The Law of Conservation of Matter Weve talked

The Law of Conservation of Matter

We’ve talked about changes in matter… • The evaporation of a puddle of water • Rust forming on a metal fence PH YS IC CH AL EM IC AL

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 it can only change forms. • 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 Reactant s Mass of Products

An Example of the L. C. M. • 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 – Hard, gray-colored metal • Rust – 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

O TW of s e om ad at m en ch yg Ea ox TAKE A LOOK AT THE NUMBERS F e F e O 2 RE AC T How many Iron atoms? O 2 AN TS F e Fe 2 O 3 O 2 How many Oxygen atoms? O R P Fe 2 O 3 What about the T C DU PRODUCT?

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 Reactant s Mass of Products

Example: Tin Fluoride • Tin + hydrogen fluoride -> tin fluoride + hydrogen 40. 02 g 118. 7 g • What’s the total mass of the reactants? – 158. 72 g • So what should the mass of the products be? – 158. 72 g

Balancing Chemical Equations Demonstrating the Law of Conservation of Matter

What is a chemical equation? • Chemical equation - Describes a chemical change. • Parts of an equation: Reactant Product 2 Ag + H 2 S Ag 2 S + H 2 Reaction symbol

Reactants and Products • Reactant - The chemical(s) you start with before the reaction. – Written on left side of equation. • Product - The new chemical(s) formed by the reaction. – Right side of equation.

Subscripts and Coefficients • Subscript - shows how many atoms of an element are in a molecule. – EX: H 2 O • 2 atoms of hydrogen (H) • 1 atom of oxygen (O) • Coefficient - shows how many molecules there are of a particular chemical. – EX: 3 H 2 O • Means there are 3 water molecules.

A Chemical Reaction • 2 H 2 + O 2 → 2 H 2 O

Law of Conservation of Mass • In a chemical reaction, matter is neither created nor destroyed. – In other words, the number and type of atoms going INTO a reaction must be the same as the number and type of atoms coming OUT. • If an equation obeys the Law of Conservation of Matter, it is balanced.

A Balanced Equation ● CH 4 + 2 O 2 → CO 2 + 2 H 2 O Reactant Side 1 carbon atom 4 hydrogen atoms 4 oxygen atoms Product Side 1 carbon atom 4 hydrogen atoms 4 oxygen atoms

An Unbalanced Equation • CH 4 + O 2 → CO 2 + H 2 O Reactant Side 1 carbon atom 4 hydrogen atoms 2 oxygen atoms Product Side 1 carbon atom 2 hydrogen atoms 3 oxygen atoms