Unit 5: Chemical Equations and Reactions Chemistry CPA Winter 2014 Callahan, Pengitore & Ricafort.

Unit 5: Chemical Equations and Reactions

Chemistry CPA Winter 2014Callahan, Pengitore & Ricafort

Unit Objectives

Unit Objectives: • To relate chemical changes and macroscopic properties (5

signs of a chemical reaction)• To demonstrate how chemical equations describe chemical

reactions• To illustrate how to balance chemical reactions by changing

coefficients• To distinguish between the five major types of chemical

reactions• To classify a reaction as belonging to one of five major types. • To classify factors that influence the rate of reaction.

Unit 5 Standards:

• NJCCCS: 5.1.12.A-D, 5.2.12.A.5, 5.2.12.B.2, 5.2.12.B.3, 5.2.12.D.5

• CCCS: ELA-Literacy.RST.11-12.3, 4, 5, 7, 8, 9, 10

Chemical Equation

• represents a chemical change or reaction

Reactants Products

Reactants – chemicals before the reactionProducts – substance formed in the reaction

5 signs of a chemical reaction

1. Color Change

5 signs of a chemical reaction

2. Precipitation of a Solid

5 signs of a chemical reaction

3. Energy Change – heat or light absorbed or released

5 signs of a chemical reaction

4. Odor Changes

5 signs of a chemical reaction

5. Gas Release

• Reactions can be described several ways:1. In a sentence

Copper reacts with chlorine to form copper (II) chloride.

2. In a word equationCopper + chlorine copper (II) chloride

3.Using formulasCu + Cl2 CuCl2

Symbols used in ReactionsSymbol Explanation

+ Used to separate two reactants or two products

Yields, separates reactants and products

(s), (l), (g) State of matter; Fe (s)

(aq) Aqueous solution – dissolved in water

Heat supplied to the rxn

Catalyst used (speeds up the rxn time)

Diatomic Elements

Diatomic Elements

• There are 7 elements on the periodic table that exist in nature bonded in pairs.

• Those elements are:– Hydrogen, Nitrogen, Oxygen, Fluorine, Chlorine,

Bromine, and Iodine.

• Whenever they are referred to as alone or as a gas, i.e, “chlorine gas”, we write them like this:– H2, N2, O2, F2, Cl2, Br2, I2

Diatomic Elements

• To help you remember them, you can use the acronym, “HONClBrIF”

• Or, with the exception of Hydrogen, they make the number “7” on the periodic table starting at Nitrogen, going down to Iodine:

Practice Problem Set:Write the word equations below as chemical equations

1. Zinc and lead (II) nitrate react to form zinc nitrate and lead.

2.Aluminum bromide and chlorine gas react to form aluminum chloride and bromine gas.

3.Sodium hydroxide and carbon dioxide react to form sodium carbonate and water (dihydrogen monoxide)

In a chemical reaction

• Law of Conservation of Matter states that matter isn’t created or destroyed.

• The way atoms are joined is changed

Because of the LAW OF CONSERVATION OF MATTER, we have to BALANCE EQUATIONS!!!

To BALANCE AN EQUATION means to make sure that the NUMBER of atoms of each

element is the same in the reactants and in the products!

Balancing Rules:

1. You may change the coefficients to any number you want, but…

2. You may NEVER change a subscript.

A coefficient is a number written in front of the compound

Balancing example #1

C + O CO2

Balancing example #2

Zn + HCl H2 + ZnCl2

Balancing example #3

CuO + C Cu + CO2

Balancing example #4

O3 O2

Balancing example #5

Al + CuCl2 Cu + AlCl3

Balancing example #6

C + H2 C3H8

Balancing example #7

CF4 + Br2 CBr4 + F2

Balancing example #8

Hg2I2 + O2 Hg2O + I2

Balancing example #9

N2 + H2 NH3

Balancing example #10

KClO3 KCl + O2

Balancing example #11

H2 + O2 H2O

Balancing example #12

AlBr3 + K2SO4 KBr + Al2(SO4)3

5 Types of Chemical Reactions

1. Synthesis2. Decomposition3. Single Replacement (Displacement)4. Double Replacement (Displacement)5. Combustion

Synthesis Reaction

Synthesis reaction – 2 substances are combined to form a single product

A + B AB

+

Synthesis Examples

2Mg + O2 2MgOMagnesium and oxygen combine to form

magnesium oxide.

2H2 + O2 2H2OHydrogen and oxygen combine to form

dihydrogen monoxide (water)

Synthesis Demo

• Ignite magnesium

Decomposition Reaction

Decomposition reaction – A single compound is broken down into 2 or more products.

AB A + B

+

Decomposition Examples

2NaCl(s) 2Na(s) + Cl2(g)

Sodium chloride (table salt) decomposes into sodium and chlorine gas.

CaCO3 CaO + CO2

Limestone (CaCO3 ) decomposes into lime (CaO) and carbon dioxide.

Decomposition Demo

• Decomposition of Hydrogen Peroxide with MnO2 as a catalyst

H2O2 O2 + H2O

Single Replacement Reaction

Single Replacement reaction – A single element takes the place of an element in a compound.

A + BC B + AC

+ +

Single Replacement Examples

Mg + BeO Be + MgOMagnesium replaces beryllium in beryllium

oxide to form magnesium oxide and beryllium.

2Na + 2HCl H2 + 2NaCl

Sodium replaces hydrogen in hydrochloric acid to yield hydrogen and sodium chloride.

Single Replacement Demo

• Magnesium reacting with Hydrochloric acid• Compared to Copper reacting with HCl

Double Replacement Reaction

Double Replacement reaction – Elements in 2 compounds switch places to form 2 new compounds.

AB + CD AD + CB

+ +

Double Replacement Examples

MgO + BeS MgS + BeOOxygen and sulfur switch places to form

magnesium sulfide and beryllium oxide.

Na2S + Zn(NO3)2 2Na(NO3) + ZnS

Sulfur and nitrate switch places to form sodium nitrate and zinc sulfide.

Double Replacement Demo

• A double replacement between barium chloride and potassium sulfate

• Potassium Iodide and lead (II) nitrate produces potassium nitrate and lead (II) iodide

Combustion Reaction

Combustion reaction – hydrocarbon (H and C) burn in oxygen to produce water and carbon dioxide. Heat is given off as energy.

CxHy + O2 H2O + CO2

Combustion Examples

CH4 + 2O2 2H2O + CO2

Methane burns in oxygen to produce water and carbon dioxide.

C12H22O11 + 12O2 11H2O + 12CO2

A sugar molecule burns in oxygen to produce water and carbon dioxide

Combustion Demo

• Burn sugar• Burn gasoline

Factors that affect reaction rates

• Temperature– Higher temperature faster reaction

• Concentration– Changing the amounts of reactants– High concentration means more particles per

volume– Example: 3% H2O2 vs. 30% H2O2

• Surface Area – Reactions occur faster when there is more surface

area• Sugar cube vs. sugar packet dissolving

• Catalyst– Used to speed up a reaction without being used

up – Enzymes are examples of catalyst

Can you slow a reaction down?

• Inhibitor – a substance added to slow reactions down– Added to food products

Limiting Reactant• Sometimes, when two or more reactants are

present in a reaction, more than the other is available for reacting.

• The one thing that there is not enough of is called the limiting reactant.

• It is the reactant that is used up first.

Limiting Reactant Example #1:Making a Bicycle

Making a bicycle requires1 frame + 2 wheels 1 bicycle

a)If you have 100 frames and 120 wheels, howmany bicycles can you manufacture? ______a)What is the limiting reactant? _________

Limiting Reactant Example #2:Grilled Cheese Sandwich

Bread + Cheese ‘Cheese Melt’

2 B + C B2C

100 bread 30 slices ? sandwiches

Limiting Reactant Example #3:Chemistry Problem

Hydrogen gas can be produced by the reaction of magnesium and hydrochloric acid, as shown by this equation:

Mg(s) + 2HCl (aq) MgCl2(aq) + H2(g)In particular, 6,000 molecules of HCl were mixed with 1,000

molecules of Mg.

a)Which reactant is limiting? ________________________

a)How many molecules of H2 are formed when the reaction is complete? ______________________________________