For questions 18-21, . in each space, classify the reaction type.
18. 2libr + pb(no3) - pbbr2 + 2lino3
19. fe + 2hcl - fecl2 + h2
20. cao + h2o - ca(oh)2
21. nicl2 - ni + cl2

 is a double displacement reaction

Explanation:

                                                       It is of the form

Both oxidation and reduction occur in this reaction. Fe gets oxidized and H gets reduced.

It is an exothermic combination reaction. Calcium reacts with water to produce calcium hydroxide and heat is released in this process.

It is a decomposition reaction where nickel chloride decomposes to nickel and chlorine.

force required to detach it is given by

charges are taken to be fundamental charge

now we have

so above is the minimum distance at which it will detach