Notes – Chemical Equations

Chapter 8, Lesson 2

Conservation of Mass• When the end of the last period bell rang,

everyone moved to a new room, and ended up in different groups in different rooms. The number of students IN the whole school did not change, only their arrangement.

• The amount of matter in a chemical reaction does not change, so the total mass at the start MUST equal the total mass at the end.

Conservation of Mass

• Matter is ALWAYS conserved in chemical reactions.

• Chemical equations show that in chemical reactions, atoms rearrange, but no atoms are gained or lost.

Conservation of Mass

• Antoine Lavoisier invented a balance to make more precise measurements.

• He also determined that the mass before and after a chemical reaction was the same.

Mass of elementsBEFORE

Mass of elementsAFTER

Conservation of Mass

• The law of conservation of mass states that the total mass before a chemical reaction is equal to the total mass after the reaction.

Chemical Equations

• Reactants are the starting materials in a chemical reaction and are placed on the LEFT side.

• Products are the ending materials in a chemical reaction and are placed on the RIGHT side.

Reactant + Reactant → Product

H2 + O2 → H2O2

Chemical Equations

• “Tin + oxygen → tin oxide” reads as “tin plus oxygen produces tin oxide.”

• Word equations can be long and do not show that mass is conserved.

Chemical Equations

• Instead of writing long word equations, scientists use symbols and formulas.– Symbols represent atoms.– Formulas represent molecules.– Molecules may be elements or compounds.

Chemical Equations

• A molecule is the combination of 2 or more atoms.

• Some molecules have atoms that are alike, called diatomic (O2).

• Most molecules are made of more than one type of atom (H2O) – a compound.

Chemical Equations

Chemical Equations

• Chemical equations use the symbols and formulas instead of words.

Balancing a Chemical Equation

• Atoms are neither gained nor lost in a reaction, so both sides of the equation must have the same number of atoms.

Balancing a Chemical Equation

• A subscript tells how many atoms of an element are in one molecule.

• A coefficient tells how many atoms, molecules, or formula units are in a reaction.

2H2O

subscript

coefficient

Balancing a Chemical Equation• (i.e.) “Oxygen reacts with hydrogen to form water.”

H2 + O2 → H2O

Reactants Products

H O H O

2 2 2 1

• This is unbalanced, and we CANNOT change the subscripts!

• We add coefficients, a number in FRONT of a chemical formula that tells you how many molecules or atoms of each reactant & product act in the reaction. We do not write “1.”

• We add a coefficient of “2” in front of the product:

H2 + O2 → 2 H2OReactants Products

H O H O

2 2 4 2

• The oxygen atoms are now balanced, but the hydrogens are not anymore. Add a coefficient of “2” in front of the H2:

2 H2 + O2 → 2 H2O

Reactants Products

H O H O

4 2 4 2

Balancing a Chemical Equation

Writing Balanced EquationsStep 1 Determine the correct symbols and formulas

for reactants and products.

Step 2 Write reactant symbols and formulas to the left of an arrow and product symbols and formulas to the right.

Step 3 Count the number of each kind of atom on both sides.

Step 4 Use coefficients to make the number of each kind of atom the same on both sides of the arrow.

Step 5 Check to see that each kind of atom balances.

Writing Balanced Equations

• Reaction of methane:

CH4 + O2 → CO2 + H2O balance hydrogen

CH4 + O2 → CO2 + 2H2O balance oxygen

CH4 + 2O2 → CO2 + 2H2O balanced equation

Writing Balanced Equations

• Baking soda and vinegar:

NaHCO3 + HC2H3O2 → CO2 + H2O + NaC2H3O2

Writing Balanced Equations

• When counting atoms inside parentheses, the subscript multiplies all atoms inside.

Ca + H2O → Ca(OH)2 + H2

Ca + 2H2O → Ca(OH)2 + H2

Which is a diatomic molecule?

A O2

B H2O

C CH4

D N

8.2 Chemical Equations

What is potassium bromide (KBr)?

A an element

B a mixture

C a compound

D a diatomic molecule

8.2 Chemical Equations

In the following equation, which is a reactant?

Ca +2H2O → Ca(OH)2 + H2

A H2

B (OH)2

C Ca

D none of the above

8.2 Chemical Equations

What is the coefficient of H2O?

Ca + 2H2O → Ca(OH)2 + H2

A 1

B 0

C 4

D 2

How many (OH) molecules are in the products?

Ca + 2H2O → Ca(OH)2 + H2

A 1

B 2

C 3

D 4

In a chemical reaction, which law states the products have the same mass as the reactants?

A Law of Conservation of Water

B Law of Conservation of Energy

C Law of Conservation of Mass

D Law of Conservation of Particles

If an equation is balanced, the number of atoms in the products is ____ the number of atoms in the reactants.

A greater than

B less than

C equal to

D none of the above

Symbols represent ____ and ____ represent molecules.

A elements; formulas

B formulas; equations

C molecules; formulas

D elements; equations