Chemisty Review

Elements are represented by 1, 2, or 3 letters. They always begin with a capitol letter, followed by lower case letters (if any). Example: Co = cobalt CO = carbon and oxygen.

These symbols are used to write the formulas for compounds. A compound is a chemical formed from 2 or more different elements. Example: NaCl

Chemical reactions form new substances. Some of the signs of a chemical reaction are:

Bubbles (production of a gas)

Temperature Change – exothermic releases heat, endothermic absorbs heat

Color change

Light is given off

Change of smell

Examples: tarnishing, rust

Formulas can have small numbers in them called subscripts A subscript tells you how many atoms of the preceding element are in the formula. If there is no subscript, the number is 1. Example: H2O = 2 atoms of hydrogen, 1 atom of oxygen.

Formulas are used in chemical equations to show what takes place in a chemical reaction. Chemical equations have 3 parts:

Reactants - Yield (arrow) – Products

2H2 + O2 2H2O

Equations work with molecules of elements and compounds. If a the formula of a molecule has a large number in front of it, that number, called a coefficient, represents how many molecules are represented in the equation. Example: 2H2O = 2 molecules of water.

When counting atoms, multiply the coefficient by the subscript to get the total number of atoms. Example: 2H2O = 2 x 2 = 4 atoms of hydrogen. 2 x 1 = 1 atom of oxygen.

The Law of Conservation of Mass states that “Matter can never be created, nor destroyed.” This means that all of the atoms present in the reactants will also be present in the products. Since a chemical reaction is represented in a chemical equation, the equation must be balanced; it must show the same number of atoms of each substance on either side of the arrow.

You can tell if a chemical equation is balanced by counting atoms:

Example:

2K + 2H2O 2KOH + H2

K 2

H 4

O 2

K 2

H 4

O 2

Balanced!

If a chemical equation is not balanced, you must balance it by adding coefficients.

Example:

Ca + O2 CaO

Ca 1

O 2Balanced!Not

Ca 1

O 1

2 2

Ca 2

O 2

2Mg + O2 2MgO

1. In the equation above, what is the coefficient of the oxygen molecule?

A. 0

B. 1

C. 2

D. 4

H2O + CO H2CO3

2. What subscripts needs to be added to the box to make the equation balanced?A. 0B. 1C. 2D. 4

3. What is a clue that a chemical change has occurred?A. A beaker suddenly becomes very

warm when you mix two room-temperature liquids in it.

B. A liquid forms bubbles when it reaches a high temperature.

C. Water changes color when you add food coloring to it.

D. A car door feels hot to the touch on a summer afternoon

4. Which of the following equations does not demonstrate the law of conservation of mass?

A. 2Na + Cl2 2NaCl

B. NaOH + HCl NaCl + H2O

C. P4 + 5O2 2P4O10

D. SnCl2 + 2FeCl3 2FeCl2 + SnCl4

5.In the balanced equation below, how many molecules of products are shown?

C12H24 + 18O2 12CO2 + 12H2O

A. 144

B. 72

C. 24

D. 2

6. Which of the following is not the result of a chemical reaction?

A. A tarnished penny

B. A rusty gate

C. An evaporated mud puddle

D. Brown guacamole

7.The chemical formula for photosynthesis is

6CO2 + 6H2O C6H12O6 + 6O2

How many atoms of oxygen are in the products?A. 18B. 12C. 8D. 6

8.What coefficients are needed to balance the following equation?

Ca + O2 CaO

A. 3, 2

B. 2, 2

C. 2, 3

D. 3, 3

Count the atoms and molecules in the following formula:

3Na2CO3

Key:1. B 9. Molecules: 3

2. C Atoms:

3. A Na = 6

4. C C = 3

5. C O = 9

6. C

7. A

8. B