Chapter 11 - The alkali metals (group 1 - lithium, sodium and potassium) and Chapter 12 - halogens (group 7 - chlorine, bromine and iodine)

What are the similarities in the reactions of lithium, sodium and potassium with water provide evidence for their recognition as a family of elements?

Group 1 metals such as potassium, sodium and lithium, react with water to produce a metal hydroxide and hydrogen. For example:

          lithium   +   water   →   lithium hydroxide   +   hydrogen

          2Li (s)   +   2H₂O (l)   →   2LiOH (aq)   +   H₂ (g)

The observations for the reaction of water with either potassium or sodium or lithium have the following similarities:

1. fizzing (hydrogen is produced)

2. metal floats and moves around on the water

3. metal disappears

In each case a metal hydroxide solution is produced.

These similarities in the reactions provide evidence that the 3 metals are in the same group of the Periodic Table (i.e. have the same number of electrons in their outer shell).

What are the differences between the reactions of lithium, sodium and potassium with air and water provide evidence and how they provide evidence for the tend in reactivity in group 1

Lithium is the first element in group 1 of the Periodic Table. The observations for the reaction of lithium and water are:

1. fizzing (hydrogen gas is released)

2. lithium floats and moves around on the water

3. lithium disappears

Sodium is the second alkali metal in the group. The reaction of sodium and water is more vigorous than lithium’s:

1. fizzing (hydrogen gas is released)

2. sodium floats and moves around on the water

3. sodium melts into a silver-coloured ball

4. sodium disappears

Potassium is the third alkali metal in the group. The reaction of potassium and water is more vigorous than sodium’s:

1. fizzing (hydrogen gas is released)

2. potassium floats and moves around on the water

3. catches fire with a LILAC flame

4. potassium disappears

When the group 1 metals react with air they oxidise, showing a similar trend in reactivity as we go down the group of the Periodic Table.

Therefore, as we go down group 1 (increasing atomic number), the elements become more reactive: Li<Na<K<Rb<Cs<Fr

use knowledge of trends in group 1 to predict the properties of other alkali metals

From the data in the table, it is possible to deduce the properties of francium from the trends in the other group 1 metals.

For example, we can predict that francium will have a melting point around 20⁰C and a density of just over 2g/cm³.

We can also predict that francium will react violently with water, producing francium hydroxide and hydrogen.

Alkali metal	Melting point (⁰C)	Density (g/cm³)	Reaction with water	Products
lithium (Li)	181	0.53	fizzing	lithium hydroxide + hydrogen
sodium (Na)	98	0.97	rapid fizzing	sodium hydroxide + hydrogen
potassium (K)	63	0.86	vigorous fizzing and lilac flame	potassium hydroxide + hydrogen
rubidium (Rb)	39	1.53	?	rubidium hydroxide + hydrogen
caesium(Cs)	29	1.88	?	caesium hydroxide + hydrogen
francium (Fr)	?	?	?	?

What are colours, physical states and trends in physical properties of Cl, Br and I

Element	Colour	State at room temp
Chlorine (Cl2)	Green	Gas
Bromine (Br2)	Red-brown	Liquid
Iodine (l2)	Grey	Solid

what would you predict for the properties of the other halogens? (fluorine and astatine)

If you look at the trends in the physical properties of the halogens, Cl₂, Br₂, I₂ you can make predictions about the properties of the other halogens.

Element	Colour	State at room temp
Fluorine (F2)	Yellow	Gas
Astatine (At2)	Black	Solid

How do displacement reactions involving halogens and halides provide evidence for the trend in reactivity in group 7

Group 7 elements are called the Halogens. As you go up group 7 (decreasing atomic number), the elements become more reactive. For example, fluorine is the most reactive and astatine is the least reactive.

 

A more reactive halogen will displace a less reactive halogen, e.g. chlorine will displace bromine:

By reacting a halogen solution with a potassium halide solution and making observations, the order of their reactivity can be deduced:

	Potassium chloride, KCl(aq)	Potassium bromide, KBr(aq)	Potassium iodide, KI(aq)
Chlorine, Cl2(aq)	No change	Colourless to orange	Colourless to brown
Bromine, Br2(aq)	No change	No change	Colourless to brown
Iodine, I2(aq)	No change	No change	No change

From the above results, chlorine displaces both bromine and iodine, and bromine displaces iodine. Therefore the order of reactivity is: chlorine is more reactive than bromine, which in turn is more reactive than iodine.