUS. Aqueous is when you dissolve a solid into a liquid. Ex: Salt water.

Take 2 minutes to label the phases of matter for the two equations above. Ask a friend if you don't know what phase something is.

Parts of an Equation Sometimes you're going to need to put some numbers in front of the substances in your chemical equation. These numbers are called COEFFICIENTS. Its just like algebra class. Use Coefficients to balance equations. O2 = One oxygen molecule, 2 oxygen atoms. 6 O2 = Six oxygen molecules, 12 oxygen atoms.

On your note sheet, you have a reaction that looks like this: __H2 + __O2 → __H2O Put COEFFICIENT(s) in the blanks to make both sides have the same # of Hydrogens and Oxygens.

Conservation of Mass There is a reason why both sides of the equation have to have the same number of atoms. Law of Conservation of Mass - “Atoms are neither created nor destroyed during a chemical reaction. The combined total mass of the reactants shall be equal to the combined total mass of the products.” That's something you learned about last semester. Basic chemistry. Even John Dalton knew about conservation of mass.

Curve Ball: Diatomic Elements Some elements are written as a pair in chemical equations. You have to memorize which ones they are... • Hydrogen • Chlorine • Nitrogen • Bromine • Oxygen • Iodine • Fluorine Some people say they remember these with the name “Br. HONClIF” but I dunno...

Six steps for writing equations: #1 Write down all the chemical formulas for the Reactant(s) AND the Products(s). It's a good idea to label each substance with the phase of matter that it is in.

Six steps for writing equations: #2 Put the formulas down in order, then draw an arrow that points from R → P like that. Leave room in case you have to add some coefficients later. If there are two or more things on one side of the arrow, put some plus signs between them.

Six steps for writing equations: #3 Count how many of each type of atom are on each side of the equation. If there is a polyatomic ion that appears on both sides, it is usually easier to count that as its own thing instead of each individual element.

Six steps for writing equations: #4 Put coefficients in front of your formulas to balance the equation. WARNING: Don't you dare try to change the formulas of your substances. That's not something you can do, so don't even think about it.

Six steps for writing equations: #5 Double check to make sure you balanced correctly. You need to have the SAME # OF EACH TYPE OF ELEMENT ON BOTH SIDES. People always mess this up, so make sure you double check.

Six steps for writing equations: #6 Simplify all your coefficients if possible. For example in math class you wouldn't write 24/48 , you would just write ½ instead. You won't get full credit on the quiz if you don't simplify your numbers.

New derived unit: Molarity Measure how much stuff is dissolved in an aqueous solution using MOLARITY. Step #1: Figure out moles of solute. Step #2: Figure out volume of solvent. Step #3: Do division.

New derived unit: Molarity Molarity formula = Moles / Liters Two options for units. - 12M -OR- - 12mol / L

Vocabulary Terms Solute – The stuff that is getting dissolved. Solvent – The stuff that is holding the dissolved particles.

Use MOLES to convert these. Keep these measurements in mind... - Mass (g) - Molar Mass (g/mol) - Volume (L) - Density (g/L) - Molarity (mol/L) Use MOLES to convert these.

Example #1: How many moles of HCl are there in 500mL of 12M solution? 500mL x 12 moles x 1L 1L 1000mL • Cancel out units. • Multiply it out.

Example #2: How many moles of NaOH are there in 700mL of 6.2M solution? 700mL x 6.2 moles x 1L 1L 1000mL • Cancel out units. • Multiply it out.

Example #3: 40 grams of CuSO4 are dissolved in 1L of water. What is the molarity? • Figure out the Molar mass of CuSO4 first. 40g CuSO4 x 1 mole 1L ?? g • Cancel out units. • Multiply it out.

Types of Chemical Reactions

Why identify reaction types? Sometimes you will have a nice teacher who tells you what the products of a reaction will be, but once in a while, you're going to be in a situation where you have to figure out the products all by yourself. Don't worry. There are a couple ways you can do that...

Why identify reaction types? Strategy #1: Use Mass % composition to figure it out. Strategy #2: Identify one of FIVE BASIC TYPES OF REACTIONS and go from there...

There are FIVE BASIC TYPES of chemical reactions. On a test/quiz, you will be expected to recognize them. #1. Synthesis #2. Decomposition #3. Single Replacement #4. Double Replacement #5. Combustion

1. Synthesis reaction Put TWO OR MORE smaller reactants together to make a bigger product. General formula: A(small) + X(small) → AX (big compound) Sometimes called a “combination reaction” because you're combining the reactants together...

Examples of synthesis rxns. 2 H2 + O2  2 H2O 2 K + Cl2  2 KCl H2O + N2O5  2 HNO3

Synthesis Reaction 2 Na (s) + Cl2 (g) → 2 NaCl (s)

Hints for identifying “Synthesis” reactions: • “Synth” means “to build” - meaning something is being put together or built in these reactions. • There is usually only one large product, and it is composed of the smaller reactants. • There are fewer things on the product side than the reactant side.

2. Decomposition Reaction Literally the exact opposite of synthesis reactions. Instead of building something, you are breaking it down. General Formula: AX(bigger) → A(small) + X(small) Start w/ one big thing, but end with the smaller parts that it was made out of.

Decomposition of nitrogen triiodide 2 NI3 → N2 + 3 I2 Key words: “Breaks down” “decomposes into”

Hints to identify “Decomposition” reactions: • “Composition” is whatever something is made out of, so decomposition is when you take these things out of whatever they were making before. • Start with a single, larger reactant that changes into two or more smaller products.

REPLACEMENT REACTIONS These come in two varieties: “Single” and “Double” replacement reactions. They USUALLY happen with IONIC compounds.

3. Single Replacement Rxns An element reacts with a compound (usually ionic) and takes the place of the element it is most similar to. A metal will replace a metal. A nonmetal will replace a nonmetal. General formulas: A + BX  B + AX (A is a metal) Y + BX  X + BY (Y is a nonmetal)

Examples of single replacement rxns. Mg + 2 AgNO3  2 Ag + Mg(NO3)2 Cl2 + 2 KBr  Br2 + 2 KCl 2 Al (s) + Fe2O3 (s)  Al2O3 (s) + 2 Fe (l)

Hey, Mr. Stephan. Tell your students that one analogy that you use to remember how Single Replacement reactions work…

Single-replacement reactions only occur if the “single” element is more reactive than the element it is trying to replace. If the “single” element is not more reactive, no reaction will occur. Indicate a non-reaction by writing NR.

4. Double Replacement Rxns Two compounds (usually ionic) react and exchange ions with each other. Remember: Cations (+) like to hang out with Anions (-). Don’t try to put two Cations together. Don’t try to put two Anions together, either. The general formula for a Double-replacement reaction is “BY + AX  BX + AY” • Anions swapped places in that example.

Examples of double replacement rxns. CaCl2 (aq) + Na2CO3 (aq)  2 NaCl (aq) + CaCO3 (s) 2 HCl (aq) + Na2S (aq)  2 NaCl (aq) + H2S (g) Pb(NO3)2 (aq) + 2 KI (aq)  2 KNO3 (aq) + PbI2 (s)

Hey, Mr. Stephan. Don’t you also have a lame analogy for Double-Replacement reactions? These guys probably want to hear that one, too!

Formation of a Precipitate If you mix two aqueous ionic compounds together, sometimes a PRECIPITATE will form. A precipitate is the solid stuff that falls out of a solution after a double-replacement reaction. There won’t always be a precipitate, but Sometimes there is. Watch this video: Lead Iodide Precipitation

5. Combustion Rxns In order for a combustion reaction to occur, a catalyst is required – either a spark or flame will do just fine. • Combustion means “to burn.” One of the reactants is a type of fuel. The other reactant is oxygen (you need O2 gas to keep a fire burning.)

Examples of combustion rxns. CH4 + 2 O2  CO2 + 2 H2O C4H9OH + 6 O2  4 CO2 + 5 H2O

Combustion Reactions always release a lot of ENERGY! 5. Combustion Rxns Combustion Reactions always release a lot of ENERGY! The products are typically H2O and CO2.

Types of Fuel Some examples of Fuel used in a Combustion reaction: OCTANE PROPANE BUTANE METHANOL