Conservation of Mass and Chemical Equations

section 4.3

Law of Conservation of Mass

In a chemical reaction, the atoms that make up the starting material(s) undergo a rearrangement to form the product(s).

Since no atoms are lost of gained in a chemical reaction, mass is conserved.

LCM: In a chemical reaction, the total mass of the reactant(s) = total mass of the product(s)

Demonstration: Law of conservation of Mass (LCM)NaHCO3(s) + HC2H3O2(aq)

NaC2H3O2(aq) + H2O(l) + CO2(g)

1. Carry out rxn in open flask:Does NOT demonstrate LCMCO2(g) escapes2. Collect CO2(g) in a balloon over the flaskDoes NOT demonstrate LCMLoss of mass = mass of room air “pushed” out of

the way by expanding balloon . . .

3. Carry out rxn in a sealed, plastic 2-L pop bottle.

LCM proven—bottle does not expand as the CO2(g) is produced

Moral of the story:Just because you think you have a good

experiment (eg #2 above) doesn’t mean that the experiment is good.

Historical NoteLCM Proposed by Antione Laviosier, the father of

modern chemistryHe also:• devised systematic chemical nomenclature• recognized and named oxygen• helped construct the metric system

• wrote the first extensive list of elements• worked closely with his wife,who translated English scientificpapers into French; she made drawings of his experiments• lost his head in the French RevolutionNOTE: He married at the age of 28;Anne-Marie was 13.

Writing Chemical Equations

Here’s one way of doing things:1. equations

2. equations

3. - ed chemical equations

Notations for Physical State

Physical state Notationsolid (s)liquid (l)gas (g)aqueous (aq)

Aside: English Lessonwhat does homonuclear mean?the same atomswhat does diatomic mean? atoms

what does homonuclear diatomic mean?two of the same atoms—bonded to each other

Need to know: the Homonuclear Diatomics

H2(g) N2(g) O2(g) F2(g)

Cl2(g)

Br2(l)

I2(s)

halogens

Balancing Chemical Equations

As we said:

word eq’n skeleton eq’n

balanced chemical eq’n (include physical states)

eg. Water is formed from the reaction of molecular hydrogen with molecular oxygen.

Write the word, skeleton and balanced eq’ns.hydrogen + oxygen water

H2 + O2 H2O

2 H2(g) + 1 O2(g) 2 H2O(l)(The 1 and 2 are called coefficients.)check your answer—atom count; physical state

eg. Gaseous ammonia (NH3) is prepared by the reaction of molecular nitrogen with molecular hydrogen.

nitrogen + hydrogen ammonia

N2 + H2 NH3

N2(g) + 3 H2(g) 2 NH3(g)

eg. Gaseous methane (CH4) reacts with oxygen to produce gaseous carbon dioxide and water vapour.

methane + oxygen carbon dioxide + water

CH4 + O2 CO2 + H2O

CH4(g) + 2 O2(g) 1 CO2(g) + 2 H2O(g)

eg. Gaseous butane (C4H10), the fuel in a disposable lighter, reacts with oxygen to produce carbon dioxide and water.

butane + oxygen carbon dioxide + waterC4H10 + O2 CO2 + H2O

C4H10 + ____ O2 4 CO2 + 5 H2O 13/2*

--or-- 2 C4H10 + 13 O2 8 CO2 + 10 H2O* fractions okay think: mol

Practice

Complete section 3.1 in the text. Do at least some of the

Learning CheckPractice ProblemsReview QuestionsAlso do the worksheet onBalancing chemical eqns.