I. Intro to Reactions

Ch. 7 – Chemical Reactions

A.Signs of a Chemical Reaction Change in heat and light Formation of a gas Formation of a precipitate Color change Odor

B.Law of Conservation of Mass In a chemical reaction, matter is not

created or destroyed. Atoms can only rearrange. Discovered by Lavoisier.

4H

2O

4H

2O

C. Chemical Reaction

A change in which one or more substances are converted to different substances.

A+BC+DREACTANTS PRODUCTS

C. Chemical Equations

Reactants = Starting substances Products = Substances Formed

C. Chemical Equationsp. 635

C. Chemical Equations

A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols.

Chemical equations written with symbols makes it much easier to tell what is happening in the reaction.

C. Chemical Equations

Aqueous lead(II) nitrate plus two units

of aqueous potassium iodide

produces solid lead(II) iodide and two

units of aqueous potassium nitrate.

Pb(NO3)2+ KI PbI2 + KNO32 2

Coefficient - # of units of each substance

C. Chemical Equations

Describing Coefficients:

individual atom = “atom”

2Mg 2 atoms of magnesium

covalent substance = “molecule”

3CO2 3 molecules of carbon dioxide

ionic substance = “unit”

4MgO 4 units of magnesium oxide

C. Johannesson

II. Balancing Equations

Ch.7– Chemical Reactions

A. Balancing Steps

1. Write the unbalanced equation.

2. Count atoms on each side.

3. Add coefficients to make #s equal.

Coefficient subscript = # of atoms

4. Reduce coefficients to lowest possible ratio, if necessary.

5. Double check atom balance!!!

B. Helpful Tips

Balance one element at a time. Update ALL atom counts after

adding a coefficient. If an element appears more than

once per side, balance it last. Balance polyatomic ions as single

units. “1 SO4” instead of “1 S” and “4

O”

Al + CuCl2 Cu + AlCl3

33 2

C. Balancing Example

Aluminum and copper(II) chloride react to form copper and aluminum chloride.

2

Determine the reactants and products

Write the reactants on the left side of the equation and the products on the right

Show an equal number of atoms for each element on both sides of the equation.

D. Writing Balanced Equations

When propane gas burns in the air, the reactants are propane (C3H8)and oxygen (O2). The products formed are carbon dioxide (CO2)and water (H2O).

Step 1: Find the reactants and products.

Example

Example

Step 2: Write a chemical equation

C3H8 + O2 CO2 + H2O

Example

Step 3: Balance the chemical equation

*In balancing a chemical equation, change only the coefficients. Never change the subscripts.

C3H8 + O2 CO2 + H2O5 3 4

Homework Assignment

Page 235: 33-36 Show inventory and balance the

reactions

D. Writing Reactions Identify the reactants and products

and write the equation for each of the following chemical reactions.

The key to getting this right is to balance each of the pieces correctly (Write the balanced formula. Remember from chapter 4?)

After writing the reactions balance them.

D. Writing Reactions

There are a few elements that need to be treated in a special way because of how they bond with each other.

Example: Elemental oxygen, is never found by itself. It is always found as O2. Oxygen is one of the diatomic elements. This means they are always paired up.

There are 7 diatomic elements: H, I, Br, O, N, Cl, F. (Remember

HI BrONClF!) These elements are all gases!

• The Seven Diatomic Elements

Br2 I2 N2 Cl2 H2 O2 F2

HIBrONClF

D. Writing Reactions

Writing Reactions

When writing the balanced formula for elemental metals they are always written by themselves.

Example: Elemental Copper is just Cu. Elemental Aluminum is just Al.

This will be the case for all elemental metals.

Writing Reactions

Example 1:

Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas.

Step 1: Find the reactants and products

Writing Reactions

The reactants are Mg metal and Water, the products are Mg hydroxide and H gas.

Step 2: Write the balanced formula for each piece.Mg (s) = Magnesium MetalH2O (l) = Water

Mg(OH)2 (s) = Magnesium Hydroxide

H2 (g)= Hydrogen gas

Writing Reactions

The reactants are Mg metal and Water, the products are Mg hydroxide and H gas.

Step 2: Write the balanced formula for each piece.

Mg (s) = Magnesium MetalH2O (l) = Water

Mg(OH)2 (s) = Magnesium Hydroxide

H2 (g)= Hydrogen gas

Writing Reactions

Step 3: Put the reactants and products into a chemical reaction.

Mg(s) + H2O(l) Mg(OH)2 (s) + H2(g)

Step 4: Balance the reaction

D. Writing Reactions

Write the following reactions and then balance them:

1. Solid Ammonium dichromate decomposes to form solid chromium (III) oxide, gaseous nitrogen, and gaseous water.

2. Gaseous ammonia (NH3) reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water.

Assignment

Page 234 #’s 9-11, 13, 18, 21, 25-28

I want you to write the question, circle the reactants, underline the products. Then write out the reaction in symbol form and balance it.

Ch. 8 – Chemical Reactions

III. Types of Chemical Reactions

A. Combustion

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

the burning of any substance in O2 to produce heat

A + O2 B

Na(s)+ O2(g)

C3H8(g)+ O2(g) 5 3 4

A. Combustion

Products:contain oxygenhydrocarbons form CO2 + H2O

CO2(g)+ H2O(g)

Na2O(s) 4 2

B. Synthesis

the combination of 2 or more substances to form a compound

only one product

A + B AB

C. Johannesson

B. Synthesis

H2(g) + Cl2(g) 2 HCl(g)

Al(s)+ Cl2(g) AlCl3(s)2 3 2

B. Synthesis

Products: ionic - cancel chargescovalent - hard to tell

C. Decomposition

a compound breaks down into 2 or more simpler substances

only one reactant

AB A + B

C. Johannesson

C. Decomposition

2 H2O(l) 2 H2(g) + O2(g)

KBr(l) K(s) + Br2(l) 2 2

C. Decomposition

Products:binary - break into elementsothers - hard to tell

C. Johannesson

D. Single Replacement

one element replaces another in a compoundmetal replaces metal (+)nonmetal replaces nonmetal (-)

A + BC B + AC

C. Johannesson

D. Single Replacement

Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

C. Johannesson

Fe(s)+ CuSO4(aq) Cu(s)+ FeSO4(aq)

D. Single Replacement

Products:metal metal (+)nonmetal nonmetal (-) free element must be more active (check activity series)

Br2(l)+ NaCl(aq) N.R.

C. Johannesson

AB + CD AD + CB

E. Double Replacement

ions in two compounds “change partners” cation of one compound combines with

anion of the other

C. Johannesson

E. Double Replacement

Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)

Pb(NO3)2(aq)+ KI(aq) PbI2(s)+ KNO3(aq)

E. Double Replacement

Products:switch negative ions

NaNO3(aq)+ KI(aq) N.R.

2 2

Decide the RXN TypeWOWO33 + 3H + 3H22 W + 3H W + 3H22OO

RbCl + 2ORbCl + 2O22 RbClO RbClO44

RbBr + AgCl RbBr + AgCl AgBr + RbCl AgBr + RbCl

2KNO2KNO33 2KNO 2KNO2 2 + O+ O22

3Mg + N3Mg + N22 Mg Mg33NN22

2C2C22HH66 + 7O + 7O22 4CO 4CO22 + 6H + 6H22OO

Chemical reactions can be Chemical reactions can be classified into 5 different types.classified into 5 different types.These 5 types are Single These 5 types are Single Displacement, Double Displacement, Double Displacement, Decomposition, Displacement, Decomposition, Synthesis, and Combustion. Synthesis, and Combustion. You will be given the general form You will be given the general form for each of the 5 reactions.for each of the 5 reactions.

Chemical Changes

In a single displacement reaction one In a single displacement reaction one element displaces or changes places with element displaces or changes places with another element in a compound.another element in a compound.General Form:General Form:Element + CompoundElement + Compound Element + Element + CompoundCompoundEx. ClEx. Cl22 + 2KBr + 2KBr 2KCl + Br 2KCl + Br22

Chlorine changes places with BromineChlorine changes places with Bromine

Single Replacement

Look at the reactants in the following single Look at the reactants in the following single displacement reaction and decide which displacement reaction and decide which elements will change places: Al + Feelements will change places: Al + Fe22OO33 Be sure to find the charges of all the Be sure to find the charges of all the elements.elements.Which ones will switch?Which ones will switch?

Single Replacement

In a double displacement reaction the In a double displacement reaction the positive and negative parts of two positive and negative parts of two compounds switch places.compounds switch places.General form: General form: Compound + Compound Compound + Compound Compound + Compound + Compound Compound Ex: PbClEx: PbCl22 + Li + Li22SOSO44 PbSO PbSO44 + 2LiCl + 2LiCl

Double Replacement

Look at the reactants for the following Look at the reactants for the following double displacement reaction and predict double displacement reaction and predict what will happen.what will happen.ZnBrZnBr2 2 + AgNO+ AgNO33

Double Replacement

In a decomposition reaction a compound In a decomposition reaction a compound break up or decompose down into simpler break up or decompose down into simpler substances.substances.General form: Compound General form: Compound two or two or more elements or compoundsmore elements or compoundsEx: 2AgEx: 2Ag22O O 4Ag + O 4Ag + O22

Decomposition

In a synthesis reaction two or more In a synthesis reaction two or more substances combine to form one new substances combine to form one new substance.substance.General form: Element or compound + General form: Element or compound + Element or compound Element or compound Compound CompoundEx: CaO + SiOEx: CaO + SiO22 CaSiO CaSiO33

Ex: 2HEx: 2H22 + O + O22 2H 2H22OO

Synthesis

Combustion

Combustion occurs when an organic Combustion occurs when an organic compound (a compound of Carbon) reacts with compound (a compound of Carbon) reacts with oxygen in the air and burns. When this occurs oxygen in the air and burns. When this occurs the products are the products are alwaysalways Carbon dioxide and Carbon dioxide and Water.Water.General form: Hydrocarbon + oxygen General form: Hydrocarbon + oxygen Carbon dioxide + Water Carbon dioxide + Water Ex: CHEx: CH44 + 2O + 2O22 CO CO22 + 2H + 2H22OO

Examples:Decide the RXN Type

WO3 + 3H2 W + 3H2O

RbCl + 2O2 RbClO4

RbBr + AgCl AgBr + RbCl 2KNO3 2KNO2 + O2

3Mg + N2 Mg3N2

2C2H6 + 7O2 4CO2 + 6H2O