Alkali metals + Halogens (and displacement reactions)

What are the properties of alkali metals? How do these properties change as you go down the group? State which ones are different or similar to typical metals.

Different from typical metals:

- They are soft (for a metal) and can be cut with a knife. Softness increases going down the group.

- They have low melting and boiling points. These decrease going down the group.

- They have low density, density increases going down the group.

- They are very reactive. Reactivity increases going down the group.

Similar to typical metals:

- They are good conductors of heat and electricity.

- They are shiny when freshly cut.

Francium is radioactive

What happens to alkali metals when in contact with the air?

They react with the oxygen (water vapor, just put oxygen) from the air, forming metal oxides which has a dull, matte grey colour coating.

How can we keep alkali metals from reacting with the air? Why is it important to not touch these metals with bare fingers?

Keeping them in a sealed jar with oil.

There could be enough sweat in skin to give a reaction, producing lots of heat and a ver corrosive metal hydroxide

Heating with a Bunsen burner an alkali metal in the air we get a much more vigorous reaction and it is more difficult to see which metal is most reactive because all the reactions are so rapid

What do we see when ignite lithium, sodium and potassium? What does it form + its colour? Equation?

Lithium burns with a red flame to form lithium oxide

Sodium burns with a yellow flame to form sodium oxide

Potassium burns with a lilac flame to form potassium oxide

The products are all white powder — the alkali metal oxide

Equation for e.g lithium

4Li(s) + O2(g) ~> 2Li2O(s)

Give me the word and chemical equation when lithium reacts with oxygen.

Lithium + oxygen ~> lithium oxide

4Li(s) + O2(g) ~> 2Li2O(s)

How does lithium react with water?

Fizzing (gives off gas)

Floats (less dense than water)/moves on the surface of water

Dissolves/disappears

How does sodium react with water?

Fizzing

Melts to form a ball

Floats and moves on the water surface

Dissapears

How does potassium react with water?

Fizzing

Floats and moves on surface of water

Catches with a lilac flame

Disappears

Why does potassium appear dull just after being taken from its container?

Because it is so reactive that it is difficult to keep it from reacting with the air even in oil.

What is the word/chemical equation for the reaction between alkali metals and water? Example using lithium.

Lithium + water ~> lithium hydroxide + hydrogen

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

If the universal indicator was previously green, after adding the alkali metals into the water what colour would it turn and what would the pH be? Why?

Blue and pH 12-14 because the metal hydroxide is alkaline (solution contains OH- ions)

TRIPLE CONTENT: Explain the trend of group one with reference to electronic configuration

Going down group 1 reactivity increases because valence electron is further away from nucleus.

Force of attraction between positive nucleus and negative electron becomes weaker the further away

because there are more shells and their shielding effect

so it is easier to lose the electron

Halogens: the term halogen means salt producing. When they react with metals these elements produce a wide range of salts

What are the colours, physical states at room temp for halogens fluorine, chlorine, bromine and iodine?

What are the characteristics of halogens? Trends going down the group?

- Toxic/poisonous vapours (so need to use a fume cupboard when handling them). Liquid bromine is also very corrosive.

- Diatomic

Going down the group:

- Boiling and melting point increases

- Reactivity decreases

- Colours get darker

Why do their melting and boiling points increase down the group?

Halogens are all simple molecular structures

As the relative molecular mass increases, the intermolecular forces of attraction becomes stronger, and therefore more energy must be put in to overcome these stronger forces of attraction.

Remember, no covalent bonds are broken when these melt/boil

What are displacement reactions? What should you remember when writing word equation for this type of reaction?

Displacement reaction is a reaction where a more reactive element will take the place of a less reactive element inside a compound.

-ide = compound (potassium chlorIDE)

-ine = by itself (chlorINE)

How can we spot a displacement reaction has occurred?

Colour change, bubbles, formation of a precipitate (ppt)

Observations on displacement reactions involving halogens and halides

TRIPLE CONTENT: Explain why potassium is more reactive than lithium (3 marks)

More shells so valence electron is further away.

Weaker attraction from positive nucleus to electron (bc of shielding effect)

Easier to lose electrons

In terms of electrons, what does reactivity depend on?

- Distance of outer shell from nucleus

- Shielding effect of shells

Size of positive charge in nucleus (more protons = more positive - gets outweighed by other factors)