ObjectiveI can write balanced equations describing chemical reactions using the law of conservation of matter

What do all of these have in common?

Rusting

What do all of these have in common?

Rusting

How do you write a chemical reaction?

How do you write a chemical reaction?

Reactants Products

VERY IMPORTANT!

Please note that the atoms themselves are neither created nor destroyed…they are merely rearranged.

VERY IMPORTANT!

Please note that the atoms themselves are neither created nor destroyed…they are merely rearranged.

The Law of Conservation of Mass—mass is neither created nor destroyed.

VERY IMPORTANT!

Please note that the atoms themselves are neither created nor destroyed…they are merely rearranged.

The Law of Conservation of Mass—mass is neither created nor destroyed.

Every atom that appears in the reactants must also appear in the products

Describing Chemical Reactions with Chemical Equations

You can use a word description… Propane gas reacts with oxygen in a Bunsen burner to

produce carbon dioxide and water vapor. Using chemical formulas in an equation is more

efficient A skeleton equation is not balanced

C3H8 + O2 CO2 + H2O A balanced equation obeys the law of

conservation of mass & shows the relative amounts of the molecules of reactants and products. C3H8 + 5 O2 3 CO2 + 4 H2O

You can indicate the physical state of a substance in the equation by putting a symbol after each formula.

(s) —solid(l) —liquid(g) —gas(aq) —aqueous solution = (dissolves in

water)“Hydrogen gas and oxygen gas can

combine to produce liquid water.”2H2(g) + O2(g) 2H2O(l)

Example #1: Rusting“solid iron reacts with oxygen gas to produce

iron(III) oxide (rust)”iron + oxygen iron(III) oxide

Example #1: Rusting“Iron reacts with oxygen to produce iron(III) oxide

(rust)”iron + oxygen iron(III) oxide

Fe3+ O2-

Fe(s) + O2(g) Fe2O3(s)

4 Fe(s) + 3O2(g) 2Fe2O3(s)

Some chemical reactions require the use of a catalyst.

Catalyst—a substance that speeds up the rate of a reaction but that is not used up in the reaction. It is neither a reactant or product.

Catalyst example:

The decomposition of hydrogen peroxide when poured onto a cut is an example of a catalyst at work.

H2O2(l) H2O(l) + O2(g)

The catalyst in this case is iron in blood that speeds up the reaction and causes oxygen gas to be released. This gas is observed as bubbles forming on the wound.

Hydrogen Peroxide water + oxygen

Example #2:Hydrogen Peroxide H2O2

“Hydrogen peroxide decomposes to form water and oxygen gas.”

Hydrogen Peroxide water + oxygen H2O2(aq) H2O(l) + O2(g)

2H2O2(aq) 2H2O(l) + O2(g)

Example #2:Hydrogen Peroxide H2O2

“Hydrogen peroxide decomposes to form water and oxygen gas.”

Balancing Chemical Equations

Balancing a chemical equation is necessary so that it obeys the law of conservation of mass.

In every balanced equation, each side of the equation has the same number of atoms of each element.

Rules for Balancing Equations1. Determine correct chemical formulas for all the

reactants and products in the reaction.2. Put reactants on the left and products on the

right with a yield sign () in between. Use plus signs to separate multiple reactants or products.

3. Count the number of atoms of each element in the reactants and products.--a polyatomic ion appearing unchanged on both sides of the equation counts as a single unit.

4. Balance the elements one at a time by adding coefficients. If no coefficient is written, it is assumed to be 1. --Do not try to change the subscripts!!!

5. Finally, make sure all the coefficients are in the lowest possible ratio that balances.

practice: “Hydrogen gas and oxygen gas combine to produce liquid water”

_____ H2(g) + _____ O2(g) _____ H2O(l)

O ________ O_______

H ________ H_______

“Hydrogen gas and oxygen gas combine to produce liquid water”

H2(g) + O2(g) H2O(l)

2H2(g) + O2(g) 2H2O(l)

Skeleton Equation

BalancedEquation

“Silver nitrate reacts with hydrogen sulfide to form silver sulfide and nitric acid – HNO3.”

____Ag(NO3) + ____H2S ____Ag2S + ____H(NO3)

Ag______ Ag______

(NO3) ______ (NO3) ______

S ______ S ______

H ______ H ______

Desk Work• Finish the desk practice balancing problems & get them checked