Chemical Properties

Syllabus Statements 3.3 Chemical properties 3.3.1 Discuss the similarities and differences in the chemical properties of elements in the same group. 3.3.2 Discuss the changes in nature, from ionic to covalent and from basic to acidic, of the oxides across period 3.

The IB expect you to be able to answer questions about: Group 0 – the noble gases Group I – the alkali metals Group VII – the halogens

Group 0 – the noble gases There’s not much to say about these! They are colourless, monatomic gases They are unreactive This is because they have a full outer shell of electrons. They cannot gain or lose electrons, so they don’t (usually) react http://en.wikipedia.org/wiki/Noble_gas_compound

Group I – the alkali metals Lithium Sodium Potassium Rubidium Caesium (US spelling Cesium) Francium (We don’t usually bother to include Francium because its so rare that only about 17 atoms of it exist) Oh yeah – someone always asks: Caesium is about $100 per gram – but its sale is carefully controlled!

There are some things that all the alkali metals have in common: Physical Properties: All good conductors of electricity All have low densities (they float on water) All have fairly low melting points All give shiny grey surfaces when freshly cut All soft enough to cut with a knife

Chemical Properties: They are all very reactive (stored under oil to stop them reacting with air) All form ionic compounds with non metals All form ions with a single positive charge

Some properties of alkali metals Symbol Ionization Energy/MJ mol–1 Density/g cm–3 Electro-negativity MeltingPoint (in °C) First Second Li 0.526 7.305 0.534 1.0 179 Na 0.502 4.569 0.97 0.9 98 K 0.425 3.058 0.86 0.8 64 Rb 0.409 2.638 1.52 39 Cs 0.382 2.430 1.87 0.7 28

Reaction with water The most famous reaction of alkali metals is with water. What are the similarities between the reactions? The metals float on water The products are hydrogen and the metal hydroxide (hence “alkali metals” )

Write the equation for potassium reacting with water Write the equation for potassium reacting with water. Include the state symbols. 2K(s) + 2H2O(l)  2KOH(aq) + H2(g) Note that because KOH is an ionic substance it splits up in water to give K+ and OH- It is the OH- that makes the solution an alkali

What are the differences? The main difference is in how vigorous the reaction is. Lithium reacts slowly. It produces hydrogen, but doesn’t get hot enough to melt. Sodium reacts more strongly. It produces hydrogen and gets hot enough to melt the sodium in to a small ball which moves around on the surface of the water. It may burn with a yellow/orange flame. Potassium reacts even more vigorously. The heat produced is enough to ignite the hydrogen, and a characteristic lilac coloured flame is seen.

For the alkali metals, reactivity increases as you go down the group. This is because larger atoms can lose electrons more easily. We talked about that before!

Question: State 2 observations you could make during the reaction between sodium and water. If you said that you would see hydrogen given off, kick yourself! You would see a gas given off – just observing it wouldn’t tell you it was hydrogen.