Three possible cases:
- If amount are equal for each reactant (for example 1 mol each), the limiting is the hydrogen and the excess reagent is the acetylene.
- When moles of Hβ are greater than CβHβ
The acetylene is the limiting reagent so the Hβ is the excess
- Β When moles of CβHβ are greater than Hβ
For this case, Hβ is the limiting reactant and the excess is the CβHβ
Explanation:
First of all we determine the reaction:
Reactants, acetylene and hydrogen
Products are ethane
Then, the balanced reaction is: CβHβ + 2Hβ β CβHβ
1 mol of acetylene reacts with 2 moles of hydrogen ir order to produce 1 mol of ethane.
If amount are equal for each reactant, the limiting is the hydrogen,
For example, 1 mol each
For 1 mol of acetylene I need 2 moles of Hβ. I've only got 1 mol, so I do not have enough Hβ. The excess reagent is the acetylene.
- When moles of Hβ are greater than CβHβ
For example, 3 moles of Hβ and 0.5 mol of CβHβ
2moles of Hβ need 1 mol of CβHβ for the reaction
Then 3 moles of Hβ will need (3 . 1) / 2 = 1.5 moles
We have 0.5 moles, so the acetylene is the limiting reagent, again.
- When moles of CβHβ are greater than Hβ
For example 1 mol of CβHβ and 0.001 moles of Hβ
If I have 1 mol of CβHβ, I definetly need the double of moles of hydrogen, so in this case, Hβ is the limiting reactant and the excess is the CβHβ
If we have 1 mol of Hβ and 0.5 mol of CβHβ, notice that moles of acetylene are lower than hydrogen
1 mol of CβHβ needs 2 moles of Hβ
So 0.5 moles of CβHβ will need 1 mol of Hβ (it's ok because we have 1 mol)
2 moles of Hβ need 1 mol of CβHβ for reaction
Then, 1 mol of Hβ will need 0.5 moles of CβHβ (it's ok because we have that amount)
In this case, there is no excess neither limiting. That's why we can choose any of them to determine the moles (or mass) for the product