The metals react vigorously with water such that even water in the air is enough to ignite sodium and potassium. Do not do this experiment on the overhead. It is not safe. There are often microscopic pits in the metal causing the metal to launch. Potassium is the most reactive, immediately produces purple sparks and flames. Be sure to use small pea-sized pieces of metal. As you go from lithium to caesium, you need to put less energy into the reaction to get a positive ion formed.
This energy will be recovered later on plus quite a lot more! This is going to be related to the activation energy of the reaction. So although lithium releases most heat during the reaction, it does it relatively slowly - it isn't all released in one short, sharp burst. Caesium, on the other hand, has a significantly lower activation energy, and so although it doesn't release quite as much heat overall, it does it extremely quickly - and you get an explosion.
Note: You need to be a bit careful about how you phrase this! You probably haven't noticed my use of the phrase "This is going to be related to the activation energy of the reaction. The reaction certainly won't involve exactly the energy terms we are talking about. The metal won't first convert to gaseous atoms which then lose an electron. But at some point, atoms will have to break away from the metal structure and they will have to lose electrons. However, other energy releasing processes may happen at exactly the same time - for example, if the metal atom loses an electron, something almost certainly picks it up simultaneously.
The electron is never likely to be totally free. That will have the effect of reducing the height of the real activation energy barrier. The values we have calculated by adding up the atomisation and ionisation energies are very big in activation energy terms and the reactions would be extremely slow if they were for real. The reactions become easier as the energy needed to form positive ions falls. This is in part due to a decrease in ionisation energy as you go down the Group, and in part to a fall in atomisation energy reflecting weaker metallic bonds as you go from lithium to caesium.
This leads to lower activation energies, and therefore faster reactions. Note: If you are a UK A level student, you will almost certainly find that your examiners will only expect you to explain this in terms of the fall in ionisation energy as you go down the Group. In other words, they simplify things by overlooking the contribution from atomisation energy.
Stick with what your examiners expect - don't make life difficult for yourself! I'm trying to be as rigorous as I can because a sizeable part of my audience is working in systems outside the UK or beyond A level. If this is the first set of questions you have done, please read the introductory page before you start. The Facts General All of these metals react vigorously or even explosively with cold water. 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.
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.
Potassium Potassium behaves rather like sodium except that the reaction is faster and enough heat is given off to set light to the hydrogen. Rubidium Rubidium is denser than water and so sinks. 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. 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.
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. Then ionise the metal by supplying its first ionisation energy. It fizzes steadily and becomes smaller, until it eventually disappears. When sodium is added to water, the sodium melts to form a ball that moves around on the surface. It fizzes rapidly, and the hydrogen produced may burn with an orange flame before the sodium disappears.
When potassium is added to water, the metal melts and floats. It moves around very quickly on the surface of the water.
The hydrogen ignites instantly.
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