What is meant when a substance in a reaction is in excess, like Mg in excess in an HCl and Mg reaction?

Answer 1

When a reactant is in excess it means that there is more than enough of the substance to completely react the other reactants….it is the other reactant which limits the yield of the reaction.

For example, if Magnesium is in excess in a reaction between Mg and Hydrochloric acid as shown below,
Mg + 2HCl --> H2 + MgCl2
According to the stoichiometry of the reaction, for every 2 moles of HCl present, 1 mole of Mg is needed. If Mg is in excess, there is more than 1 mole of Mg available to react for every 2 moles of HCl. HCl is the limiting reagent in this case.

Answer 2

the HCL is the limiting reactant...in the reaction it will be used up while there will still remain some of the Mg-this means that there's more than enough of the Mg, while the HCl being used up will stop the reaction...that's why the Mg is "in excess"

Answer 3

It means that the limiting factor is the concentration of HCl. And the number of moles of product will not be restricted by the Mg, since there is more than required for the reaction.

You would write it as

Mg + 2HCl -----> MgCl2 + H2

The H2 would be lost as a gas.

Answer 4

It means that there's more than is needed. In an equation, there will be a limiting substance (unless you're calculated things exactly and gotten the proper amounts) which says only so much of a product can be used. If only so much of a product can be used, there can only be som much of the other reactant used to make it.

Here's an example:
A+B yields C
It takes all your A to make 5 grams of C..
In order to make 5 grams of C you need 7 grams of B
If you have 10 grams of B you have 3 excess grams, becuase only 7 will be used.
Since there is left over/extra B that is the excess substance.

I hope that Helps.

Answer 5

This means that the amount of Mg does not matter when determining the amount of products you receive. The limiting reagent determines it. You basically have unlimited Mg but only a certain amount of the limiting reagent.

Answer 6

It means there are more moles of it present than whatever it is reacting with. In other words, its not the limiting reactant.