Chemical Reactions

Background

• We learned about Physical and Chemical properties and changes.

– Physical properties were those like mass, volume, density, melting point, boiling point, conductivity, malleability, state/phase of matter etc.

• Changes within these properties can be undone and the properties of the substances are not altered.

– Chemical properties were those like oxidation, flammability, and reactivity.

• Changes within these properties CANNOT be undone. Change is permanent and the product now has new properties of its own.

Remember…..

Chemical Reactions occur because of the forces between atoms and valence electrons. The groups on the periodic tables all are organized based on Chemical and Physical Properties. Chemically, we are speaking of valence electrons.

IONIC BOND- When a metal bond to a NON-metal, this is usually due to an IONIC bond where the NON-metal has taken a free electron (valence electron) from a metal. If you look…groups 1 and 2 (all metals) have 1 or 2 valence electrons. The NON-metals are in groups 13-18. They have between 3-8 valence electrons. They NEED the electrons to complete their “octets” (means 8 by the way) or shells. BY stealing away 1 or 2 from a Metal, they begin to complete their shells and leave the Metal’s shell full.

Covalent Bond- When a NON-metal and a NON-metal are close enough for electromagnetic force to attract valence electrons, the atoms SHARE electrons to complete their “octets” or shells. *See next slide.

Remember—we did Covalent in class!

Organize Your Thoughts

Chemical

reactions

Chemical

equations

Chemical

equations

• Balancing equations

• Predicting products

from reactants

• Synthesis

• Decomposition

• Single replacement

• Double replacement

• Combustion

Packard, Jacobs, Marshall, Chemistry Pearson AGS Globe, page 175

Signs of Chemical Reactions when atoms bond….

There are five main signs that indicate a chemical reaction has taken place:

change in color change in odor production of new

gases or vapor

input or release

of energy

difficult to reverse

rele

ase

inp

ut

In other words……..

ubblesproduction of new

gases or vapor

change in odor

input or

release

of energy

difficult to reverse

difficult to reversechange in color

What? New Substance!?• I thought you said Matter cannot be created nor

destroyed, only changed???

– New substance is referring to a substance that has its own chemical properties (different from those that created it).

– Example: Sometimes when 2 substances are mixed, the result is a PRECIPITATE. This is a SOLID that is the product of 2 reacting substances.

– A kidney stone is a real life example! There

Aren’t really stones in your body!

Chemical Equations

• Reactants – the substances that exist before a chemical

change (or reaction) takes place.

• Products – the new substance(s) that are formed during

the chemical changes.

• CHEMICAL EQUATION indicates the reactants and products of a reaction.

REACTANTS PRODUCTS

For Example-- 2H2

+ 02 2H

20

Do you remember 6th Grade? Combine your 6th Grade Knowledge with your 8th Grade Knowledge….

Use these to fill in the Reactants and the Products

Did you get it correct?

Common Processes using a chemical equation:

Look! They are the reverse of each other!!!

Some Symbols Used in Chemical Equations

“Yields”; indicates result of reaction

Used to indicate a reversible reaction

A reactant or product in the solid state;

also used to indicate a precipitate

Alternative to (s), but used only to indicate a precipitate

A reactant or product in the liquid state

A reactant or product in an aqueous solution

(dissolved in water)

A reactant or product in the gaseous state

(s)

(l)

(aq)

(g)

Chemical Equations

4 Al(s) + 3 O2(g) 2 Al2O3(s)

The numbers in the front are called

stoichiometric coefficients.

The letters (s), (g), and (l) are the

physical states of compounds.

aluminum oxide

Reactants Product

Characteristics of Chemical Equations

• The equation must represent known facts.

• The equation must contain the correct formulas for the reactants and products.

• The law of conservation of mass must be satisfied. (Equation must be balanced).

Chemical Equations

This equation means:

4 Al(s) + 3 O2(g) 2 Al2O3(s)

4 Al atoms + 3 O2 molecules yields 2 molecules of Al2O3

aluminum oxide

sandpaper

Visualizing a Chemical Reaction

Na + Cl2 NaCl 2 2

Chemical Equations

Because the same atoms are present

in a reaction at the beginning (reactants)

and at the end (products), the amount

of matter in a system does not change.

The Law of Conservation of Matter

Kotz web

Chemical

Factory

100% 100%80%

20%

Chemical Equations

Because of the law of the conservation of matter, an equation must be balanced.

It must have the same number of atoms of the same kind on both sides.

*Remember from class?

On both sides:

C = 1

H = 4

O = 4

Word Equations

• A WORD EQUATION describes chemical change using the names of the reactants and products.

Write the word equation for the reaction of methane gas

with oxygen gas to form carbon dioxide and water.

methane + oxygen

Reactant Product

CH4 O2 CO2 H2O+ + 22

carbon dioxide + water

*Synthesis (also known as Composition) and Decomposition reactions are considered REDOX reactions (more on this later)

NEW! Very IMPORTANT!

Synthesis Reaction –Synthesis means put together!

General form: A + B AB

element or element or compound

compound compound

Na+Cl -

Na

ClNa

Cl

Na+ Cl -

2 Na + Cl2 2 NaCl

Synthesis Reactions

CO2 + H2O C6H12O6 + O2

H2 + O2 H2O

Na + Cl2 NaCl

6 6 6

Photosynthesis

Formation of water

2 2

2 2

Formation of salt

A + B C

General Form

H

H

H

H

Decomposition Reaction –Decomposition means to breakdown!

H

H

H

HO

Decomposition reaction

2 H2O 2 H2 O2

General form: AB A B

+

+

+

compound two or more elements

or compounds

OO

O

H2O H2 + O2

electricity

Decomposition Reactions

H2O2 H2O + O2

NI3 N2 + I2

2 2

Hydrogen Peroxide

Electrolysis of water

2 2

Nitrogen triiodide

AB A + B

General Form

2 3

*Both Synthesis and Decomposition reactions (explanations to follow) are considered Oxidation-Redox reactions. Think back to our last 2 days of bonding. The placement of electrons had to be altered in order for the molecules to form.

Oxidation (redox) is a type of chemical reaction in which electrons are transferred from one substance to another. For oxidation to occur, one substance must lose electrons and a second substance must gain the lost electrons.The substance gaining electrons is said to be reduced and the substance losing the electrons is said to be oxidized. Thus an oxidation reaction is called a Redox reaction.In the Redox Reaction diagram, sodium (Na) is being oxidized and chlorine (Cl) is being reduced.The term "oxidation", with its obvious root from the word "oxygen", assumes that oxygen has an oxidation number of -2. WHY -2?? BECAUSE Oxygen must gain 2 electrons in order to fill its shellremember?

Above, Sodium (Na) must LOSE 1 electron, so therefore its oxidation number is +1

Ionic bonds results in redox

Single and Double Replacement Reactions

Double-replacement reaction

CaCO3 + 2 HCl CaCl2 + H2CO3

General form:

AB + CD AD + CB

Single-replacement reaction

Mg + CuSO4 MgSO4 + Cu

General form:

A + BC AC + B

Acid-Base

• Neutralization is a type of chemical reaction in which a strong acid and strong base react with each other to form water and salt. Have you ever been unlucky enough to be stung by a wasp or a bee? Bee stings are acidic in nature, which is why a household remedy for a bee sting is baking soda or sodium bicarbonate, which is a basic substance. A wasp sting, on the other hand, is mildly basic, so a household remedy for this will be vinegar, also known as acetic acid. These simple treatments ease these painful stings by a process called neutralization.

• Heartburn, as well as an acidic stomach due to eating too much spicy food, can be relieved by taking an antacid. The antacid is alkaline/basic and helps neutralize the stomach's acidic environment. If you look at the medicine for heartburn in the picture below, you may have used this one before to ease your heartburn via the process of neutralization.

Evidence of an Acid-Base Reaction is that a SALT and WATER are the Products

Acid- Base Reaction

Combustion

Hydrocarbons (Carbon and Hydrogen combo) Combine to react and produce Carbon Dioxide and Water. Evidence of a combustion reaction are Carbon Dioxide and Water as products.