CHEMICAL REACTIONS

PHYSICAL VERSUS CHEMICAL CHANGEA REFRESHER

Physical Change A change that alters the form or appearance of a material but DOES NOT convert the material into a new substance.

Chemical Change A change in matter that forms one or more new substances. The new substances) are made of the same elements as the original substance, but are now in different combinations.

ATOMS AND MOLECULES

Atom is the smallest particle of an element. All atoms of an element have the same chemical

properties. A molecule is a particle made of two or more

atoms bonded together. Some molecules are made of the same element

(example O2 – the form of oxygen we breathe) MOST molecules are made of more than one

type of atom. (example H20 ) Try to count the atoms: how many total atoms are

there in 1 molecule of water?

MORE ABOUT MOLECULES

All compounds are molecules. but NOT all molecules are compounds.

Think about it. A compound is a pure substance made up of two

or more elements. A molecule is two or more atoms.

OBSERVING CHEMICAL REACTIONS Chemical Reaction: The process in which

substances undergo chemical changes that results in the formation of new substance.

How can you tell if a chemical reaction has taken place?

Evidence for chemical reactions:1. Color change2. A Precipitate Forms

1. Precipitate is a solid that forms from solution during a chemical reaction.

3. Gas is produced (like when you mix baking soda and vinegar)

4. Changes in temperature5. Changes in properties

CHEMICAL REACTION VOCABULARYLook up the definitions for the following terms and

add to your notes: (Page 24-25 or glossary)Symbol, Chemical Formula, Chemical Equation, Subscript

Symbol: In chemistry, a one-letter or two-letter set of characters used to identify an element. O, Fe, He, H

Chemical formula: A combination of symbols that represent the elements in a compound. NaCl, CO, CO2

Chemical Equation: Uses symbols and formulas to show the reactants and the products of a chemical reaction.

Subscripts: Show the number of atoms of an element in a molecule or the ratio of elements in a compound. If a letter doesn’t have a subscript the number 1 is understood to

be there. For example, CO2 has 1 atom of Carbon and 2 atoms of

Oxygen, for a total of 3 atoms.

STRUCTURE OF A CHEMICAL EQUATION

A chemical equation summarizes a chemical reaction.

The REACTANTS are the materials you have at the beginning of the reaction.

The PRODUCTS are the new materials that you have when the reaction is complete.

Chemical equations have a definite structure. The reactants are always on the LEFT, followed by an

arrow (“yields”). The products are always on the RIGHT.

Reactant + Reactant Product + Product

CHEMICAL BONDS AND REACTIONS

A Chemical Bond is the force that holds atoms together. Think of it like the “glue” that makes atoms

“stick” together.

Chemical Reactions occur when chemical bonds break or when new bonds form.

LAW OF CONSERVATION OF MASS

The Law of Conservation of Mass states that the amount of matter in a chemical reaction does not change. Therefore, the total mass of the reactants MUST EQUAL the total mass of the products.

COUNTING ATOMS

1. Make a 2-column list of the different elements in each compound.

2. Write down how many atoms of each element.

3. Add all of the atoms together for a total.

Example: NaOH

Now You Try: MgODid you say Mg = 1 and O = 1 for 2 Total?

Element # of Atoms

Na 1

O 1

H 1 3 Total

COUNTING ATOMS WITH SUBSCRIPTS

MgCl2

Now You Try!C6H12O6

SubscriptMg 1

Cl 2 3 Total

C 6

H 12

O 6 24 Total

COUNTING ATOMS WITH COEFFICIENTS AND SUBSCRIPTS

4HNO3

Coefficient: number placed in front of a chemical formula in an equation. It tells you how many atoms or molecules of each reactant and product take place in the reaction.

Count Just Like Before1.List each element.2.Multiply each element by the coefficient.3.If the element also has a subscript, multiply the coefficient by the subscript.

Now You Try!6 PO4

H 4

N 4

O 12 2o total

P 6

O 24

30 total

COUNTING ATOMS WITH COEFFICIENTS, SUBSCRIPTS AND PARENTHESIS Ca (C2H2O2)2

Just like before make a chart of all elements in the chemical formula.

Start with the parenthesis. Count each atom of the elements. Multiple each element by the parenthesis subscript.

Now You Try: Mg(C2H3O2)2 What happens if we add a coefficient of 2 in front of the

entire formula?

Ca 1

C 2 X 2 4

H 2 X 2 4

O 2 X 2 4

TOTAL OF 13 Atoms

BALANCING CHEMICAL EQUATIONS

TWO SIDES TO A CHEMICAL EQUATION

REACTANTS PRODUCTS

Reactants = what goes into the chemical rxn Products = what comes out of the chemical rxn

In chemical reactions, new things are made

BASIC EQUATION

Chemical Equation CO2 + H2O C6H12O6 + O2 + H2O

LAW OF CONSERVATION OF MATTER????

“Matter cannot be created or destroyed”

-Look at the equation again, is there the same amount of matter on the REACTANTS side as there is on the PRODUCTS side? Count the number of atoms for each element in the Reactant side and the Product side.

CO2 + H2O C6H12O6 + O2 + H2OC 1

O 3

H 2

C 6

O 14

H 9

COUNT THE ATOMS NOW REMEMBER TO MULTIPLY WITH THE COEFFICIENT!

C 6

O 24

H 24

C 6

O 24

H 24

HOW TO BALANCE A CHEMICAL EQUATION1. Create a list of the atoms of each element just as if you

are counting atoms.2. Pick an element that is not equal on both sides of the

equation.3. Add a coefficient in front of the formula with that

element and adjust your original counts.4. Continue adding coefficients until you get the same

number of atoms of each element on both sides.5. Update your list (inventory) after every move.6. Save Oxygen and Hydrogen for last

PRACTICE!

H2 + O2 H2O

C + Cl2 CCl4

DEMONSTRATION - CONSERVATION OF MASS

Vinegar & baking soda reaction We need to measure the mass of all reactants BEFORE

the reaction. Vinegar and baking soda After we measure the mass of the reactants you should be

able to calculate the total mass of the product. What would it be?

We also need to find the mass of the bag and cups. Why?

We then need to close the bag to created a closed system.

Why do you think we need to create a “closed system” by closing the bag? What might be lost if we did not close the bag?

MORE PRACTICE

Complete the front of the “balancing act” worksheet.

If you finish, try these:

LAW OF CONSERVATION OF MASS

Open system versus Closed System

If a chemical reaction takes place in a closed system, no matter can enter or leave the reaction.

- the reaction takes place in a sealed container or in a vacuum.

If a chemical reaction takes place in an open system, matter can enter or leave during the reaction. The reaction DOES NOT take place in a sealed

container.