Chemical Reactions

UNIT 6. MOLE. CHEMICAL REACTIONS. STOICHIOMETRY.
CONTENTS
1. CHEMICAL CHANGES
2. CONSERVATION OF THE MASS. LAVOISIER’S LAW.
3. CONCEPT OF MOLE
4. PROUST’S LAW.
5. PERCENTAGE COMPOSITION.
6. GAY LUSSAC’S LAW. AVOGADRO
7. CHEMICAL REACTIONS. BALANCE.
8. STOICHIOMETRY.
9. FACTORS THAT AFFECT THE VELOCITY OF A REACTION.
10. SOME TYPES OF REACTION.
11.ACTIVITIES
1. Chemical changes
As you remember, we made a classification of the changes in two groups: physical and chemical. These last changes imply a change in the substances. In the beginning we have some compounds and when the process finishes we have some others.
This is very easy to notice because a lot of properties change, for example: colour, state, volume, smell and many othersmore difficult to check at first glance.

2. Conservation of the mass. Lavoisier’s Law
Lavoisier is considered to be the first chemist in the history. He stated the law of conservation of the mass: “the mass of a closed system (in the sense of a completely isolated system) will remain constant.
This law is not quite easy to check. Imagine for instance, a piece of paper burning. Is it the massthe same before and after the combustion? Of course not, but you have to take into account the gasses released in the reaction. If you do the reaction in a closed container (with oxygen enough to react) the mass will be the same.
If you put a spoon of bicarbonate in a glass and pour some vinegar inside it a reaction takes place. In this case CO2 is releasing during the reaction. So, the massdecreases, but if we put a balloon in the rim we can check that mass doesn`t change.
3. Concept of mole
If we need to do any calculation in chemistry we have to weight the compounds, as you can imagine once you have studied the previous section. But what is the unit for mass when we talk about atoms and molecules. Remember that an atom is an extremely tiny particle and it is impossible to detectwith a balance. So, we must to take a very large amount of atoms to be detectable, but how many?
Well, the answer is a mole. Mole is 6,02.1023 entities of a substance, like a dozen are 12 or a pair are two. But why have chemist chosen such a big number? Because we have to take many, many atoms to be able to weight.
Thus, we can say a mole of electrons, a mole of molecules, a mole of atoms, andso on.
Obviously the weight of a mole is completely different for every substance. A mole of Na Cl weighs 58,5 g while a mole of sulphuric acid (H2SO4) weighs 98 g.
But, how can we calculate these weights? If you have a look at the periodic table, you will see two numbers above each element. On the left, the number 11 represents the position on the table, the numbers of electrons and protons.On the right, the number 22,9898 represents the atomic weight or the weight of a mole of Na.
In the same way we can calculate the molecular weight. Molecular weights of chemical compounds are equal to the sums of the atomic weights of all the atoms in one molecule of that compound. If we have a chemical compound like NaCl, molecular weight will be.
Mas of Na + Mas of Cl = Mass of NaCl

23 +35.5 = 58,5 g

for HNO3 molecular weight will be

Mass of H + Mass of N + Mass of O * 3 = Mass of HNO3

1 + 14 + 16*3 = 63 g
4. Proust’s law
The law of definite proportions and also the elements states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samplesof a given chemical compound have the same elemental composition.
That is a very important question, because we can be sure that a compound, it does not mean where it come from or how it was obtained, is made up for the same elements and in the same percentage.
If you take a sample of water you will always find the same composition, that is, eight grams of oxygen for one gram of hydrogen...