10 Science Chapter 6 Notes

Word Equations are used to represent chemical reactions. Their advantage is you do not have to know the formulas of the chemicals involved. All word equations have the form:

Reactant 1 + Reactant 2 Product 1 + Product 2

Example:

Silver Nitrate + Barium Chloride Silver Chloride + Barium Nitrate

Of course there may be a different number of reactants and products. (Practice on p.219)

The Law of Conservation of Mass states that the total mass of all the products in a chemical reaction is equal to the total mass of all the reactants. That this is a law suggests that scientist believe it is true everywhere (in the universe) and always. There are no exceptions. Figure 3 on p. 222 depicts this law at the particle level. Note that every atom that exists before the reaction, exists after the reaction. It is for this reason that we say matter cannot be created or destroyed. In chemical reactions molecules are broken down into atoms and then those atoms form new molecules.

So what are the implications of burning down a massive forest?

Balancing Chemical Equations

Word equations are limited in that they do not convey the proportions of reactants and products. Consider the example above. A word equation doesn't convey that 2 "molecules" of silver nitrate are needed for every one "molecule" of barium chloride. To do so we first write a skeleton equation (one that replaces the names of the chemical compounds with formulas):

AgNO3 + BaCl2 AgCl + Ba(NO3) 2

But there seems to be a violation of the Law of Conservation of Mass. We are missing a chloride ion on the products side of the equation and we have too many nitrate ions on the product side. It would appear that matter has been both created and destroyed. To correct this we must introduce coefficients:

2AgNO3 + BaCl2 2AgCl + Ba(NO3) 2

And now the equation is "balanced"

Try p. 229 # 1 - 4 and Handout called Balancing Equations

Chemical Reactions (I)

One classification of chemical reaction is combustion reactions. A combustion reaction is a very rapid reaction that takes the form:

fuel + oxygen oxides + water + energy

The most common fuels burned are called hydrocarbons (compounds containing hydrogen and carbon also called fossil fuels because of their origins in living matter).

Complete combustion of hydrocarbons produces only water and carbon dioxide

hydrocarbon + oxygen carbon dioxide + water + energy

Incomplete combustion occurs when there is not enough oxygen available. With incomplete combustion, two additional products are formed, carbon (soot) and carbon monoxide.

Carbon monoxide is a clear, colorless, odorless gas that is highly poisonous.

How do matches work? Read p. 231

Problems p. 232 # 3,4 & 6


Chemical Reactions (II)

Another classification of reactions has four categories:

i) synthesis

ii) decomposition

iii) single replacement

iv) double replacement

Synthesis Reactions

Synthesis reactions involve combining smaller atoms and/or molecules into larger molecules. They are also called combination reactions and take the form

A + B AB

A and B do not have to be elements but if they are, it is definitely a synthesis reaction.

i.e. hydrogen + oxygen water

Write a balanced chemical equation for this reaction

Decomposition Reactions

Decomposition reactions involve the splitting of a large molecule into elements or smaller molecules and take the form

AB A + B

If AB is a binary compound and the only reactant, the reaction must be a decomposition reaction.

i.e. water hydrogen + oxygen


Chemical Reactions (III)

Single Displacement Reactions

Single displacement reactions are those where either a metal displaces the cation of a compound

i.e. Mg + 2AgNO3 2Ag + Mg(NO3)2

or where a nonmetal displaces the anion of a compound

i.e. Br2 + CaI2 I2 + CaBr2

in general, A + BC B + AC (A is a metal)

or A + BC C + BA (A is a nonmetal)

Double Displacement Reactions

Double displacement reactions take this form

AB + CD AD + CB

i.e. Pb(NO3) 2 + 2KI 2KNO3 + PbI2

Practice p. 241 # 1 - 3