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

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

The Law of Conservation of Mass 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 Products

Open System There is exchange of matter and energy with surroundings Mass is lost to the environment; once gone, that mass cannot be measured. Example: Burning Wood

Closed System No matter can enter/leave the system only energy

An Example of the Closed System. 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.

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 REACTANTS PRODUCT O2 Fe2O3 Fe Fe O2 O2 What about the PRODUCT? How many Oxygen molecules? How many Iron molecules?

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

What are Chemical Equations A chemical change is represented by a chemical reaction. A chemical equation identifies the starting and ending chemicals as reactants and products: reactants products

Balancing Equations Reactants Products Chemical reactions result in chemical changes. Chemical changes occur when new substances are created. The original substance(s), called reactants, change into new substance(s) called products. Reactants Products (c) McGraw Hill Ryerson 2007 See pages 202 - 203

Balancing Equations Reactants Products (c) McGraw Hill Ryerson 2007 See pages 202 - 203

Balancing Equations Chemical reactions can be written in different ways. A word equation: Nitrogen monoxide + oxygen  nitrogen dioxide A symbolic equation: 2NO(g) + O2(g)  2NO2(g) STATE OF MATTER Letters indicate the state of each compound. (aq) = aqueous/dissolved in water (s) = solid ( ) = liquid (g) = gas COEFFICIENTS Indicates how many of each molecule there is. Ie: there are 2 molecules of NO. (c) McGraw Hill Ryerson 2007

Law of Conservation of Mass When a chemical reaction occurs, new compounds are created, BUT… No new matter is created or destroyed; atoms are just rearranged as the atoms change partners to form new compounds. If there are 3 atoms of oxygen in the reactants, there MUST be 3 atoms of oxygen in the products. Number of each atom in reactants = number of each atom in products. The law of conservation of mass: Mass of reactants = mass of products If you could collect and measure all of the exhaust from this car, you would find that mass of reactants (gas + O2) = mass of products (exhaust). (c) McGraw Hill Ryerson 2007

Writing & Balancing Equations The simplest form of chemical equation is a word equation. Potassium metal + oxygen gas  potassium oxide A skeleton equation shows the formulas of the elements/compounds. A skeleton equation shows which atoms are involved, but not how many molecules are involved. K + O2  K2O

Writing & Balancing Equations A balanced chemical equation shows all atoms and the coefficients tells us how many molecules (and atoms) there are. Balancing ensures that the number of each atom is the same on both sides of the reaction arrow. 4K + O2  2K2O K K O O K O K K K K O K

Counting Atoms to Balance Equations Using the law of conservation of mass, we can count atoms to balance the number of atoms in chemical equations. Word equation: methane + oxygen  water + carbon dioxide Skeleton equation: CH4 + O2  H2O + CO2 To balance the compounds, take note of how many atoms of each element occur on each side of the reaction arrow. See Page 207 (c) McGraw Hill Ryerson 2007

Balancing Equations CH4 + O2 H2O + CO2 C C H H O O

Balancing Equations CH4 + O2 H2O + CO2 C- 1 C -1 H - 4 H- 2 O - 2 O -3

Balancing Equations CH4 + O2 2 H2O + CO2 C- 1 C -1 H - 4 H- 2 4 O - 2 O -3 4

Balancing Equations CH4 + 2 O2 2 H2O + CO2 C- 1 C -1 H - 4 H- 2 4 O – 2 4 O -3 4

Counting Atoms to Balance Equations Skeleton equation: CH4 + O2  H2O + CO2 Carbon = 1 Carbon = 1 Hydrogen = 4 Hydrogen = 2 Oxygen = 2 Oxygen = 3 Balanced equation: CH4 + 2O2  2H2O + CO2 Carbon = 1 Carbon = 1 Hydrogen = 4 Hydrogen = 4 Oxygen = 4 Oxygen = 4 The same number of atoms must be on each side. (c) McGraw Hill Ryerson 2007 See Page 207

Balancing Equations H2 + O2  H2O hydrogen + oxygen  water Reactants Products H 2 O 2 1

YOU CANNOT CHANGE THE SUBSCRIPTS Balancing Equations X hydrogen + oxygen  hydrogen peroxide H2 + O2  H2O2 YOU CANNOT CHANGE THE SUBSCRIPTS Reactants Products H 2 O

Balancing Equations 2 H2 + O2  H2O hydrogen + oxygen  water Reactants Products H 2 O 1

Balancing Equations H2 + O2  H2O 2 hydrogen + oxygen  water Reactants Products H 2 4 O

Balancing Equations 2 H2 + O2  H2O 2 hydrogen + oxygen  water Reactants Products H 4 O 2

ON BALANCING EQUATIONS WATCH THIS LESSON ON BALANCING EQUATIONS Turn sound “on” http://www.youtube.com/watch?v=_B735turDoM

Balance the following: Fe + Br2  FeBr3 C2H6 + O2  CO2 + H2O

____Ba + ____H2O  ____Ba(OH)2 + ____H2    ____CO2 + ____H2O  ____H2CO3    ____Fe2O3 + ____C  ____Fe + ____CO ____Fe + ____H2O  ____H2 + ____Fe2O3