A graph of potential energy versus inter-nuclear distance for two chlorine atoms is given below

Explanation:

The sketch of the curve for two Br atoms can be seen in the image attached below. From the image below, we can deduce the following:

On the periodic table, the Bromine (Br) atom is the fourth member in the halogen family group. The atomic size increases from top to bottom down the group, thus Br atom posses a larger atomic size than Cl atom. As a result of that, the bond length formation between the two Br atoms will be larger compared to that of the two Cl atoms.

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2