# Ease of reaction of hydrogen with alkali and alkaline earth metals

The ease of reaction of group 1 metals with hydrogen decreases as we go down the group.

But why lithium requires high temperature than sodium, potassium and rubidium?

What is exactly "ease of reaction"? I think it is related to kinetics (rate) of reaction and not related to thermodynamics of reaction. If so, is there any chemical data table from which I can predict the ease of reaction for group 2 metals with hydrogen?

Also, at what temperature, the ease of reaction should be compared?

The following processes involve in formation of $$\ce{MH(s)}$$ from $$\ce{M(s)}$$ and $$\ce{H2(g)}$$:

$$\ce{M(s)\rightarrow M(g)\rightarrow M+(g)}$$ $$\ce{1/2H2(g)\rightarrow H(g)\rightarrow H-(g)}$$ $$\ce{M+(g) + H-(g)\rightarrow MH(s)}$$

I know that it can be done by measuring and then comparing the formation of products over time, i.e. $$\ce{\frac{\Delta [MH]}{\Delta t}}$$.

References:

1. Page 297, Concise Inorganic Chemistry, 5E, J. D. Lee (Wiley)

2. Page 115, Descriptive Inorganic Chemistry, 3E, House (Elsevier)

3. Page 342, Concise Inorganic Chemistry, 5E, J. D. Lee (Wiley)

• Have you expected the same or lower temperature for lithium ? // Ease: It is rather a linguistic problem. Ease as a verb has 2 listed meanings: 1/ to make or become less severe, difficult, unpleasant, painful, etc. 2/ to move or to make something move slowly and carefully in a particular direction or into a particular position // so in context of comparing temperatures for the reaction, it looks like 2/ ="the hesitation with which they do so decreases from lithium to caesium" Sep 13 '21 at 8:34
• @Poutnik I was thinking, if Li reacts at higher temperature than Na, then at a particular temperature, the rate of reaction for Li should be slow, or the ease of reaction should be low for Li. But both are exothermic reactions, so increase of temperature is not favourable. So, at what temperature, the ease of reaction to be compared? I added a sentence in the question. Also, in the first reference it is given that "the hesitation with which they do so, increases from lithium to caesium." Sep 13 '21 at 12:00
• No, the first reference says "ease(supposed ~= hesitation) decreases from lithium to caesium", what is in agreement with 700 deg C for Li. Lithium has higher ionization energy, therefore also the reaction activation energy, so slower at the same temperature, so higher "ease = hesitation". Sep 13 '21 at 12:04
• @Poutnik I think at first instance, it should take the meaning of "easily" as "which reaction is easier to proceed?" as mentioned by you in 1/ "become less difficult". If it is taking the meaning of "hesitation", then it is very confusing to me, considering English is not my mother-tongue. Sep 13 '21 at 12:48
• I took it from the Oxford dictionary - see the link. 1/ =easily is in contrary to the mentioned chemical fact 700 vs 400 and IE 2/=hesitate is in agreement with it, so I have voted for 2/. ( I am not a native speaker either so US/UK guys may correct us ). Sep 13 '21 at 12:51