Chemistry 11 - Weebly

[Pages:43]Chemistry 11

1

Unit 5 ? The Mole Concept

2 1. Atomic mass and Avodagro's hypothesis

It has been studied that during a chemical reaction, atoms that make up the starting material rearrange to form new and different molecules which are called compounds.

There is a question: how do we know how many atoms/particles of each type of starting materials will be involved in a chemical reaction?

3

John Dalton was the first who tried to determine how much of one

element could combine with a given

amount of another element.

He made an assumption: all elements react to form compound in 1:1 ratio.

Based on this assumption, he assigned an arbitrary masses to each element. He gave the value of 1 to hydrogen as it is the lightest. Other elements were given the arbitrary masses with respect to hydrogen.

English chemist John Dalton FRS

(1766 ? 1844)

4 Example: The reaction between 2.74 g of hydrogen gas and 97.26 g of chlorine gas makes 100 g of hydrogen chloride. What is the mass of chlorine?

Solution: Assuming hydrogen chloride contains one atom each of hydrogen and chlorine, the chlorine atom is therefore 97.26 2.74 = 35.5

times heavier than the hydrogen.

Since hydrogen is assigned a mass of "1", chlorine has

a mass of "35.5"

5 By investigating some simple compounds between elements, Dalton developed a list of relative masses for some elements, in which all mass values were determined assuming that hydrogen has the mass of "1".

This mass scaling worked for some elements only. The source of error is the assumption that compounds contain one atom of each element. Obviously, it is not correct! (We can find many counter examples from Chapter 4!)

6 Joseph Gay-Lussac, on the other hand, explored the reactions between gases at same temperature and pressure.

He observed that many gases combined in simple whole number ratios.

Example: 1L of hydrogen gas reacts with 1L of chlorine gas to produce 2L of hydrogen chloride gas.

This leads to the Gay-Lussac law of combining volume.

French chemist Joseph Gay-Lussac

(1778 ? 1850)

7 Observations by Gay-Lussac seemed to be unrelated to masses of elements.

However, Amedeo Avogadro interpreted his observations from a different angle, and proposed the following hypothesis to explain the whole number ratios.

Avogadro hypothesis: Equal volumes of different gases, at the same temperature and pressure, contain the same number of particles.

Italian chemist Amedeo Avogadro

(1776 ? 1856)

8 That means if 1L of gas A reacts with 1L of gas B, then the compound formed will have the formula AB. Similarly, if 2L of gas A reacts with 1L of gas B, then the molecule formed will have the formula of A2B.

Despite being neglected by his contemporaries for many years, Avogadro's hypothesis was finally proven correct, and it is known nowadays as the Avogadro's law!

Mathematically it looks like: =

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download