Today-- Welcome Back!• Turn in “Chapter 6 Extra Credit”• Unexpected Changes Lab• Introduce Chapter 9 (LAST CHAPTER)• Outlining – NOPE!

Chapter 9 – Chemical Reactions and Equations

Learning Objectives:■ Be able to balance chemical equations by

applying the law of conservation of mass.

■ Be able to recognize synthesis, decomposition, single replacement, double replacement, combustion and neutralization reactions.

Law of Conservation of Matter• Conservation of Matter: in all chemical and

physical changes, matter is neither created or destroyed ■ The total mass in a chemical reaction remains constant

• Antoine Lavoisier:■ Made accurate and precise measurements

during chemical reactions

Reactants ―› Products• Reactants: the substances that enter

into the reaction

• Products: the substances that areproduced by the reaction

Why Do Reactions Occur• Think back to what we know about atoms

and specifically their electrons?

• Through chemical reactions, atoms have the opportunity to obtain complete sets of valence electrons and thus become more stable.

Five General Types of Chemical Reactions

• Direct Combination (Synthesis)• Decomposition• Single-Replacement• Double-Replacement• Combustion

By knowing the type of reaction that is occurring, you can predict the products that will be formed.

Chemical Equations• A method of describing chemical reactions

■ Word Equations• Calcium + Oxygen ―› Calcium Oxide

■ Formula Equations• 2Ca + O2 ―› 2 CaO

the arrow → separates the reactants from the products

Completing the Chemical Equation

• Complete the chemical equation by describing the physical state of each substance:

■ Solid (s)■ Liquid (l)■ Gas (g)■ Aqueous (aq) means dissolved in water

Symbols used in equations

• Double arrow indicates a reversible reaction

• shows that heat is supplied to the reaction

• is used to indicate a catalyst is supplied, in this case, platinum.

∆

Pt

What is a catalyst?

• A substance that speeds up a reaction, without being changed or used up by the reaction.

• Enzymes are biological catalysts.

• How can you physically speed up a reaction?

I. Direct Combination Reactions (also called synthesis reactions).

General form: A + B → AB (two reactants make a single product)

A, B = elements or compoundsAB = compound consisting of A

and B

■ This is the only type of chemical reaction in which there is a single product formed. This single product is always more complex than the reactants.

Examples of Synthesis Reactions

■calcium + oxygen yields calcium oxide 2Ca + O2 → 2CaO

■ Notice: All equations show two (or more) reactants, but only one product.

■ http://www.ric.edu/ptiskus/reactions/Index.htm

II. Decomposition Reactions

General form: AB → A + B (one reactant makes two or more

products)

AB = compoundA, B = elements or simpler

compounds

▪ This is the only type of chemical reaction in which there is a single reactant. This single reactant is always more complex than the products.

Decomposition Reactions: Examples

■ water yields hydrogen and oxygen2H2O → 2H2 + O2

■ marble (calcium carbonate) yields calcium oxide and carbon dioxideCaCO3 → CaO + CO2

■ Notice: all equations show a single reactant decomposing into two (or more) products.

■ http://www.ric.edu/ptiskus/reactions/Index.htm

Balancing Chemical Equations

• The Law of Conservation of Matter states that:■ Matter is neither created nor destroyed!

■ For mass to remain constant both before and after a reaction, the number of atoms must remain constant

Step 1: Balancing Equations

• Write the word equation that describes the reaction.

iron + oxygen ―› iron oxide

Step 2: Balancing Equations

2. Replace the words in the equation withsymbols and formulas.

Fe + O2 ―› Fe2O3

Do we have the same numbers of each atoms on both sides of arrow?

Does this follow the law of conservation of matter?

Step 3: Balancing Equations

3. Count the # of atoms of each elementon both sides of the equation.

Fe + O2 ―› Fe2O3

Step 4: Balancing Equations

4. Starting with elements that only occurin one substance on each side of theequation, make sure that each side of theequation has an equal # of that element.

Proceed with all elements.

Remember that changing the # of one element may alter elements that have already been balanced.

Fe + O2 ―› Fe2O3

Let’s try:

CH4 + O2 ―› CO2 + H2O

• 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.

Never

Balancing Equations: Examples

■ H2 + O2 → H2O

■ Co + O2 → Co2O3

■ Pb(NO3)2 + K2S → PbS + KNO3

■ C2H6 + O2 → H2O + CO2

Balance the following

iron(II) chloride + sodium phosphate → sodium chloride + iron (II) phosphate

FeCl2 + Na3PO4 → NaCl + Fe3(PO4)2

Today

■ Look at Single-Replacement Reactions.

■ Begin “Single-Replacement Lab” set-up.

Single-Replacement Reactions

■ Copper metal and silver nitrate:

Cu(s) + AgNO3(aq) → Ag(s) + CuNO3(aq)

■ What do you observe about the reaction?

■ What do you notice about the chemical equation?

■ Cu must be more reactive than Ag in order for the reaction to take place.

Single-Replacement Reactions

General Form: A + BX → AX + BOne element and one compound

recombine (switch partners)AX, BX = ionic compounds

A, B = MetalsX = ion that switches partners

*Metal ‘A’ must be more reactive than ‘B’ for this to occur

Single-Replacement Lab

Today you will do the following:1. Formulate a question for the lab2. Formulate a hypothesis3. Design procedures4. Create a data table.

IV. Double-Replacement Reactions

General form: AX + BY → AY + BX(Positive ions in two compounds are exchanged)

A,B = positive ionsX,Y = negative ions

■ This is the only type of chemical reaction with two compounds as reactants and two compounds as products.

Double Replacement Examples

■ calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid

CaCO3 + 2HCl → CaCl2 + H2CO3

■Notice: in this reaction, two ionic compounds exchange ions to form two new ionic compounds

www.ric.edu/ptiskus/reactions/Index.htm

IV. Double-Replacement Reactions

General form: AX + BY → AY + BX(Positive ions in two compounds are exchanged)

A,B = positive ionsX,Y = negative ions

■ This is the only type of chemical reaction with two compounds as reactants and two compounds as products.

Double Replacement Examples

■ calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid

CaCO3 + 2HCl → CaCl2 + H2CO3

■Notice: in this reaction, two ionic compounds exchange ions to form two new ionic compounds

www.ric.edu/ptiskus/reactions/Index.htm

Rules of Double-Replacement Reactions

■ Reactants must be dissolved in water (releasing the ions).

■ Will occur if one of the products :• is a molecule (covalent), • a precipitate (solid comes out of

solution), or• an insoluble gas.

V. Combustion Reactions

General Form: CxHy + O2 → H2O + CO2

(hydrocarbon and oxygen react to form carbon dioxide and water)

■ This is the only type of chemical reaction where something reacts with oxygen and forms carbon dioxide and water

Combustion Examples

▪ Methane reacts with oxygen:CH4 (methane) + O2 → H2O + CO2

▪ Gasohol reacts with oxygen:C2H5OH (ethanol) + O2 → H2O + CO2

▪ Notice: in both cases, water and carbon dioxide are the products.

www.ric.edu/ptiskus/reactions/Index.htm

1. Write the word equation2. Write the balanced formula equation■ Solid iron (III) sulfide reacts with

gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.

1. Write the word equation2. Write the balanced formula equation

■ Nitric acid reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate.