Unit 7Chemical Reactions

Writing Chemical Equations• All chemical reactions have two parts:• ReactantsReactants - the substances you start with (on

left side of arrow)• Products 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):

#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 namesCopper + 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.

heat ,

Pt

Must Memorize Elements THAT EXIST AS DIATOMIC MOLECULES Must Memorize Elements THAT

EXIST AS DIATOMIC MOLECULES

Remember Remember HON17HON17 !!! !!!

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: a) the 5 major types.• We will be able to: b) predict the

products.

Types of Equations

Synthesis (or Combination)Equation in Symbols: A + B ABSample Equation:

2Cu (s) + O2 (g) 2 CuO (s)

2 or more elements combine to form only 1 product

Types of Equations

DecompositionEquation in Symbols: AB A + BSample Equation:

2 NH3 (g) N2 (g) + 3 H2 (g)

1 reactant breaks down into more than 1 product

Types of EquationsSingle ReplacementEquation in Symbols: A + BC AB + C

– Metal replacing metal– Nonmetal replacing nonmetal

Sample Equation: Mg (s) + CuCl2 (aq) Cu (s) + MgCl2 (aq)

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”

Types of Equations

CombustionEquation 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.1. 2 AgNO3 + Ni Ni(NO3)2 + 2 Ag

2. BaCO3 BaO + CO2

3. 4 Cr + 3 O2 2 Cr2O3

4. Ca + 2 HCl CaCl2 + H2

5. Cu(OH)2 + 2 HC2H3O2 Cu(C2H3O2)2 + 2 H2O

6. 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.

Balancing EquationsChemical “bookkeeping” of atoms

involved in the reaction:

H2(g) + O2(g) H2O(g)

H – 2 O – 2Reactants

H – 2 O – 1Products

COEFFICIENTS must be added so reactant atoms EQUAL product atoms!

Note the imbalance in oxygen atoms

Rules for Balancing Chemical Equations1) Count the number of atoms of each type

of element appearing on both sides

2) 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 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 Must Memorize Elements THAT

EXIST AS DIATOMIC MOLECULES

Remember HON17 !!!Remember HON17 !!!

Balancing Equations Practice

Balance the following chemical equations using the appropriate coefficients:

____ Al(s) + _____ Br2 (l) _____ Al2Br6 (s)

____ Na3PO4 + ____ Fe2O3 ____ Na2O + ____ FePO4

Reaction Stoichiometry

CS2 + 3O2 CO2 + 2 SO2

Interpretation in terms of moles:1 mole of CS2 + 3 moles of O2 form:

1 mole of CO2 + 2 moles of SO2

Predicting Products - Single Replacement Rxns.

• 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

• 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 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