CHEMICAL REACTIONS Chem I: Chapter 6 Chem IH: Chapter 11 Reactants: Zn + I2 Product: Zn I2

Chemical Reactions… occur when bonds between valence electrons of atoms are formed or broken involve changes in matter, the making of new materials with new properties & energy changes

Signs of Chemical Reactions (These signs usually indicate a chemical rxn has occurred.) 1. Color change (ex: KMnO4) 2. Formation of precipitate 3. Odor change (ex: spoiled milk) 4. Gas release (baking soda & vinegar) 5. Energy change- (vermiculite) heat absorbed heat or light released

Equations represent chemical rxns. They show what is happening. Using Equations Equations represent chemical rxns. They show what is happening.

Rules All Equations Follow Show reactants (“before”) on the left, products (“after”) on the right Use an arrow() to separate reactants from products Use + signs between reactants Use + signs between products Ex:aluminum+ oxygenaluminum oxide

2 Ways to Write an Equation Word Equations- use names of compounds & elements. Ex: aluminum+oxygenaluminum oxide don’t tell you how much reactant or product is involved. Write this equation first

2. Chemical Equations 4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s) Show the relative amounts of reactants and products in a reaction Symbols represent elements Ex: Al Formulas represent compounds Ex: O2 & Al2O3 Coefficients are the numbers in front of the symbols or formulas. They show how many units (atoms, molecules, etc.) 4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)

Additional Rules Chemical Equations Follow Physical state of chemicals is shown after the formula or symbol (s) = solid (l) = liquid (g) = gas (aq) = in aqueous solution Energy is shown when it is an important part of a rxn. With the reactants in endothermic rxns With the products in exothermic rxns Balance-equations must be balanced

Word eq’n: carbon + oxygen  carbon dioxide Charcoal, which is basically carbon, reacts with oxygen to yield carbon dioxide. Word eq’n: carbon + oxygen  carbon dioxide + energy Chemical equation: C(s) + O2(g)  CO2(g) has a quantitative meaning as well.

Balancing Chemical Equations Because of the principle of the conservation of matter, an equation must be balanced. Def: It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788

Balancing Chemical Equations, cont. 4 Al(s) + 3O2(g)--->2 Al2O3(s) This equation means 4 Al atoms + 3 O2 molecules ---produces---> 2 units of Al2O3

Balancing Equations, cont. When balancing a chemical equation you may ONLY add coefficients in front of the compounds & elements to balance the reaction. You may not change subscripts b/c this changes the compound.

Subscripts vs. Coefficients Subscripts tell you how many atoms of an element are in a compound. Ex: the “2” in O2. Coefficients tell you the quan-tity, or number, of units of a chemical. Ex: the “3” in front of CO2.←

4 Steps to Balancing Equations Step #1: Write the correct symbols & formulas for the reactants and the products. (DO NOT TRY TO BALANCE IT YET! Just write the correct formulas first. And most importantly, once you write them correctly, DO NOT CHANGE THE FORMULAS!)

HINT: WHEN COMPLETING STEP #1 When writing formulas, remember The 7 diatomic elements H2, N2, O2, F2, Cl2, Br2, I2

4 Steps to Balancing Equations Step #2. Count the number of atoms for each element on the left side. Count the number of the atoms of each element on the right side.

4 Steps to Balancing Equations Step #3: Determine where to place coefficients in front of symbols & formulas so that the left side has the same number of atoms as the right side for EACH element.

Steps to Balancing Equations Step # 4: Check your answer to see if: The numbers of atoms on both sides of the equation are now balanced. The coefficients are in the lowest possible whole number ratios. (reduced)

Some Suggestions to Help You Helpful Hints for balancing equations: Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last. For polyatomic ions that appear on both sides of the equation count ions, not atoms. Water may be considered ionic, made up of H+ and OH-, if helpful.

Practice Problem

Balancing Equations What Happened to the Other Oxygen Atom????? 2 2 __ H2(g) + ___ O2(g) ---> __ H2O(l) LEFT SIDE RIGHT SIDE H=2 H=2 O=2 O=1 What Happened to the Other Oxygen Atom????? This equation is not balanced!

ANSWER TO PRIOR SLIDE _2_H2(g) + ___O2(g) --->_2_H2O(l) Re-Count the atoms _2_H2(g) + ___O2(g) --->_2_H2O(l) LEFT SIDE RIGHT SIDE H=4 H=4 O=2 O=2

Balancing Equations 2 3 ___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)

Practice Balancing Equations ____C3H8(g) + _____ O2(g) ----> _____CO2(g) + _____ H2O(g) ____B4H10(g) + _____ O2(g) ----> ___ B2O3(g) + _____ H2O(g)

Balancing Equations Na3PO4 + Fe2O3 ----> Na2O + FePO4 Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate Na3PO4 + Fe2O3 ----> Na2O + FePO4

5 Types of Reactions Combination(Synthesis)-2 or more reactants combine to form 1 product. Ex: 2H2 + O2  2H2O 2. Decomposition- 1 reactant breaks down into 2 or more products. Ex: 2H2O  2H2 + O2

5 Types of Reactions, cont. 3. Single Replacement- 1 element replaces another in a compound Ex: 2K + H2O  2KOH + H2 (K replaced H in water)

5 Types of Reactions, cont. 4. Double Replacement-exchange of cations be-tween 2 ionic compounds Ex: 2NaCN +H2SO42HCN + Na2SO4

5 Types of Reactions, cont. 5. Combustion-the rapid combination of a substance w/oxygen to form heat & light. Reactants are made of C,H, and sometimes O. Products are CO2, H2O, (and energy).

5 Types of Reactions, cont. 5. Combustion Ex 1: CH4 + 2O2  CO2 + 2H2O + energy Ex 2: Mg + O2 2MgO +energy (note: Ex2 is also a synthesis rxn)

Practice Problems: What type of reaction? 1. Na3PO4 + Fe2O3 Na2O +FePO4 2. C3H8(g)+O2(g)CO2(g) + H2O(g) 3. 2Al(s) + 3Br2(l) ---> Al2Br6(s)

Answers to Practice Problems: What type of reaction? Na3PO4 + Fe2O3 Na2O +FePO4 Double replacement 2. C3H8(g)+O2(g)CO2(g) + H2O(g) combustion 3. 2Al(s) + 3Br2(l) ---> Al2Br6(s) synthesis

Activity Series (Single Replacement) Not all combinations of chemicals will produce a reaction. An ion that is more reactive will replace a less reactive ion in a compound. Copy the Activity Series of Metals Table onto your P. Table. (also on p333 of text)

Activity Series of Metals Activity Series of Metals (element can displace any element below it on chart) Decreasing Reactivity NAME SYMBOL CAN REPLACE Lithium Li Will replace H from acids & water Potassium K Calcium Ca Sodium Na Magnesium Mg Will replace H from acids only Aluminum Al Zinc Zn Iron Fe Lead Pb (Hydrogen) (H) Copper Cu Will NOT replace H from water or acids Mercury Hg Silver Ag       

Activity Series of Halogens Decreasing Reactivity     NAME SYMBOL CAN REPLACE Fluorine F An element higher on the P. Table (more reactive) can displace any element below it (less reactive) on the P. Table. Chlorine Cl Bromine Br Iodine I

Practice Problems Complete Q#1- from p ____ of text. Q& A please!