6: Chemical Reactions - Mole and Mass Relationships - Chemistry LibreTexts
It is the quantitative relation between the number of moles (and therefore mass) of various products and reactants in a chemical reaction. Chemical reactions. A balanced chemical equation gives the identity of the reactants and the products as well as the accurate number of molecules or moles of each that are. How to use mole ratios from a balanced reaction to calculate amounts of reactants. Using a balanced chemical equation to calculate amounts of reactants and products is called . Example: Using mole ratios to calculate mass of a reactant.
Exercise Alchemists produced elemental mercury by roasting the mercury-containing ore cinnabar HgS in air: Given g of cinnabar, how much elemental mercury can be produced from this reaction?
- Steps in Converting between Masses of Reactant and Product
- Stoichiometry Problems
Consequently, none of the reactants was left over at the end of the reaction. This is often desirable, as in the case of a space shuttle, where excess oxygen or hydrogen was not only extra freight to be hauled into orbit but also an explosion hazard.
Mole-Mole Relationships in Chemical Reactions - Chemistry LibreTexts
More often, however, reactants are present in mole ratios that are not the same as the ratio of the coefficients in the balanced chemical equation. As a result, one or more of them will not be used up completely but will be left over when the reaction is completed.
In this situation, the amount of product that can be obtained is limited by the amount of only one of the reactants. The reactant that restricts the amount of product obtained is called the limiting reactant The reactant that restricts the amount of product obtained in a chemical reaction.
The reactant that remains after a reaction has gone to completion is in excess. Assume you have invited some friends for dinner and want to bake brownies for dessert. You find two boxes of brownie mix in your pantry and see that each package requires two eggs. The balanced equation for brownie preparation is thus Equation 3. Because each box of brownie mix requires two eggs and you have two boxes, you need four eggs.
Twelve eggs is eight more eggs than you need. Although the ratio of eggs to boxes in is 2: Hence the eggs are the ingredient reactant present in excess, and the brownie mix is the limiting reactant.
Mass Relationships in Chemical Equations
Even if you had a refrigerator full of eggs, you could make only two batches of brownies. In order to do this we will need to use fractional coefficients. Typically a stoichiometric coefficient of "1" is not explicitly included when writing the chemical equation. We can get rid of the fractional coefficients by multiplying by 2 even though this is a perfectly acceptable balanced chemical equation. Balanced, but without fractional coefficients At the very beginning of this problem, perhaps you could see this was the answer.
If you can see the balanced equation by sight, you don't need to go by the guidelines. Remember they are only guidelines to help if you run into trouble.
You can see by simply adding a 2 in front of NO, we violate the first guideline even though it leads us to a balanced equation. Balance the given chemical reaction.
6: Chemical Reactions - Mole and Mass Relationships
This one may not be as easy to see the final answer so we will use the guidelines to balance the equation. N2O3 is the most complex species so we will add a 1 for its coefficient. Now we can balance the remaining single element species. In order to balance the number of atoms we need 2 atoms of N and 3 atoms of oxygen on the left side of the equation.
Balanced The equation is now balanced.
However, we can get rid of the fractional coefficient by again multiplying by 2. Balanced, without fractional coefficients Notice that in these two examples N2 and O2 react with a different stoichiometry to obtain different products. Is it necessary for the number of moles of the reactants to be equal to the number of moles of products?
Answer Not only does the stoichiometry tell us the mole relation between product and reactants but it will also tell us the mass relation. How many grams of CO2 are produced by the complete combustion of 1.
Remember that a combustion reaction is one in which the reactant glucose in this case reacts with O2 to produce CO2 and H2O.
So first let's write the skeleton equation unbalanced equation. Use the guidelines to balance the equation. We will assign a 1 to glucose.
We will balance the oxygen last because it is contained in a single-element species O2. This means we need to balance the C and H atoms next. So we need 6 atoms of C and 12 atoms of H on the right side of the reaction. So now we have 6 atoms of C and 6 atoms of H2 which accounts for twelve H atoms.
Now we can balance the oxygen. We have a total of 18 oxygen atoms on the right side. In order to have 18 atoms of oxygen on the left side we will need to assign a 6 to O2.