The Law of Conservation of Mass

We’ve talked about changes in matter… The evaporation of a puddle of water Rust forming on a metal fence PHYSICAL CHEMICAL

HOWEVER… No matter is created or destroyed by such changes! The total amount of matter remains constant.

The Law of Conservation of Matter During a chemical reaction, matter cannot be created or destroyed. Even though the matter may change from one form to another, the same number of atoms exists before and after the change takes place!

Since the overall mass of the matter does not change… The mass of the reactants – the substances there before the reaction occurs – must equal the mass of the product – the substances there after the reaction takes place. Mass of Reactants Mass of Products

An Example of the Law of Conservation of Mass Test tube containing lead nitrate is placed upright in a flask that contains potassium iodide. What are these? REACTANTS! They don’t mix!

A stopper is put in place to create a closed system – an environment where matter cannot enter or escape. The mass of the system is found.

After the mass is found, the system is turned upside down so the lead nitrate can mix with the potassium iodide. They create lead iodide and potassium nitrate.

Although the matter changed in form, the total mass of the system remains unchanged.

The Formation of Rust Iron Rust Hard, gray-colored metal Flaky, orange-red solid

When iron reacts with oxygen in the air, they form rust or IRON OXIDE. This is written as: Iron + Oxygen Iron Oxide

TAKE A LOOK AT THE NUMBERS Each made of TWO oxygen atoms Fe Fe O2 Fe2O3 Fe2O3 Fe Fe O2 O2 What about the PRODUCT? How many Oxygen molecules? How many Iron molecules? PRODUCT REACTANTS

All Chemical Reactions follow the Law of Conservation of Matter With Iron Oxide, the MASS STAYS THE SAME! The total number of iron atoms and oxygen atoms in the reactants is the same as that in the product. The atoms were just rearranged! No new atoms were created and none were destroyed!

Determining the Mass of Reactants and Products REMEMBER: The mass of the reactants is always equal to the mass of the products. Mass of Reactants Mass of Products

Example: Tin Fluoride Tin + hydrogen fluoride -> tin fluoride + hydrogen What’s the total mass of the reactants? 158.72 g So what should the mass of the products be? 118.7 g 40.02 g

Total Mass of Reactants: 158.72 g Tin + hydrogen fluoride -> tin fluoride + hydrogen If we know the mass of the tin fluoride, can we figure out how much hydrogen was produced? Subtract the mass of one product from the total mass. 156.71 g 2.01 g Total Mass of Reactants: 158.72 g

ANY QUESTIONS?