Objective  I can write balanced equations describing chemical reactions using the law of conservation of matter

What do all of these have in common?

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  C 3 H 8 + O 2  CO 2 + H 2 O  A balanced equation obeys the law of conservation of mass & shows the relative amounts of the molecules of reactants and products.  C 3 H O 2  3 CO H 2 O

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.” 2H 2 (g) + O 2 (g)  2H 2 O(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 Fe 3+ O 2- Fe(s) + O 2 (g) 

Example #1: Rusting “solid iron reacts with oxygen gas to produce iron(III) oxide (rust)” iron + oxygen  iron(III) oxide Fe 3+ O 2- Fe(s) + O 2 (g)  Fe 2 O 3 (s)

Example #1: Rusting “solid iron reacts with oxygen gas to produce iron(III) oxide (rust)” iron + oxygen  iron(III) oxide Fe 3+ O 2- Fe(s) + O 2 (g)  Fe 2 O 3 (s) 4 Fe(s) + 3O 2 (g)  2Fe 2 O 3 (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. H 2 O 2(l)  H 2 O (l) + O 2(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 H 2 O 2 “Hydrogen peroxide decomposes to form water and oxygen gas.”

Hydrogen Peroxide  water + oxygen H 2 O 2 (aq)  H 2 O(l) + O 2 (g) Example #2: Hydrogen Peroxide H 2 O 2 “Hydrogen peroxide decomposes to form water and oxygen gas.”

Hydrogen Peroxide  water + oxygen H 2 O 2 (aq)  H 2 O(l) + O 2 (g) 2H 2 O 2 (aq)  2H 2 O(l) + O 2 (g) Example #2: Hydrogen Peroxide H 2 O 2 “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 Equations 1. 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” _____ H 2 (g) + _____ O 2 (g)  _____ H 2 O(l) O ________  O_______ H ________H_______

“Hydrogen gas and oxygen gas combine to produce liquid water” H 2(g) + O 2(g)  H 2 O ( l ) 2H 2(g) + O 2(g)  2H 2 O ( l ) Skeleton Equation Balanced Equation

“Silver nitrate reacts with hydrogen sulfide to form silver sulfide and nitric acid – HNO 3.” ____Ag(NO 3 ) + ____H 2 S  ____Ag 2 S + ____H(NO 3 ) Ag______ Ag______ Ag______ Ag______ (NO 3 ) ______  (NO 3 ) ______ (NO 3 ) ______  (NO 3 ) ______ S ______ S ______ S ______ S ______ H ______ H ______ H ______ H ______