I. Writing and Balancing Equations II. Identifying Reaction Types Unit 6 Chemical Reactions

Reactants the substances you start with Products the substances you end up with “Yields” or Produces Writing a chemical equation

In a chemical reaction The atoms are rearranged. –But the atoms aren’t created nor destroyed. – ( in other words: whatever you start with you end with ) – this is a LAW – Law of Conservation of Matter (Mass) Example: Copper reacts with chlorine to form copper (II) chloride. In a word equation Copper + chlorine  copper (II) chloride In a chemical equation Cu + Cl 2 CuCl 2

Balancing Chemical Equations

Balanced Equation Atoms can’t be created or destroyed. All the atoms we start with we must end up with. A balanced equation has the same number of each element on both sides of the equation.

Rules for balancing  Write the correct formulas for all the reactants and products  By Inspection: Count the number of atoms of each type appearing on both sides  Balance the elements one at a time by adding coefficients (the numbers in front)  Check to make sure it is balanced.

Never Change a subscript to balance an equation. –If you change the formula you are describing a different reaction. H 2 O is not the same as H 2 O 2 Never put a coefficient in the middle of a formula »2 NaCl is okay, Na2Cl is not.

Example H 2 +H2OH2OO2O2  **If necessary- make a table to keep track of the #s on each side I.By inspection determine the #of each element on each side of the yields arrow.**

Example: H 2 +H2OH2OO2O2  RP H O 2221 Need 2 but only have 1 Can’t change subscript! Must add coefficient!! 2 but the hydrogen changes to: Must make 4 hydrogen! Add coefficient:

Balanced Equation: H 2 +H2OH2OO2O2  22 The equation is now balanced, has the same number of each kind of atom on both sides

C + O 2  CO 2 This equation is already balanced C + O O  C O O C + O 2  CO 2 **If the equation is already balance – circle it

C + O 2  CO We need one more oxygen in the products. Can’t change the formula, because it describes what is C + O  C O O

Must be used to make another CO But where did the other C come from? C + O  C O O O C

Must have started with two C 2 C + O 2  2 CO C + O  C O O O C C

Types of Reactions

There are millions of reactions. Can’t remember them all. Fall into several categories. »We will learn 5 basic types Will recognize them by the reactants and the resulting products

#1. Combination Reactions (composition or direct union) Compose, Combine - put together 2 elements, or compounds combine to make one compound. Ca + O 2  CaO H 2 O + SO 3  H 2 SO 4  22 ???are these equations balanced???

#2 Decomposition Reactions decompose = fall apart one reactant falls apart into two or more elements or compounds. NaCl  Na + Cl 2 CaCO 3  CaO + CO 2  ???are these equations balanced??? 22

Balance and Identify Ca + Cl 2  CaCl 2 Iron(II) oxide  Fe + O 2 Al + O 2  Al 2 O 3 Ca + Cl 2  CaCl 2 2FeO  2Fe + O 2 4Al + 3O 2  Al 2 O 3 combination decomposition combination

#3: Single Replacement One element replaces another Reactants must be an element and a compound. Products will be a different element and a different compound. Na + KCl  K + NaCl F 2 + LiCl  LiF + Cl 2  C  C 

#4: Double Replacement Two things replace each other. Usually in aqueous solution. NaOH + FeCl 3  The positive ions change place. NaOH + FeCl 3  Na +1 Cl -1 +  Fe +3 OH - NaOH + FeCl 3  NaCl  +  Fe(OH) 3  C  D  D CC

Li + KCl  K + LiCl Balance and Identify 22 NaOH + FeCl 3  NaCl  +  Fe(OH) 3 33 single replacement double replacement F 2 + LiCl  LiF + Cl 2

Examples Fe + O 2  Fe 2 O 3 Cu + AgNO 3  Ag + Cu(NO 3 ) 2 NO 2  N 2 + O 2

Answers: examples 4Fe + 3O 2  2Fe 2 O 3 Cu + 2AgNO 3  2Ag + Cu(NO 3 ) 2 2NO 2  N 2 + 2O 2

Examples AgNO 3 + Cu  Cu(NO 3 ) 2 + Ag Mg + N 2  Mg 3 N 2 P + O 2  P 4 O 10 Na + H 2 O  H 2 + NaOH CH 4 + O 2  CO 2 + H 2 O

Answers: examples 2AgNO 3 + Cu  Cu(NO 3 ) 2 + Ag 3Mg + N 2  Mg 3 N 2 4P + 5O 2  P 4 O 10 2Na + 2H 2 O  H 2 + 2NaOH CH 4 + 2O 2  CO 2 + 2H 2 O

# 5: Combustion Combustion: fuel + oxygen A fuel: composed of (C x H x ) and maybe O »is reacted with oxygen Complete combustion: the products will be CO 2 and H 2 O. Incomplete combustion: the products will be CO and H 2 O.

Examples C 4 H 10 + O 2  CO 2 + H 2 O C 3 H 8 + O 2  CO 2 + H 2 O C 6 H 12 O 6 + O 2  CO 2 + H 2 O C 5 H 12 + O 2  CO 2 + H 2 O

Complete and Balance

The Process Predict the reaction type. – by looking at the reactants. Arrange the new product pieces***. – next to each other. –Determine the charges of each. Write the new product formulas. –(crisscross method!!) Balance the equation. ***don’t forget HONClBrIF

Reminder: 7 Diatomic Molecules HONClBrIF H 2 O 2 N 2 Cl 2 Br 2 I 2 F 2 Examples Oxygen gas = O 2 Hydrogen gas = H 2 Write the following equation: Hydrogen chloride decomposes into hydrogen gas and chlorine gas. Remember: chlorine gas = Cl 2 !! HCl H 2 + Cl 2 2

Complete and balance CaCl 2 + NaOH  Zn + H 2 SO 4  CuCl 2 + K 2 S  HgO 

Complete and balance CaCl 2 + NaOH  Zn + H 2 SO 4  CuCl 2 + K 2 S  HgO  Na +1 Cl -1 +  Ca +2 OH - Zn +2 SO H 2 ** Cu +2 S -2 +  K +1 Cl -1 Hg +2 + O 2 ** **don’t forget HONClBrIF

More examples H 2 + O 2  CuF 3  Mg(OH) 2 + H 2 SO 3  K + Fe(NO 3 ) 3 

Symbols used in equations the arrow separates the reactants from the products Read “reacts to form” + reads as “and” (s) after the formula - (g) after the formula - (l) after the formula - solid gas liquid Al(s) + O 2 (g)  Al 2 O 3 (s)

Symbols used in equations indicates a reversible reaction is used to indicate a catalyst is used in this case, platinum Pt Mn manganesein this case, Al(s) + O 2 (g) Al 2 O 3 (s) MnO

What is a catalyst? A substance that controls the rate of a reaction without being a part of the reaction. Catalysts are neither reactants nor products.

Symbols used in equations (aq) after the formula - dissolved in water, an aqueous solution. ∆ heat  used after a product indicates a gas  (same as (g))  used after a product indicates a  precipitate - (same as (s)) shows that heat is supplied to the reaction

Activity Series and: #3 Single Replacement Can metals replace metals (and hydrogen)? Use the Activity series Something above replaces below. K + AlN  Zn + HCl  Think of water as HOH Na + HOH  Metals replace one of the H, combine with hydroxide.

What does the equation say? Fe(s) + O 2 (g) Fe 2 O 3 (s) Cu(s) + AgNO 3 (aq)  Ag( ) + Cu(NO 3 ) 2 (aq) NO 2 N 2 (g) + O 2 (g)

Examples Copper(II) wire is placed in an aqueous solution of silver chloride producing an aqueous solution and a precipitate. AgNO 3 (aq) + Cu (s)  Cu(NO 3 ) 2 (aq) + Ag (s) Sodium pellets are placed in water resulting in the production of hydrogen gas and a solution of sodium hydroxide. Na (s) + H 2 O (l)  H 2 (s) + NaOH (aq)

Convert these to equations Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (II) chloride and hydrogen sulfide gas. Nitric acid (HNO 3 ) dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

Chapter 7 Summary

An equation Describes a reaction Must be balanced because to follow Law of Conservation of Matter Can only be balanced by changing the coefficients. Has special symbols to indicate state, and if catalyst or energy is required.

How to recognize which type Look at the reactants or products E + E Combination C C Decomposition E and/or C E + CSingle replacement C and E C + CDouble replacement C and C Fuel + O 2 Combustion carbon dioxide and water C x H x + O 2

Reactions Come in 5 types. Can tell what type they are by the reactants. Single Replacement happens based on the activity series using activity series. Double Replacement happens if the product is a solid, water, or a gas.