Stoichiometry

According to dictionary of chemistry "Stoichiometry" is the numerical relationship of elements and compounds as reactants and products in chemical reactions. So in this article, I would like to present the basic concept of Stoichiometry.

A. Relative atomic mass (A_r)

The relative mass of an atom based on a scale in which a specific carbon atom (carbon-12) is assigned a mass value of 12.

Example

A sample of gallium contains 60% of atoms of ⁶⁹Ga and 40% of atoms of ⁷¹Ga. Calculate the relative atomic mass of this sample of gallium.

B. Relative formula mass (M_r)

Since atoms are so tiny, we use large groups of atoms (moles of atoms) to east our calculation. The formula for a compound contains much information of use to the chemist, including the number of atoms of each element in a formula unit of a compound.

In this article I will not use the term “Relative molecular mass”. The word molecule or the general term formula unit may applied to one unit of H₂O. The word atom or the term formula unit may applied to one unit of uncombined Pt. However, there is no special name for one unit of NaCl. Formula unit  is the best designation. (Some texts refer to “molecule” of NaCl, and especially to “molecular mass” of NaCl, because the calculations done on formula units do not depend on the type of bonding involved. However, the terms molecule and molecular mass should be reserved for substances bonded into molecules).

The term “formula mass” is correctly used for either ionic or molecular substances. Relative formula mass of a compound is the sum of the atomic masses of all atoms in the formula.  

Example :

1. What is the formula mass of CaSO­₄?

2. What is the formula mass of Mg(NO₃)₂

C. The mole

The amount of substance that contains the mass in grams numerically equal to its formula mass in amu (atomic mass unit) contains 6.022 × 10²³ formula units, or one mole of the substance. This is sometimes called the molar mass of the substance. Molar mass is numerically equal to the formula mass of the substance (the atomic mass for atoms of elements) and has the units grams/mole.

1. How many moles of substance are there in 111 grams of F₂?

2. Calculate the mass of 2.50 mol NaClO₄.

Changes of State

The physical state of a substance depends on energy being absorbed and released by the substance. Changes of states are reversible. This process can be explained by using the kinetic theory of particles proposed by Robert Hooke (1635-1703).

Two diagrams below will show the changes of states of a substance X.

Explanation:
A-B : Solid                                              C-D : Liquid                                      D-F : Gas
    B-C : Mixture of solid and liquid             D-E : Mixture of liquid and gas

In point A, the temperature of solid X increases, heat energy is absorbed by the particles in the solid. The heat energy is converted into kinetic energy. The particles start to vibrate faster about their fixed position.

In point B, when the temperature is high enough, the vibration of the particles become sufficient to overcome the attractive forces between them (the melting point of solid X). The particles begin to break away from their fixed positions.  Solid X begins to melt and change to liquid. During the melting process, the temperature of substance X does not rise even though heating continues. The temperature remains constant because all heat energy taken in by the particles is used to overcome the attractive forces holding the particles together.

In point C, all the solid X has melted. The particles are no longer in their fixed positions. The particles slide over one another. The substance is now a liquid. The temperature rises as heating continues. Energy is absorbed by particles in the liquid. The particles gain kinetic energy and start to move faster as the temperature rises.

In point D,  the temperature increases until it reaches 70^oC. This is the boiling point of substance X. The liquid have enough energy to overcome the forces holding them together.  The temperature remains constant during this boiling process until all the substance X has changed into vapour.

In point E, The particles now spread far apart and move rapidly in all direction. The substance is now a gas.

    Explanation:
    G-H : Gas                                            I-J : Liquid                                      K-L : Solid
    H-I : Mixture of gas and liquid            J-K : Mixture of liquid and solid

In point H, When a gas is cooled sufficiently, it changes into a liquid. This process is called condensation. As the temperature drops, the gas particles lose energy and move more slowly. eventually, the movement of the particles becomes slow enough for the gas to change into a liquid. During the condensation process, the temperature of substance X remains constant even though cooling continues. This is because heat energy is released as the particles are attached to each other to form a liquid. The heat energy is completely given out to the surrounding.

In point I, all substance X has condensed. The temperature of liquid X continues to drop as cooling continues until it reach 30^oC

In point J, liquid X starts to freeze and change into a solid. During the freezing process, the temperature of substance X remains constant even though cooling continues. This is because heat energy is released as the particles are attached to each other to form a solid. The heat energy is completely given out to the surrounding.

In point K,  all substance X has solidified. The temperature of solid X  continues to drops as it is allowed to cool.


Reference:

Toon, T Y., Kwong, C L., Sadler, J., & Clare, E., 2007, G. C. E. 'O' Level Chemistry Matters, Singapore, Marshall Cavendish International (Singapore) Private Limited. p. 6-13

Basic Concepts (All About Matter)

Chemistry is the study of matter and energy and the interaction between them ¹. What is matter? the word "matter" is used to cover all substances and materials from which the physical universe is composed ². So basically matter is anything that has mass and occupies space. Any material object, no matter how large and small, is composed of matter. Matter can exist as a solid, a liquid or a gas. These three forms of matter are called the state of matter. But then if I raise a question regarding this concept such as " are light, heat and sound included as matter?". The answer is NO. They are forms of Energy. Energy is the ability to produce change ¹. So we can conclude that whenever the matter changes from one state to another, energy is involved.

Every substance has certain characteristics that distinguish it from other substance. Those characteristics that serve to distinguish and identify a specimen of matter are called the properties of a substance. Physical properties are the properties related to the appearance of the substance. The example of these physical properties are density, color, melting point, boiling point, freezing point, hardness and state at room temperature. Chemical properties are the properties related to the chemical reaction (how it reacts). The example of these chemical properties are flammability, rust resistance, reactivity, etc ¹.

The theory used to explain the states of matter is the kinetic theory of particles. In order to use this theory properly you should imagine that all substances are made up of tiny particles.  To help you to do it, let me give you a simple picture of it.

The main points of this theory are:

1. All matter is made up of tiny particles, that cannot be seen through naked eye. Different substances have different size of particles
2. The particles move all the time. The higher the energy, the faster they move on average
3. The heavier the particles, the slower they move on average ².

By using this theory we can explain the states of matter (its behaviors and properties). The summary can be seen in a table below.

	Solid	Liquid	Gas
Arrangement of Particles	They are close together	They are close together	They are far apart
	They are in a regular pattern called a crystal lattice*	They are in a random pattern	They are in a random pattern
	Strong attractive forces	Strong attractive forces (aren't as strong as they are in a solid)	Weak attractive forces
Movement of Particles	They vibrate in their fix position	They move around each other	They move quickly in all directions
Properties	A Solid has a definite shape and volume	A Liquid has a fixed volume and take up space of its container	It takes up space of its container
	Slightly increase in size when heated	Expand when heated and contract when cooled.**	The volume of gas are affected quite markedly by changes in temperature.
	Slightly decrease in size when cooled	They can't be compressed***	They can flow
	They cannot be compressed or squashed	They can flow
	They cannot flow

*      Organized structure of repeating patterns ³.

**    Water will contract when heated from zero degree to four degree Celsius but expands above four degree Celsius. It will expand when frozen due to the strange shape of water (the presence of hydrogen bonds)⁴.

 ***  For advance physical chemistry liquid still can be compressed. Its compressibility constant (k) is very small. The liquid has a very low compressibility simply because there is very little empty space⁵.

Reference:

1 Goldberg, D E., 2005, Beginning Chemistry 3rd Edition, United States of America, McGraw-Hill Companies, Inc. p.1.

2 Earl, B and Wilford, L D R., IGCSE Chemistry, Dubai, Hodder Murray. p.1.
3 Moore, J T., 2010, Chemistry Essentials for Dummies, Canada, Wiley Publishing, Inc. p.7.
4 Why does water expand when it freezes?, (2009, October 11), Retrieved from http://www.thenakedscientists.com/HTML/questions/question/2435/
5 Castellan, G W., 1983, Physical Chemistry 3rd Edition, United States of America, Addison-Wesley Publishing Company, Inc. p. 87.