Download - Conservation of Mass and Chemical Equations
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.