What happens to the speed when the two small people are placed on one side and the two taller people are placed on the other side? what about when it is one small person on one side versus all four people on the other side?

People of lighter weight are sitting on one side and taller people are sitting on the other side. There will be a difference in the balance of weight in the vehicle, if this continues to remain the same.

Then the speed increases the short people having lesser weight would feel a lesser gravitational pull. while the other one with heavier weight will feel more gravitational pull due to which the vehicle can turn over due to imbalance.  

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