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Rabu, 01 Desember 2010

Chemical Equations

atom.gif (1130 bytes)

A chemical equation represents what happens during a chemical reaction in symbol form. The "reactants" are on the left. The "products" are on the right and an arrow represents the reaction.

For example when Carbon burns in Oxygen it produces Carbon Dioxide. An equation would represent this reaction as:

C + O2 arrow.gif (871 bytes) CO2

The equation would be read as: Carbon plus Oxygen gives Carbon dioxide. (The arrow means "gives".)

If you count the numbers of atoms of each element on each side of the arrow (1 carbon atom and 2 Oxygen atoms on the left and 1 Carbon and 2 Oxygen atoms on the right) you will see that the numbers are equal. This is where the term "equation" comes from.

The number of atoms of each element on the left of the arrow must equal the number of atoms of each element on the right of the arrow.

Consider the situation where Hydrogen gas burns with Oxygen gas to produce water vapour:

H2 + O2 arrow.gif (871 bytes) H2O ... This 'equation' is NOT balanced!

There are 2 atoms of Hydrogen on the left and 2 on the right, but while there are 2 atoms of Oxygen on the left there is only 1 on the right. Something is not correct. Atoms can't just disappear.

You might be tempted to change the formula of water to H2O2, but H2O2 is Hydrogen Peroxide, a very strong bleach. If you drank a bottle of H2O2 it would probably kill you! You can't change the formula of a substance to try to balance the equation. You might also be tempted to write Oxygen as just O, but Oxygen is a di-atomic gas. It exists as two atoms bonded together. It must be written as O2.

NOTE: Keep in mind that the equation is meant to represent what happens when atoms interact. The equation above suggests that one molecule of Hydrogen will react with one molecule of Oxygen to produce one molecule of water. If you actually conducted this experiment you would be dealing with millions of molecules, not just one, or two.

We need to think in terms of many molecules reacting together. For example if we put a "2" in front of the H2 and a "2" in front of the H20 we get:

2H2 + O2 arrow.gif (871 bytes) 2H2O

What we now have is an equation that says two molecules of Hydrogen gas will react with one molecule of Oxygen gas to produce two molecules of water vapour. Is it "balanced"? Are there the same number of atoms of each element on each side of the arrow? ...

On the left On the right Is the equation Balanced?
4 Hydrogens, 2 Oxygens 4 Hydrogens, 2 Oxygens

YES

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When balancing an equation one trick is to focus on just ONE element at a time. Don't try and do it all in your head. Choose an element, balance it and then in the next step worry about how your attempt at balancing has effected other elements.

Consider the reaction between water and Sodium metal to produce Sodium Hydroxide and Hydrogen gas. (This is a VERY violent reaction. The Hydrogen gas usually ignites, resulting in several reactions occurring at the same time..)

Na + H2O arrow.gif (871 bytes) NaOH + H2 ... This 'equation' is NOT balanced!

Steps in Balancing ...

The Na is OK. We will look at the Hydrogen ..
Hydrogen has two atoms on the left and three on the right. Another trick in balancing equations is to eliminate 'odd' numbers.

1. To eliminate the odd number of Hydrogens we put a '2' in front of the NaOH on the right.

Na + H2O arrow.gif (871 bytes) 2NaOH + H2

We now have two hydrogens on the left and four on the right. (Ignore the Sodiums for a while.)

2. To balance the Hydrogens we put a two in front of the H2O on the left.

Na + 2H2O arrow.gif (871 bytes) 2NaOH + H2

We now have four Hydrogens on the left and four on the right. We now need to look at the Sodiums.

There is one Sodium on the left and two on the right.

3. Put a two in front of the Na on the left.

2Na + 2H2O arrow.gif (871 bytes) 2NaOH + H2

On the left On the right Is the equation Balanced?
2 Sodiums, 4 Hydrogens, 2 Oxygens 2 Sodiums, 4 Hydrogens, 2 Oxygens

YES

This means that when Sodium metal reacts with water two atoms of Sodium combine with two molecules of water to produce two molecules of Sodium Hydroxide and one molecule of Hydrogen gas.

IMPORTANT: This section builds upon everything you have done in the previous sections. You will only be able to understand it if you have successfully completed the earlier work.

Things you MUST know:
- Atomic Structure
- Symbols of Elements
- Valencies
- Chemical Formulae

If you don't know these basics, go back and learn them. If you don't successfully learn all the material presented so far, you will NEVER be able to do Chemistry. Put some real effort in and learn the basics now! (For those who have - congratulations!)


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