CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2 Chapter 6

Introduction –Chemical rxns occur when bonds between valence electrons of atoms are formed or broken –Chemical rxns involve changes in matter, the making of new materials with new properties&energy changes –Symbols represent elements, –Formulas represent compounds, –Chemical equations represent chemical rxns

Signs of Chemical Reactions 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: Show what is happening during a reaction

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 aluminum+oxygen  aluminum oxide Ex: aluminum+oxygen  aluminum oxide

There are 2 ways to write an equation for a rxn. Word Equations- use names of compounds & elements. Chemical Equations-use formulas of compounds & symbols of elements.

Word Equations (Label example from prior slide) aluminum+oxygen  aluminum oxide Problem: word eq’ns don’t tell us enough.

Chemical Equations Their Job: Show the relative amounts of reactants and products in a reaction. 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) The numbers in the front are called Coefficients & show how many units (atoms, molecules, etc.)

Additional Rules Chemical Equations Follow Physical state of chemicals is shown after the formula or symbolPhysical 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.Energy is shown when it is an important part of a rxn. With the reactants in endothermic rxnsWith the reactants in endothermic rxns With the products in exothermic rxnsWith the products in exothermic rxns Balance-equations must be balancedBalance-equations must be balanced

Charcoal, which is basically carbon, reacts with oxygen to yield carbon dioxide. Word eq’n: carbon + oxygen  carbon dioxide + energy Chemical equation: C(s) + O 2 (g)  CO 2 (g) + energy 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) + 3O 2 (g)--->2 Al 2 O 3 (s) This equation means 4 Al atoms + 3 O 2 molecules ---produces---> 2 units of Al 2 O 3 2 units of Al 2 O 3

Balancing Equations, cont. –When balancing a chemical equation you may ONLY add coefficients in front of the compounds & elements to balance the reaction. not –You may not change the subscripts. (Changing the subscripts changes the compound.)

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

4 Steps to Balancing Equations Step #1: Write the correct formula 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 – H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2

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. (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units.

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

ANSWER TO PRIOR SLIDE Re-Count the atoms _2_H 2 (g) + ___O 2 (g) --->_2_H 2 O(l) LEFT SIDERIGHT SIDE H=4H=4 O=2O=2

Balancing Equations ___ Al(s) + ___ Br 2 (l) ---> ___ Al 2 Br 6 (s) 23

Practice Balancing Equations ____C 3 H 8 (g) + _____ O 2 (g) ----> _____CO 2 (g) + _____ H 2 O(g) ____B 4 H 10 (g) + _____ O 2 (g) ----> ___ B 2 O 3 (g) + _____ H 2 O(g)

Balancing Equations Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate Na 3 PO 4 + Fe 2 O > Na 2 O + FePO 4 Na 3 PO 4 + Fe 2 O > Na 2 O + FePO 4

5 Types of Reactions 1. Synthesis-2 or more reactants combine to form 1 product. Ex: 2H 2 + O 2  2H 2 O 2. Decomposition- 1 reactant breaks down into 2 or more products. Ex: 2H 2 O  2H 2 + O 2

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

5 Types of Reactions 4. Double Replacement- exchange of cations be- tween 2 ionic compounds Ex: 2NaCN +H 2 SO 4  2HCN + Na 2 SO 4

5 Types of Reactions 5. Combustion-the rapid combination of a substance w/oxygen to form heat & light. Ex 1: CH 4 + 2O 2  CO 2 + 2H 2 O + energy Ex 2: Mg + O 2  2MgO +energy (note: Ex2 is also a synthesis rxn)

What type of reaction? 1. Na 3 PO 4 + Fe 2 O 3  Na 2 O +FePO 4 2. C 3 H 8 (g)+O 2 (g)  CO 2 (g) + H 2 O(g) 3. 2Al(s) + 3Br 2 (l) ---> Al 2 Br 6 (s)

What type of reaction? 1.Na 3 PO 4 + Fe 2 O 3  Na 2 O +FePO 4 Double replacement 2. C 3 H 8 (g)+O 2 (g)  CO 2 (g) + H 2 O(g) combustion 3. 2Al(s) + 3Br 2 (l) ---> Al 2 Br 6 (s) synthesis

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