In a chemical reaction, why can't you add the masses of two reactants to determine the total mass of product?

Question

In a chemical reaction, why can't you add the masses of two reactants to determine the total mass of product?

Answer 1

Because before the reaction takes place, the chemicals of one mass will mix or cancel-out the other masses chemical body.Hence Chemical reaction, and the properties of this final product will be different.

Answer 2

The Law of Conservation of Energy states that:

For any chemical reaction, the total mass of the reactants equals the total mass of the products.

Hence you CAN add simply find the total mass of the reactants to arrive at the total mass of the products.

However some chemical reactions result in a gas being evolved which may not be visible, so the solid/liquid/aqueous product you are left with will have a smaller mass.

Consider the following equation:

Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g)

The hydrogen gas released will mean that some mass is lost, hence the magnesium chloride formed will weigh less than the total mass of magnesium and hydrochloric acid beforehand.

Answer 3

You could do this with stoichiometric quantities of the reactants. The laws of conservtion of mass will always apply. So if the question was to say:

What is the mass of NaCl produced when 45.980 g of Na react with 70.906g of Cl2? If you check this out you will see that both reactants are in the exact mass ratio according to the equation:

2Na + Cl 2 → 2NaCl

Therefore the mass produced will be 116.886g which is obtained by addingthe reactant masses.

However, if you are asked:

What is the mass of NaCl produced when 37.500g of Na react with 85.75g of Cl2? You cannot add the reactants and get an answer of 123.25g NaCl. In this case the Na is clearly the limiting reactant and you would ahve to calculate the mass of Cl2 that would be consumed in the reaction, and the mass of unreacted Cl2 remaining .It is therefore no possible merely to add thereactants. You have to check each time whether one is limiting.

Answer 4

There might be more than one product, but the masses will equal because of the law of conservation of mass.

Answer 5

it is simply because you are not sure that your reactants follow mass equilibrium. that is, a specific amount of one reactant must react with another specific amount of the other. to react, they must have a proportion with each other.

most of the time, an excess of the other is left since the excess amount is not enough to react with the other.

Answer 6

possible answers (comes down to a semantic argument)

-some mass is lost as energy

-one reactant may not fully react, therefor mass of product = (sum of reactants - reactants left)