Chemistry 112

Chapter 4 Class Notes

Overview: This chapter will introduce:

· types of chemical change,

· the kinetic molecular theory,

· how to balance chemical equations, and

· a classification for chemical reactions

( 5 general types).

Introduction:

What Chemical Equations do you conduct every morning?

Brushing your Teeth – fluoride compounds in your toothpaste react with compound in your teeth

Eating Breakfast – acids and other materials in your digestive system

Driving to School – hydrocarbons in the fuel react with oxygen in the air

Other – clothing, antibiotics, plastics

Reflections:

1. What happens to matter in chemical changes?

2. How can we represent chemical reactions?

3. How can chemical reactions be classified or grouped?

4. What are some examples of different types of chemical reactions?

Types of changes in Matter:

(1) Physical Changes - fundamental particles remain unchanged at a molecular

level.

- no change in written chemical formula

- involves relatively small amounts of energy changes

- example: melting, evaporation, tearing, grinding

(2) Chemical Changes - involves some type of change in the chemical bond

within the fundamental particles

- there is a change in chemical formula

- larger energy change with new compounds being formed

- example: burning

(3) Nuclear Changes - create entirely new atomic particles

- new chemical formulas

- involve extremely large changes in energy

- “radioactivity”

The Kinetic Molecular Theory:

Three points: all matter is made up of atoms

particles of a substance are in constant random motion

all collisions are elastic

The energy created by the motion of the particles is called KINETIC ENERGY (KE). The average kinetic energy of a substance is the measure of temperature.

States of Energy:

(1) solids – definite shape and volume, incompressible, do not flow readily and

have low KE. The motion is described as vibrational

(2) liquids – assume the shape of the container, but have a definite shape, are

Incompressible, flows readily and have higher KE. The motion is

described as vibrational, rotational, and translational.

(3) gases – assume the shape and volume of their container, are highly

compressible, flows easily, and has the highest energy. The motion is

described as translational

Evidence of chemical reactions:

1. color change

2. odor change

3. state change –precipitate

4. energy change – if the form of heat, light, sound or electricity

ENDOTHERMIC REACTIONS: needs an input of energy for the reaction to occur. Products have less energy that the products.

EXOTHERMIC REACTIONS: gives an output of energy as the reaction is completed. The reactants more energy that the products.

The Law of Conservation of Mass States in a chemical reaction, the total mass of the reactants always equals the total mass of the products.

Atoms in a chemical reaction do not change- the number of each kind of atom is the same before and after the equation. In a chemical reaction the atoms of the reactants are simply rearranged.

methane + oxygen à carbon dioxide + water

CH₄ + 2O₂ à CO₂ + 2H₂O

This Law has implications far beyond the lab.

Pollution, We burn gas….where do the atoms go?

Why do reactions occur?

Due to the COLLISION –REACTION THEORY

The particles of a substance are in constant random motion. This motion inevitably results in collisions among particles. If different substances are present, all the different particles will collide randomly with each other. If the collision has two things:

1. enough energy (moving fast enough)

2. proper orientation

The atoms of the substances will rearrange to form a new substance.

Word Equations!

Chemical reactions may involve sophisticated chemicals, such as explosive reactions of dynamite or simple household materials as in the reactions of the bathroom cleaner with a stain.

A WORD EQUATION is one way of representing a chemical reaction. It tells you what reacts and what is produced.

Format:

All the reactants à All the products

Reactants: the substances you start with

Products: the new substances made in the end

Reactant #1 + reactant #2 à product #1 + product #2

Examples : iron + oxygen à iron (III) oxide

copper + silver nitrate à silver + copper (II) nitrate (DEMO)

Balancing Chemical Equations

We need to explain chemical equations in symbolic form.

Skeleton equation – formulas of the reactants are connected to the formulas of the

products by an arrow

methane + oxygen à carbon dioxide + water

symbols = CH₄ + O₂ à CO₂ + H₂O

but there is a problem…..the number of individual atoms are not equal on both sides (law of conservation of mass)

Atom	Reactant’s side	Product’s side
Carbon	1	1
Hydrogen	4	2
Oxygen	2	3

*we can not change the type or formulas of the molecules so how can we solve the imbalance?

Answer…..change the number of molecules rather than their formulas.

CH₄ + O₂ +O₂ à CO₂ + H₂O + H₂O

CH₄ + 2O₂ à CO₂ + 2H₂O

This is called a balanced chemical equation

Coefficient – the number written in from of a chemical formula to indicate how many are

being used in the reaction.

Subscript - indicates the number of atoms or ions present in a substance

States of matter at SATP – s,l,g,aq

Do the worksheet on counting atoms in chemical compounds

How to Balance a Chemical Equation

Steps: 1. Write a word equation for the reaction

2 Write the skeleton equation with chemical formulas

3 Count the number of atoms of each type on the reactant and product side

(use a table)

4. determine the number of each substance needed.

Examples :

1. Iron reacts with oxygen to form magnetic iron oxide (Fe₃O₄ ) A mixture of

the two most common oxides iron (II) oxide and iron (III) oxide

2. Magnesium reacts with hydrogen nitrate to produce hydrogen gas and magnesium nitrate.

3. Methane reacts with oxygen to produce carbon dioxide and water.

4. Solid aluminum reacts with bromine gas to produce aluminum bromide

5. nitrogen gas reacts with hydrogen gas to produce ammonia gas

MOLE CONCEPT:

Since atoms, ions and molecules are extremely small particles, a convenient number of them must be much greater than a dozen.

Mole – (mol) an amount of substance with the number of particles corresponding to

Avogadro’s Number (6.02 x 10²³)

Example: 1 mole of Na = 6.02 x 10²³ atoms of Na

1 mole of Cl₂ = 6.02 x 10²³ atoms of Cl₂

1 mole of NaCl = 6.02 x 10²³ molecules of NaCl

Classifying Chemical Reactions

(1) Formation Reaction

Is the reaction of two or more elements to form either an ionic or molecular compound.

2Mg + O₂  2MgO

Cl₂ + H₂  2HCl

(2) Single Decomposition Reaction

Is the breakdown of a compound into its components (elements). It is the reverse of a formation reaction.

2H₂O  2H₂ + O₂

2CuO  2Cu + O₂

(3) Combustion reaction

Is the burning of a substance with sufficient oxygen available to produce the most common oxides of the elements present in the compound being burned. These reactions are always exothermic.

The most common oxides are:

Carbon: CO₂ nitrogen: NO₂

Hydrogen: H₂O metals: most common valence charge

Sulfur: SO₂ joined to oxide

Example: 2C₄H₁₀ + 13O₂ à 8CO₂ + 10H₂O

(4) Single Replacement Reaction

Is the reaction of an element with a compound to produce a new ionic compound and element.

Example: Cu + 2AgNO₃ à Cu(NO₃)₂ + 2Ag

Cl₂ + 2NaI à NaCl + 2I₂

(5) Double Replacement Reaction

Occurs between two ionic compounds in solution. The ions “change partners” to form new products.

Example: CaCl₂ + Na₂CO₃  CaCO₃ + 2NaCl

HCl + KOH  HOH + KCl

***** copy summary chart done in class