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) • Carry out rxn in open flask: Does NOT demonstrate LCM CO2(g) escapes • Collect CO2(g) in a balloon over the flask Does NOT demonstrate LCM Loss of mass = mass of room air “pushed” out of the way by expanding balloon . . .

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 Note LCM Proposed by AntioneLaviosier, the father of modern chemistry He 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 scientific papers into French; she made drawings of his experiments • lost his head in the French Revolution NOTE: He married at the age of 28; Anne-Marie was 13.

Writing Chemical Equations Here’s one way of doing things: • equations • equations • - ed chemical equations

Notations for Physical State

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

Need to know: the HomonuclearDiatomics 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,skeletonand 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)  1CO2(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 + water C4H10 + 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 Check Practice Problems Review Questions Also do the worksheet on Balancing chemical eqns.

Conservation of Mass and Chemical Equations