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Lesson 4: The alkali metals
What you need to know:
3. How the alkali metals react in water and with group 7 elements. The ‘alkali metals’ is the general name for any element that is found in group 1.
They are called ‘alkali’ metals because when they react with water they form
alkaline solutions. You may have seen demonstrations of adding lithium (Li),
sodium (Na) and potassium (K) to water at school and have seen that the
reactions are very violent. Rubidium (Rb) and caesium (Cs) are so reactive it is
very hard to find them naturally and francium (Fr) is radioactive.
In your exam you will need to recognise the chemical symbols for lithium, Unlike other metals, the alkali metals are soft and can be cut very easily.
They also have very low melting points and are not very dense (not very
heavy). If the alkali metals are left out in the air then they will look grey and
dull, but if you cut a chunk off, you will see the shiny metal underneath. This
shiny surface quickly changes as the alkali metal reacts with air, forming an
oxide. As we move down group 1, the boiling and melting points get lower.
In your exam you may be asked to describe the properties of the alkali metals. The alkali metals react violently with water, and you may have seen this at school. If you drop some lithium into a bowl of water you will see the lump of lithium fizzing around and it will slowly dissapear. The lithium is reacting with the water to form a new chemical called lithium hydroxide and hydrogen: Lithium + water lithium hydroxide + hydrogen
Writing this as a balanced equation we would get: 2Li + 2H2O 2LiOH + H2
Sodium also reacts with water but the reaction is more violent. Drop a small lump of sodium into a bowl of water and the water will fizz around the surface of the water, sometimes giving off sparks, which can make the hydrogen catch fire. This means that you can see a small ball of fire floating around on the water. The sodium is reacting with the water to form a new chemical called sodium hydroxide and hydrogen: sodium + water sodium hydroxide + hydrogen
Writing this as a balanced equation we would get: 2Na + 2H2O 2NaOH + H2
You can see that both lithium and sodium react with water in the same way,
producing a hydroxide and hydrogen gas. In fact all the alkali metals react
this way
and if we were to add a lump of potassium to a bowl of water, then we
get potassium hydroxide and hydrogen gas. The reaction would also be even
more violent and the hydrogen would definitely catch fire and bits of the metal
might even be thrown out of the bowl.
We can write the reaction of any alkali metal with water simply as: 2X + 2H2O 2XOH + H2
Where X is whatever alkali metal you have placed in the water. In your exam you may need to say in words and write out an equation, showing what happens when an alkali metal reacts with water. As we move down the group from lithium to francium the alkali metals
become more reactive
and so react more violently with water and other
chemical and elements. The reason why the alkali metals get more violent
as we move down the group is because of the location of the outer most
electron
. All group 1 metals have 1 electron in their outer shell, but as the
number of electron shells increase, the distance of the outer most electron gets
further from the nucleus. This means that it is easier for the outer electron to
escape and so the element will be more reactive. This means that the most
reactive group 1 element will be francium (Fr).
In your exam you may need to state how the reactivity of the group 1 metals changes as you go down the group. You may also be asked to explain why the group 1 metals become more reactive as you move down the group. The alkali metals also react very violently with the group 7 elements. This is because the group 7 elements are desperate to gain 1 electron (to gain a full outer shell) and the group 1 elements are desperate to lose 1 electron (to gain a full outer shell). This means that they are very happy to react together. If we react sodium and chlorine together we would get sodium chloride, which is also known as common table salt. sodium + chlorine Sodium chloride
And writing this as a balanced equation we get: 2Na + Cl2 2NaCl k
All the group 1 metals react with chlorine (or any of the group 7 elements) in the same way. We can write the reaction of any alkali metal with chlorine simply as: 2X + Cl2 2XCl [p
Where X is whatever alkali metal you want to react with the chlorine gas. In your exam you may need to say in words and write out an equation, showing what happens when an alkali metal reacts with chlorine (or any group 7 Alkali metals can also react with oxygen, which is why they tarnish when they are cut, as the metal will react with the oxygen in the atmosphere. So if we react sodium and oxygen together we would get sodium oxide: sodium + oxygen sodium oxide o
And writing this as a balanced equation we get: 4Na + O2 2Na2O
All the group 1 metals react with oxygen in the same way. We can write the reaction of any alkali metal with oxygen simply as: 4X + O2 2X2O
Where X is whatever alkali metal you want to react with the oxygen gas. In your exam you may need to say in words and write out an equation, showing what happens when an alkali metal reacts with oxygen. It’s also important to know that a more reactive alkali metal will displace a less reactive alkali metal. So for example, if you had a solution of lithium hydroxide and added some sodium, then the sodium would displace the lithium and you would get sodium hydroxide and lithium. sodium + lithium hydroxide sodium hydroxide + lithium
Na + LiOH NaOH + Li
This happens with all alakli metals, a more rective alkali metal will always displace a less reactive alkali metal in a compound. This means that francium will displace all other group 1 metals. Alkali metal
Replaces
ceasium, rubidium, potassium, sodium and lithium. In your exam you may need to know that more reactive alkali metals will 1. The group 1 elements are called the alkali metals. 2. The alkali metals are soft and can be cut easily. As we move down the group, the boiling and melting points of the elements become lower. 3. As we move down the group, the alkali metals become more reactive. This means lithium is the least reactive and francium is the most reactive. 4. The alkali metals react with water to produce a hydroxide and hydrogen. 5. The alkali metals react with chlorine to produce an alkali metal chloride, e.g. sodium 6. The alkali metals react with oxygen to form an alkali metal oxide, e.g. sodium oxide. 7. A more reactive alkali metal will displace a less reactive alkali metal. Further information:
1. This is a video clip of lithium reacting with water: 2. This is a video clip of sodium reacting with water. This is a very good clip: 3. This is a video clip of potassium reacting with water: 4. This is a video showing the reactions of rubidium and caesium reacting with water. As you can see from these videos, the reactions of the alkali metals with water get more violent as we move down the group.

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