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Intro to Reactions

Ch. 8 – Chemical Reactions

Signs of a Chemical Reaction

 Evolution of heat and light  Formation of a gas  Formation of a precipitate  Color change

Law of Conservation of Mass

 mass is neither created nor destroyed in a chemical reaction

4 H

2 O

4 H

2 O 4 g 32 g

36 g

  total mass stays the same  atoms can only rearrange

Chemical Equations

A+B → C+D REACTANTS PRODUCTS

Symbols used in Equations

Writing Equations

  Identify the substances involved.  Use symbols to show:

2H2(g) + O2(g) → 2H2O(g)

• How many? - coefficient • Of what? - chemical formula • In what state? - physical state

 Remember the diatomic elements.

Writing Equations

Two atoms of solid aluminum react with three units of aqueous copper(II) chloride to produce three atoms of solid copper and two units of aqueous aluminum chloride.

•  How many? •  Of what? •  In what state?

Al 2 (s) + 3 CuCl2 (aq) → 3 Cu (s) + 2 AlCl3 (aq)

Describing Equations

 Describing Coefficients: • individual atom = “atom”

• covalent substance = “molecule”

• ionic substance = “formula unit”

3 molecules of carbon dioxide 2 atoms of magnesium

4 formula units of magnesium oxide

3CO2 ⇒ 2Mg ⇒

4MgO ⇒

Describing Equations

to produce

•  How many? •  Of what? •  In what state?

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

One atom of solid zinc reacts with two molecules of aqueous hydrochloric acid one unit of aqueous zinc chloride and one molecule of hydrogen gas.

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Balancing Equations

Ch. 8 – Chemical Reactions

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!!!

Helpful Tips

 Balance one element at a time.   If Hydrogen and Oxygen appear,

balance them last. Balance Hydrogen before Oxygen.

 Update ALL atom counts after adding a coefficient.

  If an element appears more than once per side, balance it last.

Al + CuCl2 → Cu + AlCl3

Al

Cu

Cl

1 1

1 1

2 3

2 ⁄

3 ⁄

6 ⁄

⁄ 3

3 3 2

Balancing Example

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

2

⁄ 2

⁄ 6

Balancing Example

 Aqueous nitric acid reacts with solid magnesium hydroxide to produce aqueous magnesium nitrate and water

  (2,1,1,2)  Solid Calcium metal reacts with water

to form aqueous calcium hydroxide and hydrogen gas.

  (1,2,1,1)

Balancing example (hydrocarbons)

 Ethane gas reacts with oxygen gas to form carbon dioxide and water.

 Ethene gas reacts with oxygen gas to form carbon dioxide and water.

 Butane gas reacts with oxygen gas to form carbon dioxide and water.

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Ch. 8 – Chemical Reactions

Types of Chemical Reactions

Combustion

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

  the burning of a hydrocarbon in O2 to produce heat

A + O2 → B

C3H8(g)+ O2(g) → 5 3 4

Combustion

  Products: •  form CO2 + H2O • Form carbon monoxide and/or carbon if

there is limited oxygen

CO2(g)+ H2O(g)

Synthesis (Combination)

  the combination of 2 or more substances to form a compound

 only one product

A + B → AB

Synthesis (Combination)

H2(g) + Cl2(g) → 2 HCl(g)

Al(s)+ Cl2(g) → AlCl3(s) 2 3 2

Synthesis (Combination)

  Products: •  If products are ionic - cancel charges •  If covalent - hard to tell

Decomposition

 a compound breaks down into 2 or more simpler substances

 only one reactant

AB → A + B

Decomposition

2 H2O(l) → 2 H2(g) + O2(g)

KBr(l) → K(s) + Br2(l) 2 2

Decomposition

  Products: • binary - break into elements • others - hard to tell (more on this later)

Synthesis Reactions to know

 Almost all metals react with oxygen to form metal oxides.

 Ex/ Magnesium reacts with oxygen to form Magnesium oxide

Synthesis reactions to know

 Nonmetals also form oxides.  Sulfur reacts with oxygen to form

Sulfur dioxide  Carbon reacts with oxygen to form

Carbon dioxide

Synthesis reactions involving water to know

 Oxides of active metals react with water to produce metal hydroxides. • Ex/ Calcium oxide reacts with water

to form calcium hydroxide.

Synthesis reactions to know involving water

 Many oxides of nonmetals in the upper right portion of the periodic table react with water to produce oxyacids (acids with oxygen). • Ex/ sulfur dioxide reacts with water

to form sulfurous acid

Decomposition reactions to know

 Metal carbonates break down to produce a metal oxide and carbon dioxide gas. • Ex/ calcium carbonate decomposes

to produce calcium oxide and carbon dioxide.

Decomposition reactions to know

 All metal hydroxides except those containing group 1 metals decompose when heated to yield metal oxides and water. • Ex/ calcium hydroxide decomposes

to produce calcium oxide and water.

Decomposition reactions to know

 Metal chlorates decompose to produce a metal chloride and oxygen. • Ex/ potassium chlorate

decomposes in the presence of the catalyst Manganese dioxide to produce potassium chloride and oxygen.

Decomposition reactions to know

 Sulfites decompose into the cation oxide and sulfur dioxide gas • Calcium sulfite decomposes to form

calcium oxide and sulfur dioxide gas.

Decomposition reactions to know

 Peroxides decompose into the oxide of the cation and oxygen gas.

• Hydrogen peroxide decomposes to form water (dihydrogen monoxide) and oxygen gas. • Sodium peroxide decomposes to

form . . .

Decomposition reactions to know

 Certain acids decompose into nonmetal oxides and water. • Ex/ Carbonic acid is unstable and

decomposes readily to produce carbon dioxide and water. • Sulfurous acid decomposes to

produce sulfur dioxide and water.

Decomposition reactions to know

 Ammonium salts decompose into ammonia and the acid of the anion. • Ex/ Ammonium acetate

decomposes to form ammonia and acetic acid. • Ammonium nitrate decomposes to

form ammonia and nitric acid.

Single Replacement

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

A + BC → B + AC

Single Replacement

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

Fe(s)+ CuSO4(aq) → Cu(s)+ FeSO4(aq)

Single Replacement

  Products: • Metal atom → metal cation(+) • Nonmetal atom → nonmetal anion(-) •  free element must be more active (check

activity series)

Br2(l)+ NaCl(aq) → N.R.

AB + CD → AD + CB

Double Replacement

  Occurs between two aqueous ionic compounds

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

anion of the other

Double Replacement

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

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

Double Replacement   Products:

•  In order for a reaction to occur on of the following must occur -  one product must be insoluble (a precipitate)

(check solubility table) - A gas is produced - A covalent compound (molecular) is produced

NaNO3(aq)+ KI(aq) → N.R. 2 2

Examples

 Solutions of Hydrochloric acid and Sodium hydroxide are mixed

 Solutions Rubidium carbonate and acetic acid are mixed

Writing net ionic equations

  1. Write the skeleton equation • Make sure charges ions are balanced.

  2. Write the total ionic equation   3. Cancel out ions that appear on both

sides   4. These are called spectator ions.   Spectator ions do not take part in the

overall net reaction.

Writing net ionic equations

 A solution of Lead (II) nitrate is mixed with a solution of potassium iodide.

 A solution of silver nitrate is mixed with a solution of sodium hydroxide.

 View clip

Predicting products from reactions

  If given only reactants • Figure out what type of reaction it is • Write the products. If one of the

products is ionic, balance charges. • Balance the equation • Ex/ Hydrochloric acid is mixed with

zinc metal.

Predicting (cont’d)

  Calcium metal burns in the presence of oxygen.

  Sodium carbonate is heated vigorously.   Potassium chlorate is heated vigorously.   Pentane burns in oxygen.   Aqueous silver nitrate is mixed with

aqueous sodium chloride.

Predicting products

 Hydrochloric acid is mixed with Sodium hydroxide.

 Gold metal is dropped into hydrochloric acid.