If you don't know about Hess's Law, you probably aren't likely to be making much sense of all this bit of the page anyway. All we are doing is inventing an imaginary route from the start to the end point of the reaction, and using Hess's Law to say that the overall enthalpy change will be exactly the same as we can calculate using this imaginary route. Note: There is no suggestion that the reaction actually happens by this route. Then ionise the metal by supplying its first ionisation energy.Īnd finally, you would get hydration enthalpy released when the gaseous ion comes into contact with water. 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.įirst, you would need to supply atomisation energy to give gaseous atoms of the metal. Overall, what happens to the metal is this: In each case, you start with metal atoms in a solid and end up with metal ions in solution. When these reactions happen, the differences between them lie entirely in what is happening to the metal atoms present. 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. The lithium value agrees almost exactly with a value I found during a web search. 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). Note: Apart from the lithium value, I haven't been able to confirm these figures. They are all fairly similar and, surprisingly, lithium is the metal which releases the most heat during the reaction! You will see that there is no pattern at all in these values. 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. The table gives estimates of the enthalpy change for each of the elements undergoing the reaction: 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. Looking at the enthalpy changes for the reactions The Group 1 metals become more reactive towards water as you go down the Group. Caesium hydroxide and hydrogen are formed Rubidium hydroxide solution and hydrogen are formed.Ĭaesium explodes on contact with water, quite possibly shattering the container. It reacts violently and immediately, with everything spitting out of the container again. Rubidium is denser than water and so sinks. This time the normal hydrogen flame is contaminated by potassium compounds and so is coloured lilac (a faintly bluish pink). Potassium behaves rather like sodium except that the reaction is faster and enough heat is given off to set light to the hydrogen. The colour is due to contamination of the normally blue hydrogen flame with sodium compounds. If the sodium becomes trapped on the side of the container, the hydrogen may catch fire to burn with an orange flame. The sodium moves because it is pushed around by the hydrogen which is given off during the reaction. 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. 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. The reaction generates heat too slowly and lithium's melting point is too high for it to melt (see sodium below). It gradually reacts and disappears, forming a colourless solution of lithium hydroxide. Lithium's density is only about half that of water so it floats on the surface, gently fizzing and giving off hydrogen. 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. This equation applies to any of these metals and water - just replace the X by the symbol you want. In each case, a solution of the metal hydroxide is produced together with hydrogen gas. It uses these reactions to explore the trend in reactivity in Group 1.Īll of these metals react vigorously or even explosively with cold water. This page looks at the reactions of the Group 1 elements - lithium, sodium, potassium, rubidium and caesium - with water. REACTIONS OF THE GROUP 1 ELEMENTS WITH WATER Reactions of the Group 1 elements with water
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