Describe what happens in a single replacement reaction. what can replace what?

The one element is substituted in place of other element in compound.

Explanation:

Single replacement:

It is the reaction in which one elements replace the other element in compound.

AB + C → AC + B

For example:

Consider the reaction between zinc and silver nitrate. In this chemical reaction the zinc is substituted in place of silver in silver nitrate compound.

Chemical equation:

2AgNO₃ + Zn → ZnNO₃ + 2Ag

The starting material consist of compound ( AgNO₃) and pure element (Zn).

After reaction a new compound ( ZnNO₃) and pure element (Ag) is generated.

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