Science Infoblog » metals

Year 11 chemistry:balancing chemical equations, reactions of metals and acids

john west — Thu, 26 Mar 2009 10:29:34 +0000

Chemistry has its own language with words and sentences.

To a chemist the words are chemical formulae. When chemical formulae are joined together in a sentence we call this a chemical equation.

Hydrogen and oxygen mixtures explode when lit. In a test tube this explosion is heard as a “pop”. We call the test for hydrogen “the pop test”

The space shuttle is launched into space with three main engines. Each engine produces 1.8 Meganewtons of thrust. The engines are powered by liquid hydrogen and oxygen fuel stored in the large central rust coloured tank.

The space shuttle is blasted into space on the back of a mega “pop test” The two tanks on either side of the shuttle are solid fuel boosters.

Hydrogen molecules and oxygen molecules react with each other to form water molecules. A chemist rewrites this sentence as a word equation

hydrogen + oxygen —–> water

Chemists use chemical formulae instead of words. Finally chemists need to count the numbers of atoms involved. We need to have the same numbers of atoms after the reaction that we had at the start.

When we do this we end up with a fully balanced chemical equation. As far as NCEA level 1 is concerned here are the steps.

word equation…..achievement
with chemical formulae……merit
a fully balanced chemical equation…..excellence

Check out the video to see how we get a fully balanced equation for the “pop test”

try balancing the equations that describe metals reacting with acids.

NCEA Level 1 Chemistry: writing chemical formulae:

john west — Wed, 25 Mar 2009 19:04:51 +0000

In ionic compounds the total charge of the positive ions is balanced by the total charge of the negative ions.

write down the formula of the metal ion
write down the formula of the negative ion
if the charge is unbalanced add another ion of the opposite charge.
repeat until charges balance

Watch the video to see how this works.

When you master this skill you are well on your way to becoming a chemistry geek!. Good luck

……………. and final finally……… a mars bar to the first person who can identify the video music.

Year 11 chemistry: reactions of metals with water

john west — Tue, 24 Mar 2009 09:01:05 +0000

…….and the students in the next video are successfully studying sixth form chemistry. If they can do it so can you……..its OK to be a chemistry geek!

Look at the three videos of the reactive metals sodium and calcium when small pieces are dropped into water. Observations allow us to work out what is happening and develop a general word equation for the reactions.

Some stuff for experts:

sodium dissolves in water. A new compound must be formed in the solution.
calcium dissolves in water. Another compound must be formed in solution
What could this compound be? Acidic solutions contain lots of hydrogen ions. Basic solutions contain lots of hydroxide ions.
Phenolphthalein indicator turns from colourless to pink when it is in a basic solution. When phenolphtalein turns pink there are hydroxide ions present.
As the metals dissolve in water hydroxide ions are produced
When sodium dissolves in water sodium hydroxide is formed in solution.
When calcium dissolves in water calcium hydroxide is formed.
A gas is given off which pops when lit
When sodium and calcium dissolve in water hydrogen gas is formed

If you have followed all of that well done, if not don’t worry. For achievement in level 1 chemistry you simply have to remember the word equations and use them.

reactive metal + water —–> metal hydroxide + hydrogen

Sodium + water —–> sodium hydroxide + hydrogen

Calcium + water —–> calcium hydroxide + hydrogen

For merit and excellence you need to be able to write underneath the word equation the chemical formulae and then balance. See the exercises in your workbook.

Leave any questions you want answered by clicking on the response link. Good luck

and finally definitely DO NOT TRY THIS AT HOME

Year 11 Chemistry: Reactions of metals with oxygen

john west — Mon, 23 Mar 2009 10:11:31 +0000

Lithium sodium and potassium tarnish quickly in air when the protective oil coating is wiped of. They are extremely reactive metals. Reactive metals get coated in a layer of metal oxide when they are left exposed to air long enough

Magnesium is another very reactive metal particularlyl when its oxide layer is is scraped off. The flame is difficult to extinguish and the metal has been used as the casing for incendiary shells. ( the casing will explode if you try and put the fire out with water)

…….and finally even iron can burst into flames. The iron has to be in the form of iron wool ( it won’t work with an iron nail for instance….why not?). It has to be heated and will only burn in an atmosphere of pure oxygen. Can you remember how to make oxygen gas and how to test for it?

Finally word equations. Word equations are a kind of chemical shorthand to stop you writing heaps of words. For instance if we were to summarise all of this stuff we might write a sentence like…

Metals tarnish in air. The oxygen in the air will chemically combine with the atoms of the metal to form a new compound The compound formed when a metal reacts with oxygen is called a metal oxide.

And the word equation:

reactive metal + oxygen —–> metal oxide

Being able to write to basic word equations will help you with achievement question.

The next two steps will help you achieve merit and excellence. For instance if we complete the process for magnesium burning in oxygen

1. general word equation: Reactive metal + oxygen —–> metal oxide

2. word equation: magnesium + oxygen —–> magnesium oxide

3. inserting chemical formulae: Mg + O₂ —–> MgO

4. fully balanced equation: 2 Mg + O₂ —–> 2 MgO