put the group 1 elements caesium lithium potassium rubidium and sodium in their order of reactivity with water. Put the most reactive element first.
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Answer:
REACTIONS OF THE GROUP 1 ELEMENTS WITH WATER
This page looks at the reactions of the Group 1 elements - lithium, sodium, potassium, rubidium and caesium - with water. It uses these reactions to explore the trend in reactivity in Group 1.
The Facts
General
All of these metals react vigorously or even explosively with cold water. In each case, a solution of the metal hydroxide is produced together with hydrogen gas.
This equation applies to any of these metals and water - just replace the X by the symbol you want.
In each of the following descriptions, I am assuming a very small bit of the metal is dropped into water in a fairly large container.
Details for the individual metals
Lithium
Lithium's density is only about half that of water so it floats on the surface, gently fizzing and giving off hydrogen. It gradually reacts and disappears, forming a colourless solution of lithium hydroxide. The reaction generates heat too slowly and lithium's melting point is too high for it to melt (see sodium below).
Sodium
Sodium also floats on the surface, but enough heat is given off to melt the sodium (sodium has a lower melting point than lithium and the reaction produces heat faster) and it melts almost at once to form a small silvery ball that dashes around the surface. A white trail of sodium hydroxide is seen in the water under the sodium, but this soon dissolves to give a colourless solution of sodium hydroxide.
The sodium moves because it is pushed around by the hydrogen which is given off during the reaction. If the sodium becomes trapped on the side of the container, the hydrogen may catch fire to burn with an orange flame. The colour is due to contamination of the normally blue hydrogen flame with sodium compounds.
Potassium
Potassium behaves rather like sodium except that the reaction is faster and enough heat is given off to set light to the hydrogen. This time the normal hydrogen flame is contaminated by potassium compounds and so is coloured lilac (a faintly bluish pink).
Rubidium
Rubidium is denser than water and so sinks. It reacts violently and immediately, with everything spitting out of the container again. Rubidium hydroxide solution and hydrogen are formed.
Caesium
Caesium explodes on contact with water, quite possibly shattering the container. Caesium hydroxide and hydrogen are formed
Summary of the trend in reactivity
The Group 1 metals become more reactive towards water as you go down the Group.
Explaining the trend in reactivity
Looking at the enthalpy changes for the reactions
The overall enthalpy changes
You might think that because the reactions get more dramatic as you go down the Group, the amount of heat given off increases as you go from lithium to caesium. Not so!
The table gives estimates of the enthalpy change for each of the elements undergoing the reaction:
Note: That's the same equation as before, but I have divided it by two to show the enthalpy change per mole of metal reacting.
enthalpy change (kJ / mol)
Li -222
Na -184
K -196
Rb -195
Cs -203
You will see that there is no pattern at all in these values. They are all fairly similar and, surprisingly, lithium is the metal which releases the most heat during the reaction!
Note: Apart from the lithium value, I haven't been able to confirm these figures. For lithium, sodium and potassium, they are calculated values based on information in the Nuffield Advanced Science Book of Data (page 114 of my 1984 edition). The lithium value agrees almost exactly with a value I found during a web search. The values for rubidium and caesium are calculated indirectly from the Li, Na and K values and other information which you will find in a later table on this page.
Digging around in the enthalpy changes
When these reactions happen, the differences between them lie entirely in what is happening to the metal atoms present. In each case, you start with metal atoms in a solid and end up with metal ions in solution.
Overall, what happens to the metal is this:
You can calculate the overall enthalpy change for this process by using Hess's Law and breaking it up into several steps that we know the enthalpy changes for.
First, you would need to supply atomisation energy to give gaseous atoms of the metal.
Then ionise the metal by supplying its first ionisation energy.
And finally, you would get hydration enthalpy released when the gaseous ion comes into contact with water.
If we put values for all these steps into a table, they look like this (all values in kJ / mol):
at. energy 1st IE hydr. enthalpy total
Li +161 +519 -519 +161
Na +109 +494 -406 +197
K +90 +418 -322 +186
Rb +86 +402 -301 +187
Cs +79 +376 -276 +179
Note: Remember that these aren't the overall enthalpy changes for the reactions when the metal reacts with water. They are only for that part of the reaction which involves the metal. There are also changes going on with the water present - turning it into hydrogen gas and hydroxide ions. To get the total enthalpy changes, you would have to add these values in as well.
Elements caesium lithium potassium rubidium and sodium in their order of reactivity with water is sodium < potassium < rubidium < caesium.
- Group 1 elements are known as alkali metals.
- Alkali metals react with water to form hydroxides and hydrogen gas is released.
- The hydroxides of alkali are basic.
- Down the group reactivity of alkali metals like sodium, potassium, rubidium and caesium increases because ionisation energy decreases down the group.
- The reaction of Na + H2O ----> NaOH + 1/2H2.
- The ionisation energy of caesium is the lowest and is the most reactive element. so, it is most reactive toward the water.
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