Chemical Reactions

Quick Review

mass volume

Matter is the “stuff” that makes up the universe.

All matter has ______ and ________.

All matter is composed of basic building

blocks known as _______.atoms

Quick Review

We know that matter can undergo changes both _________ and __________.

physicalchemical

Quick Review

When matter undergoes a chemical change or reaction the composition of the substance is changed and chemical bonds are formed.

When matter undergoes a physical change the composition of the substance is unchanged.

What happens during a chemical reaction?

• Atoms in the reactants are rearranged to form one or more different substances (products).

• Old bonds are broken; new bonds form.

Reactants Products

MgO + C CO + Mg

How do you know a chemical reaction has occurred?

• Evolution of heat and light

• Formation of a gas

• Formation of a precipitate (formation of a solid in a solution)

• Color change

• Temperature changes (endothermic/exothermic)

• A new odor is produced

Learning Check 1

Classify each of the following as a

1) physical change or 2) chemical change

A. ____ a burning candle

B. ____ melting ice

C. ____ toasting a marshmallow

D. ____ cutting a pizza

E. ____ polishing silver

Solution 1

Classify each of the following as a

1) physical change or 2) chemical change

A. __2__ a burning candle

B. __1_ melting ice

C. __2__ toasting a marshmallow

D. __1__ cutting a pizza

E. __2__ polishing silver

A Chemical Reaction

Reactants Products

Learning Check 2

A. How does an equation indicate a change in

the identity of the reacting substances?

B. How did the black and white reactants

combine?

C. Did all the reactants form product? Why or

why not?

Solution 2A. How does an equation indicate a change in the identity of

the reacting substances?

The formulas of the reactants are different than the formulas of the products.

B. How did the black and white reactants combine? 1 black combined with 1 white.

C. Did all the reactants form product? Why or why not?

No. There were more black reactants than white.

Mind Jogger

• When an object burns, the quantity of ashes that remain is smaller than the original object that was burned. How do you account for the change in mass? Where is the matter that appears to be lost?

Law of Conservation of Mass• mass is neither created nor destroyed in a

chemical reaction

4 H

2 O

4 H

2 O

4 g 32 g

36 g

• total mass stays the same (mass of reactants = mass of products)

• atoms can only rearrange

Law of Conservation of Mass

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Analyze the ProblemThe law of conservation of mass will allow us to determine the amount of amissing reactant or product that was involved in a chemical reaction.

For example, if you were told that 4 grams of hydrogen reacted with someoxygen to make 36 grams of water then you could figure out how muchoxygen was used.

Write a word equation Hydrogen + Oxygen Water

Write the masses ofeach substance givenin the question

4 g ? 36g

Use the law of conservation of mass (mass of reactants = mass of products)

4 + ? = 36

Solve to find the missing value ? = 36 - 4? = 32So 32 grams of oxygen were used

Learning Check 3

• In the complete reaction of 22.99 g of sodium with 35.45 g of chlorine, what mass of sodium chloride is formed?

• A 10.0 g sample of magnesium reacts with oxygen to form 16.6 g of magnesium oxide. How many grams of oxygen reacted?

Solution 3

• In the complete reaction of 22.99 g of sodium with 35.45 g of chlorine, what mass of sodium chloride is formed?

58.44 g of Sodium chloride

• A 10.0 g sample of magnesium reacts with oxygen to form 16.6 g of magnesium oxide. How many grams of oxygen reacted?

6.6 g of Oxygen

How do we represent these chemical reactions?

• Chemists use statements called equations.

• Chemical equations do not express numerical equalities as in math.

• Chemical equations show the direction in which the reaction progresses. Instead of an equal sign (=), an arrow is used

(means yield)

Word Equations

• A statement using words to describe a chemical reaction.

Sodium (s) + Chlorine (g) Sodium Chloride (s)

The statement reads sodium and chlorine react to produce or yield

sodium chloride.

• Useful but lacks important information

Chemical Symbols

Skeletal Equation

• Uses chemical formulas instead of words to identify reactants and products.

• Important step in writing a complete equation

Na(s) + Cl(g) NaCl(s)

Na(s) + Cl2(g) NaCl(S)

Learning Check 4

• Write skeleton equations for the following word equations.

hydrogen(g) + bromine(g) hydrogen bromide(g)

carbon(s) + sulfur(s) carbon disulfide(s)

Solution 4

• Write skeleton equations for the following word equations.

hydrogen(g) + bromine(g) hydrogen bromide(g)

H2(g) + Br2(g) HBr(g)

carbon(s) + sulfur(s) carbon disulfide(s)

C(s) + S(s) CS2(s)

Writing a Chemical Equation

Chemical symbols give a “before-and-after” picture of a chemical reaction

Reactants Products

MgO + C CO + Mg

magnesium oxide to form carbon monoxide

reacts with carbon and magnesium

Balancing a Chemical Equation

Balanced means that “conservation of mass” is upheld. Same numbers of each type of atom on each side of the equation

Al + S Al2S3 Not Balanced

2Al + 3S Al2S3 Balanced

Matter Is Conserved

H2 + Cl2 2 HCl

+ +Total atoms = Total atoms

2 H, 2 Cl 2H, 2 Cl

Total Mass = Total Mass

2(1.0) + 2(35.5) 2(36.5)

73.0 g = 73.0 g

Balance Equations with Coefficients

Use a number before the compound/element symbol

to indicate how many of them are needed called a

COEFFICIENT. Coefficients in front of formulas balance

each type of atom.

4NH3 + 5O2 4NO + 6H2O

4 N = 4 N

12 H = 12 H

10 O = 10 O

How to write and balance Chemical Equations

1. Write the skeleton equation: Al + O2 → Al2O3

2. Count Number of atoms for each element on both sides

This is not balanced because the numbers don’t match

3. Multiply coefficients until they match – multiply the entire units

2 Al + O2 → Al2O3

Go to 6 oxygens

Balancing Equations

Al + O22 Al2O3

4 3Al + 2 Al2O3 O2

Multiply each atom by 2

Learning Check 5

Balance each equation.

A. Mg + N2 Mg3N2

B. Al + Cl2 AlCl3

Solution 5

A. 3 Mg + N2 Mg3N2

B. 2 Al + 3 Cl2 2 AlCl3

What not to do when balancing Chemical Equations

• Never change a subscript to balance an equation (You can only change coefficients)– If you change the subscript (formula) you are

describing a different chemical.

– H2O is a different compound than H2O2

• Never put a coefficient in the middle of a formula; they must go only in the front.

2NaCl is okay, but Na2Cl is not.