[Ahmed Abdullah]

Strontium(Sr) and barium (Ba) both are group ii metal and lie below Calcium. As for metal we know that reactivity increases as we go down in the same group but calcium is placed higher in the reactivity series as in http://en.wikipedia.org/wiki/Reactivity_...
WHY?

[AWK]

Reactivity is one of most unprecision terms.
Compare electrochemical series
http://en.wikipedia.org/wiki/Table_of_standard_electrode_potentials

[Ahmed Abdullah]

What do you mean by "unprecision terms"? A bit explanation please...

[AWK]

What do you mean by "unprecision terms"? A bit explanation please...

Chemical reactivity is much much more complex problem, eg;
you have two samples of common steel and stainless steel. Which steel is more reactive in your opinion? Then put both samples to bleach. Which sample is more reactive - stainless steel dissolves much faster.

Take sample of silver and put it into diluted hydroiodic acid. Can you see any reaction? Change acid to concentrated hydroiodic acid. You can see bubles of hydrogen though Table_of_standard_electrode_potentials says this is impossible.

Take samples of such metals as aluminium and silver. Which metal is more reactive in your opinion? Put both samples to concentrated HNO3. Which metal will dissolve? - silver!

[Mitch]

More reactive with what? Be specific. Saying "Most reactive" is so general to render it a completely meaningless question

[djay]

enthalpy?

Do a research on the internet if you don't know what it is but you should it since it THE fundamental concept of chemistry.