4.5.A.1 Conservation of Atoms in Chemical Reactions:

1. Law of Conservation of Mass:

The Law of Conservation of Mass simply states a basic principle of chemistry. It simply means that mass cannot be created or destroyed in a chemical reaction. In other words, the sum of the total mass of the reactants; the chemical substance used to begin the reactions; is always equivalent to the total mass of products; the products obtained.

In other words, the atoms are only rearranged and not lost or gained in a chemical reaction. The reactant atoms are only rearranged to become the product.

For example, when you burn a piece of wood, it appears to be gone. But it’s not. It is still there in the sense that the mass has been rearranged into the form of carbon dioxide and water vapor, with some ash thrown in for good measure. If you could somehow gather all of these products and gases, the total mass of all of these would equal the mass of the original wood.

This law is very important because it enables chemists to balance chemical equations and predict what happens to the atoms during a reaction. It also shows that chemical transformations have to conserve atoms and molecules.

Here’s a simple example:

Reactants: 2 H₂ + O₂ → Products: 2 H₂O (water)

The mass of 2 molecules of hydrogen, H₂ and 1 molecule of oxygen, O₂ before reaction is equal to the mass of the 2 molecules of water, H₂O after the reaction. The hydrogen atoms and oxygen atoms are simply rearranged to form new bonds in water.

2. Balancing Equations & Stoichiometry:

• Balancing Chemical Equations:

Chemical equation balancing satisfies the Law of Conservation of Mass. In a balanced equation, the number of atoms of each element on the left side (reactants) equals the number of atoms of that element on the right side (products). Here’s how you can balance chemical equations:

i. Steps to Balance a Chemical Equation:

a. Write the Unbalanced Equation:
Write an unbalanced equation from the reactants and products.

Example: Unbalanced Equation:  H2​+O2​→H2​O

b. Identify the Elements:
Determine the elements present in the reaction. The elements are hydrogen (H) and oxygen (O).

c. Count the Atoms:
Determine how many of each element are present on each side of the equation.
Left side: 2 hydrogen (H) atoms and 2 oxygen (O) atoms.
On the right side: 2 hydrogen (H) atoms and 1 oxygen (O) atom.

d. Balance One Element at a Time:
Start by balancing one element. In this reaction, oxygen is unbalanced: there are 2 atoms on the left but only 1 on the right. So, we put a coefficient of 1 in front of on the right:H2​+O2​→H2​O

Now, the oxygen is balanced since it appears both on the right and the left side, 2, so the hydrogen has not been balanced. It almost looks like this: 2 on each side; that is okay.
e. Equal All Elements:
Check each element has equal numbers of atoms on each side.
The equation is balanced.

• Stoichiometry:

It’s the calculation of the amounts of reactants and products in a chemical reaction that is based on the balanced chemical equation. In this case, it involves mole ratios, or the proportions of reactants and products involved in the reaction.

Stoichiometric Calculation Steps:
In this case:
i. Write and balance the equation. Make sure your chemical equation is balanced.
Example: 2H2​+O2​→2H2​O

ii. Convert Given Quantities to Moles:
You’ll likely want units of moles for your stoichiometric calculations, because there is one common way to count particles. Because you are given mass or volume, you must convert these to moles through molar mass or molar volume.

For example, if you have 4 grams of

${\mathrm{<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;\; font-size:\; 16px;">H2<span\; style="font-size:\; 16px;">,\; convert\; this\; to\; moles\; by\; dividing\; by\; the\; molar\; mass\; of </span></span>}}_{}$ $\text{H}_2$

(which is 2 g/mol):

$$4 \, \text{g} \, \text{H}_2 \times \frac{1 \, \text{mol} \, \text{H}_2}{2 \, \text{g} \, \text{H}_2} = 2 \, \text{mol} \, \text{H}_2$$

iii. Use Mole Ratios:
Derive the mole ratios between the reactants and the products from the balanced equation. For the above equation:
2 moles of

$\text{H}_2$

H2​ react with 1 mole of O2​ to form 2 moles of H2​O..
With this ratio, you can calculate the quantity of some other substance that is involved in the reaction.

iv. Convert Moles to Desired Units:
From the mole ratio, you now can convert moles of the substance of interest to grams using molar mass or liters using molar volume.

3. Limiting Reactants:

In a chemical reaction, the limiting reactant (or limiting reagent) is the substance that is completely consumed first and thus limits the amount of product that can be formed. The other reactants are in excess, meaning there’s more of them available than needed for the reaction to complete.

The amount of product formed is measured by the amount of limiting reagent. It carries out one reaction when that substance is totally consumed, but does not keep track of remaining reactants once that is exhausted.

i. Determining the Limiting Reactant:

a. Balance the Chemical Equation: You need a balanced chemical equation to compare the mole ratios of the reactants.

Example: 2H2​+O2​→2H2​O

b. Convert All Given Quantities to Moles: Convert any given quantities of the reactants from grams, liters, or other units to moles by using their molar masses for solids and their molar volume for gases under standard conditions.

c. Use the Mole Ratios:
From the balanced equation, obtain the mole ratios between the reactants. This will inform you of how much of each reactant is required to completely react with the other.

d. Compare the Available Moles of Each Reactant:
Determine how much of each reactant is required to completely react with the other reactants. The first reactant to be depleted is the limiting reactant.

ii. Role of the Limiting Reactant:
The limiting reactant determines the quantity of product made. At this point, there is no more reaction, even though the other reactant remains in the reaction flask.

a. Excess Reactants: Those that are not used by the reaction because there is an excess amount beyond what is required by the balanced equation. They have not all reacted at the conclusion of the reaction.
b. Limiting Reactant: The reactant that gets consumed first and controls how much product will be formed.
The limiting reagent gives you information about what possible products will come from a reaction. This is going to be very important in deciding whether yields should be maximized and waste products minimized in industries. It tells you how much reactant will be needed to carry out a given reaction, apart from estimating the yield from the reactant.