Chapter 7

Chemical Reactions

Objectives

• Explain what a chemical reaction is

• Describe indications of chemical reactions

• Use state symbols in reactions

• Write balanced chemical equations

What is a Chemical Reaction?

• One or more substances are converted into new substances – New substances

must be formed!!!

All Chemical Reactions

• have two parts

• Reactants - the substances you start with

• Products- the substances you end up with

• The reactants turn into the products.

• Reactants Products

Indications of Reactions

• Change in Heat - Exothermic or Endothermic Reactions

• Light• Production of a Gas – Does not need to smell• Formation of a Precipitate

-Precipitate is a solid that is produced as a result of a chemical rxn in solution

Precipitate!

In a Chemical Reaction

• The way atoms are joined is changed• Atoms aren’t created of destroyed.• Can be described several ways• In a sentence • Copper reacts with chlorine to form copper

(II) chloride.• In a word equation• Copper + chlorine copper (II) chloride

Chemical Equation

• Represents with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical rxn.

Symbols In Equations

• An arrow separates the reactants from the products

• Read “reacts to form” or “yields”

• The plus sign = “and”

• (s) = solid

• (g) = gas

• (l) = liquid

• (aq) = aqueous solution– Dissolved in water

Symbols In equations• = reversible reactions

– Equilibrium (More later)• = Reaction is heated

• = Catalyst is used (Copper)– Catalysts speed up reactions but are not

consumed.– Enzymes are biological catalysts

• = Specific Pressure – 2.00 atmospheres (1 atm is normal)

Δ

Cu

2.0 atm

How Do Reactions Happen?

• Simple View

– Particles must collide

Reactions Continued

• Particles are moving (Kinetic Energy) -Higher temperature means a higher speed

• Particles collide-Energy is absorbed by particles-Bonds are broken-New bonds are formed

-Energy is released

Diatomic Elements

• 7 elements ALWAYS exist in diatomic state

– Diatomic = 2 atoms

• H2 , N2 , O2 , F2 , Cl2 , Br2 , I2

• Elements in –ogen and –ine

• 1 + 7 pattern on the periodic table

Diatomic Elements

Converting To Formula Equ.

• You will often have to convert word equations to formula equations.

• Determine the reactants and products

• Covert the words to equations

• Include any state symbols that are given

• If no state are given don’t worry about them.

Converting To Formula Equ.• Sodium metal and chlorine gas react to

form solid sodium chloride

Converting To Formula Equ.• A solution of hydrochloric acid and solid

sodium carbonate react to form solid sodium chloride and gaseous carbon dioxide and water vapor.

Convert To a Sentence

• Fe(s) + O2(g) Fe2O3(s)

• Solid Iron and gaseous oxygen yields solid iron (III) oxide

Convert To a Sentence

• Cu(s) + AgNO3(aq)

Ag(s) + Cu(NO3)2(aq)

• Solid Copper and a solution of silver nitrate yields solid silver and a solution of copper (II) nitrate

Balanced Equation

• Law of Conservation of Mass states “Mass cannot be created or destroyed.”

• Thus, atoms can’t be created or destroyed

• So, a balanced equation has the same number of each element on both sides of the equation.

• Balance equations with coefficients– Number in front of a formula (Multiplier)

Is This Equation Balanced?

H2 + I2 HI

NO WAY!• Hydrogen and Iodine – 2 reactant atoms

only 1 product atom

Balance With Coefficients

• If there are 2 HI molecules the equation is balanced. H2 + I2 2HI

Don’t Change the Formula•You make a different compound!!!!!

Writing Balanced Equations

1. Write the correct formulas for all the reactants and products

2. Count the number of atoms of each type appearing on both sides

3. Balance the elements one at a time by adding coefficients (the numbers in front)

4. Check to make sure it is balanced.

Never

• Never change a subscript to balance an equation.

– If you change the formula you are describing a different reaction.

– H2O is a different compound than H2O2

• Never put a coefficient in the middle of a formula

– 2 NaCl is okay, Na2Cl is not.

Examples

• H2 + O2 H2O

Examples

• Ca(NO)3 + NaI CaI2 + NaNO3

Examples

• C2H6 + O2 CO2 + H2O

Homework

• p. 264 #23,25,26,28,30-33

Objectives

• Predict a reaction type

• Predict the products of a reaction

• Use the activity series

• Predict solubility of compounds

Types of Reactions

Predicting the Products

Types of Reactions

• There are millions of reactions.

• Can’t remember them all

• Fall into several categories.

• We will learn 5 types.

• Will be able to predict the products.

• For some we will be able to predict whether they will happen at all.

• Will recognize them by the reactants

#1 Synthesis Reactions

• Combine - put together

• 2 elements, or compounds combine to make one compound.

• Ca +O2 CaO

• SO3 + H2O H2SO4

• We can predict the products if they are two elements.

• Mg + N2 Mg3N2

#2 Decomposition Reactions

• decompose = fall apart

• one reactant falls apart into two or more elements or compounds.

• NaCl Na + Cl2

• CaCO3 CaO + CO2

#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

• F2 + LiCl LiF + Cl2

#3 Single Replacement

• We can tell whether a reaction will happen

• Some are more active than other

• More active replaces less active

• Higher on the list replaces lower.

• If the element by itself is higher, it happens, in lower it doesn’t

Activity Series Lithium

Potassium Calcium Sodium

Magnesium

Aluminum Zinc

Chromium Iron Nickel Lead HydrogenHydrogen Bismuth Copper Mercury Silver Platinum Gold

Halogens -

•F2

•Cl2

•Br2

•I2

#3 Single Replacement

• What does it mean that Au And Ag are on the bottom of the list?

• Nonmetals can replace other nonmetals

• Limited to F2 , Cl2 , Br2 , I2

• The order of activity is that on the table.

• Higher replaces lower.

Solubility

• Some compounds dissolve in water.– We say they are Soluble– Examples

• Sodium Chloride, Potassium Nitrate

• Some compounds do not dissolve– We say they are Insoluble– Form Precipitates

Solubility Rules

• Solubility Rules are a general list that tells us what kind of compounds are soluble or insoluble.

• Follow from beginning to end

• Rule one has precedence over rule two

Solubility Rules• All Acids are soluble

• Most nitrate and acetate salts are soluble.

• Most salts containing the alkali metal ions (Li+, Na+,

K+, Cs+, Rb+) and the ammonium (NH4+) ion are

soluble.

• Most chloride, bromide and iodide salts are soluble.

Exceptions are salts containing the ions Ag+, Pb2+,

and Hg2+.

• Most sulfate salts are soluble. Notable exceptions

are BaSO4, PbSO4, HgSO4 and CaSO4.

Solubility Rules

• Most hydroxide salts are insoluble.

• Most sulfide (S2-), carbonate (CO32-),

chromate (CrO42-) and phosphate (PO4

3-)

salts are insoluble

Solubility Rules

• M I Soluble?

• Potassium Bromide– Yes

• Iron (III) Sulfate– Yes

• Calcium Phoshate– No

• Zinc Acetate– Yes

#4 Double Replacement

• Two things replace each other.

• Reactants must be two ionic compounds or acids.

• Usually in aqueous solution

• NaOH + FeCl3 Fe(OH)3 + NaCl

#4 Double Replacement

• Will only happen if one of the products

– doesn’t dissolve in water and forms a solid

– or is a gas that bubbles out.

– or is a covalent compound usually water.

Examples

• H2 + O2

• H2O

• Zn + H2SO4

• HgO

• KBr +Cl2

• AgNO3 + NaCl

• Mg(OH)2 + H2SO3

Last Type

• Combustion

• A compound composed of only C H and maybe O is reacted with oxygen

• If the combustion is complete, the products will be CO2 and H2O.

• CH4 + O2 CO2 + 2H2O

How to recognize which type

• Synthesis – Only one product• Decomposition – Only one reactant• Single replacement – Element and

Compound as reactants• Double replacement – Two compounds• Combustion – Something reacting with

oxygen

Homework

• p. 264 #'s 35 37-40, 46 without net ionic

Objectives

• Predict the type of double replacement reaction

• Write net ionic equations

More on Double Replacement

• Three Main Types– Precipitation (A solid forms)– Gas Forming (A gas forms)– Neutralization

• Reaction of Acid and Base

Precipitation

• One of the products will be a solid

• Determined by solubility rules

• That’s it.

Gas Forming

• You see bubbles!• Usually from decomposition of

– Carbonic Acid - H2CO3 CO2 + H2O

– Sulfurous Acid - H2SO3 SO2 + H2O

• When you get one of these as a product replace it with the gas and water.

• Ex. HNO3 Na2CO3 H2CO3 + NaNO3

• Products become CO2 + H2O + NaNO3

Neutralization

• Reaction of Acid and a Base

• Acid usually starts with H

• Base usually has hydroxide OH-

• Product is salt and water

• Ex HF + KOH HOH +KF

Dissociation• Ionic Compounds separating into ions

– AgNO3(aq)+Na2S(aq)Ag+ +NO3- +2Na+ +S-2

Net Ionic Equations

• Used to remove “unimportant” reactants and products

• What’s “unimportant”

– Whatever is not used to make precipitates, liquids, or gases

– They are spectator ions

Net Ionic Rules

• Dissociate all soluble compounds– According to balanced equation

• Leave all solids, liquids, and gasses

• Cross out common terms (Same)

• Rewrite as net ionic

Example• Solutions of lead (II) nitrate & hydrochloric

acid are in beakers. Draw what in is in the beakers.

Example• Solutions of lead (II) nitrate & hydrochloric

acid are mixed. Write the balanced equation

Example• Draw what is in the beaker when the two

solutions are mixed.

Example• Solutions of lead (II) nitrate & hydrochloric

acid are mixed. Write the net ionic equation

Example• Solutions of copper (II) nitrate and sodium

sulfide are mixed. Write the net ionic equation.

Homework

• p. 266 #'s 46,62ab,68,75,82,98