REACTIONS

Reactions

Chemical equation: equation that shows the rearrangement of atoms that occurs in a chemical reaction

Reactants: original substances Products: resulting substances

A + B Yields AB

Reactants Products

Symbols You’ll See

Symbol Meaning

(s) Solid state

(l) Liquid state

(g) Gas state

(aq) Aqueous solution

Yields

+ Two reactants or two products

⇄ Reversible reaction

Law of Conservation of Mass Recall that mass cannot be created or destroyed. In other words

reactant mass = product mass

28.014 amu + 6.0474 amu

34.061 amu=

14.007 amu 1.0079 amu

Four Indications of Chemical Change

1. Energy absorbed or released in the form of heat or light.

2. Change in color or odor.3. Production of gas (bubbling).4. Formation of precipitate (solid).

Balancing Chemical Equations

Balanced chemical equations have equal numbers of atoms of each element in reactant and products.

Never change subscripts! Coefficient: number of

particles of a substance (can change).

Subscript: number of atoms in a substance (does not change).

Subscript• Indicate number of

atoms of an element in a substance

Ex) Three atoms of H• Changing changes

chemical identity

2 NH3Coefficient• Indicate the number of

particles of a substance

Ex) Two molecules of NH3

• Change to balance formulas

2 atoms N & 6 atoms H

Steps for Balancing Chemical Equations

1. Write skeleton equation.

2. Write all elements present.

3. Count and write number of atoms of each element on reactant side.

4. Repeat step 3 for product side.

5. Add coefficients to balance.

6. Recount atoms to check work.

7. Make sure that coefficients are in lowest whole number ratio.

A2B + C2 CB2 + A2ABC

212

22

1

2

42

2

4

2

24

4

48 8

4

How to Reduce Error

Go in this order:1. Metals2. Polyatomic Ions3. Nonmetals4. Hydrogen and

Oxygen last!H2S + AgNO3 HNO3 +

Ag2S

HS

AgNO3

2111

1121

2

22

2

2

2

H2S + AgNO3 HNO3 + Ag2S

Day 2—Types of Reactions

Types of Chemical Reactions

1. Synthesis2. Decomposition3. Single Replacement4. Double Replacement5. Combustion

Synthesis

Two or more reactants form one product.A + B AB

Decomposition

One reactant forms two or more products.AB A + B

Single Replacement

A more reactive element replaces a similar, but less reactive, element in a compound.

A + BC AC+ BTo check if a reaction will occur, check

your reactivity chart.

Double Replacement

Two compounds exchange ions and form two new compounds.

AB + CD AD + CB

Combustion

Substance reacts with oxygen and releases energy in the form of heat and light.

CxHy + O2 CO2 + H2O

Day 3—Reactivity Series

Reactivity Series

Please turn to the page in your notebook with the activity series. This is the same page as the list of polyatomic ions.

The activity series shows a list of metals (and hydrogen). The top metals are highly reactive and will take the place of another metal in a bond.

The bottom metals are fairly unreactive and will usually be found by themselves. (Notice that gold is down by the bottom.)

Activity Series

We’ve learned about single replacement reactions. These can only occur if the metal that is being added to the compound is more active than the original.

So for example, Al + AgNO3 Ag + Al(NO3)3 will occur. But Cu + FeSO4 CuSO4 + Fe will not. Iron is

more active than Copper, and will not give up its bond with sulfate.

Solubility

Another important table is the solubility chart. This shows whether a compound is soluble (or can be dissolved in water), or insoluble (and will form a precipitate).

This chart is important in double-replacement reactions.

When you show the double replacement products, you can determine what phase they will be in.

Day 4—Acid-Base Reactions

Acid-Base Reactions

Salt: ionic compound composed of the cation of a base and anion of acid

Acid Base Reactions: are also double replacement, and also called “neutralization”

Acid + Base Water + Salt

Acid Base SaltHCl(aq) NaOH(aq) NaCl

Water(aq) + HOH(l ) + Ex)

Types of Reactions

Synthesis A + B AB Decomposition AB A + B Single Replacement AB + C A + CB Double Replacement AB + CD AD + CB Combustion CxHy + O2 CO2 + H2O Acid-Base Neutralization Hanion +

cationOH H2O + salt

Day 5—Redox Reactions

Oxidation-Reduction ReactionsOxidation: Loss of electronsReduction: Gain of electrons

Memory Aids:OIL RIG: Oxidation I Lose, Reduce I GainLEO goes GER: Lose Electrons, Oxidize…

Gain Electrons, Reduce

Oxidation Numbers

Oxidation Number: value used to represent the number of electrons transferred.

Identify “redox” reactions by observing changes in oxidation number.

Assigning Oxidation NumbersIf a compound were composed of ions,

oxidation numbers would be the charges.

Ex) H2SO4

+1 –2

= 0

x = +6

(2 ∙ +1) + x + (4 ∙ –2 )

General Rules

Neutral elements = 0 Monatomic ions = ionic charge Oxygen (when not by self) = -2 Hydrogen with nonmetal = +1 Hydrogen with metal = -1 Sum of states = the chemical formula’s

overall charge

General Rules

Ask, “What was oxidized? What was reduced?”

1. Assign oxidation numbers2. Compare oxidation numbers on each side

a) Oxidation number decreases = reduced, gained electrons

b) Oxidation number increases = oxidized, lost electrons

c) No change = oxidation didn’t occurEx) Mg + H2SO4 MgSO4 + H2

–2+6+10 0–2+2 +6

gained 1 e–lost 2 e–

H was reduced

Mg was oxidized

S & O were not oxidized or reduced

Oxidation Reduction ReactionsOxidizing Agent: substance that causes

oxidation (whatever was reduced).Reducing Agent: substance that causes

reduction (whatever was oxidized).

Ex) Net ionic equation: Mg + 2H+ Mg2+

+ H2

H+ was reduced

Mg was oxidized

H+ was the oxidizing agent

Mg was the reducing agent

What Are Redox Reactions?

Always: combustion, single replacement, synthesis of ions from pure elements.

Never: double replacement—the oxidation numbers will remain the same.