Unit 6 Chemical Reactions

Writing Chemical Equations All chemical reactions have two parts: Reactants - the substances you start with (on left side of arrow) Products - the substances you end up with (on right side of arrow) The reactants turn into the products. Reactants  Products

In a chemical reaction… The way atoms are joined is changed. Atoms aren’t created or destroyed; they just combine together in new ways. Law of Conservation of Mass Can be described using sentences, word equations, or skeleton equations (symbols):

Copper reacts with chlorine to form copper (II) chloride. #1. In a sentence every item is a word Copper reacts with chlorine to form copper (II) chloride. #2. In a word equation symbols used w/ element or compound names Copper + chlorine  copper (II) chloride #3. In a skeleton equation formulas and symbols are used to describe a reaction Cu + Cl2  CuCl2 These are NOT balanced (# of atoms on each side is not necessarily equal)

Symbols used in Equations The arrow () separates the reactants from the products; means “reacts” or “yields” The plus sign (+) = between products or between reactants; means “and” Subscripts are used to describe the number of atoms in a FORMULA. Coefficients are used to describe the number of molecules in the REACTION. They are the only things changed when balancing a reaction.

Subscripts vs. Coefficients 3 CuCl2 + 2 Al  2 AlCl3 + 3 Cu On the reactant side of the equation: How many copper (II) chloride molecules react? How many total chlorine atoms are there? How many aluminum atoms are there?

States of Matter Solid--(s) after the formula Precipitate -- a solid formed in a reaction Gas--(g) after the formula Liquid—(l) after the formula Aqueous-- (aq) after the formula - dissolved in water.

Other Symbols used in Equations indicates a reversible reaction (More later) show that heat is supplied to the reaction is used to indicate a catalyst used or supplied, in this case, platinum.

Must Memorize Elements THAT EXIST AS DIATOMIC MOLECULES Remember HON17 !!!

Write a skeleton equation for: Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form solid iron (III) chloride and hydrogen sulfide gas.

Types of Reactions There are probably millions of reactions. We can’t remember them all, but luckily they will fall into several categories. We will learn the 5 major types. We will then be able to predict the products of a reaction when given just the reactants!

2 or more elements combine to form only 1 product Types of Equations Synthesis (or Combination) Equation in Symbols: A + B  AB Sample Equation: 2Cu (s) + O2 (g)  2 CuO (s) 2 or more elements combine to form only 1 product

1 reactant breaks down into more than 1 product Types of Equations Decomposition Equation in Symbols: AB  A + B Sample Equation: 2 NH3 (g)  N2 (g) + 3 H2 (g) 1 reactant breaks down into more than 1 product

Types of Equations Single Replacement Equation in Symbols: A + BC  AB + C Sample Equation: Mg (s) + CuCl2 (aq)  Cu (s) + MgCl2 (aq) Metal replaces metal; Nonmetal replaces nonmetal Positive ion replaces positive ion; negative ion replaces negative ion!

Types of Equations Double Replacement – Equation in Symbols: AX + BY  BX + AY Sample Equation: 2AgNO3(aq) + CuCl2 (aq)  Cu(NO3)2 (aq) + 2AgCl (s) 2 ionic compounds switching “partners”

CH4(g) + O2 (g)  CO2 (g) + H2O (l) Types of Equations Combustion Equation in Symbols: CxHy + O2 CO2 + H2O Sample Equation: CH4(g) + O2 (g)  CO2 (g) + H2O (l) Hydrocarbons react to form CO2 and H2O

Examples: Identify the Type of Rxn. 2 AgNO3 + Ni  Ni(NO3)2 + 2 Ag BaCO3  BaO + CO2 4 Cr + 3 O2  2 Cr2O3 Ca + 2 HCl  CaCl2 + H2 Cu(OH)2 + 2 HC2H3O2  Cu(C2H3O2)2 + 2 H2O C8H18 + O2  CO2 + H2O

Balancing Chemical Equations Atoms can’t be created or destroyed in an ordinary reaction: All the atoms we start with we must end up with (meaning: balanced!) A balanced equation has the same number of atoms of each element on both sides of the equation.

Chemical “bookkeeping” of atoms involved in the reaction: Balancing Equations Chemical “bookkeeping” of atoms involved in the reaction: H2(g) + O2(g)  H2O(g) H – 2 O – 2 Reactants H – 2 O – 1 Products Note the imbalance in oxygen atoms COEFFICIENTS must be added so reactant atoms EQUAL product atoms!

Rules for Balancing Chemical Equations Count the number of atoms of each type of element appearing on both sides Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST! (HINT: save O until the very last) 3) Double-Check to make sure it is balanced.

Never Never change a subscript to balance an equation (You can only change coefficients) If you change the subscript (formula) you are describing a different chemical. H2O is a different compound than H2O2 Never put a coefficient in the middle of a formula; they must go only in the front 2NaCl is okay, but Na2Cl is not.

Hints & Tips for Balancing Equations Take one element at a time, working from left to right Save H for next to last and O for last. (Shortcut) polyatomic ions that appear on both sides of the equation should be balanced as independent units IF EVERYTHING BALANCES EXCEPT FOR O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is a diatomic element) Same goes for HON17

Must Memorize Elements THAT EXIST AS DIATOMIC MOLECULES Remember HON17 !!!

Balancing Equations Practice Balance the following chemical equations using the appropriate coefficients: ____ Al(s) + _____ Br2 (l)  _____ Al2Br6 (s)

Balancing Equations Practice Balance the following chemical equations using the appropriate coefficients: ____ Na3PO4 + ____ Fe2O3  ____ Na2O + ____ FePO4

Predicting Products - Single Replacement Rxns. One element replaces another Reactants must be 1 element and 1 ionic compound. Products will be a different element and a different compound. Na + KCl ® K + NaCl F2 + LiCl ® LiF + Cl2 (Cations switched) (Anions switched)

Predicting Products - Single Replacement Rxns. Metals will replace other metals (and they can also replace hydrogen) K + AlN ® Zn + HCl ® Think of water as: HOH Metals replace the first H, and then combines with the hydroxide (OH). Na + HOH ®

Single Replacement Rxns. Practice: Fe + CuSO4 ® Pb + KCl ® Al + HCl ®

Predicting Products - Double Replacement Rxns. Two things replace each other. Reactants must be two ionic compounds, in aqueous solution NaOH + FeCl3 ® The positive ions change place. NaOH + FeCl3 ® Na+1 Cl-1 + Fe+3 OH-1 NaOH + FeCl3 ® Fe(OH)3 + NaCl

Complete and balance: __CaCl2 + __NaOH ® __CuCl2 + __K2S ® __KOH + __Fe(NO3)3 ® __(NH4)2SO4 + __BaF2 ®

Predicting Products – Combustion Rxns. Combustion means “add oxygen” Normally, a compound composed of only C, H, (and maybe O) is reacted with oxygen (O2) – usually called “burning” In a combustion reaction, the products will ALWAYS be CO2 and H2O. C3H8 + O2  CO2 + H2O

Mixed Practice Examples: H2 + O2 ® H2O ® Zn + H2SO4 ® HgO ® KBr + Cl2 ® AgNO3 + NaCl ® Mg(OH)2 + H2SO3 ®