4.3 Conservation of Mass and Chemical Equations

Learning Goals

• Learn what is the law of conservation of mass

• Learn how to represent chemical reactions by word equations and skeleton equations

Reactants and Products

In chemical reactions we have

– Reactants: pure substance undergoing a chemical change• products that we combine (or start the reaction with)

– Products: pure substance formed by a chemical change• the result of the reaction• Contain different properties than the reactants

Chemical Reactions

• Are represented as either– Word equations– Skeleton equations (periodic symbol)

What are the states that an element can exist in?- Solid (s)- Liquid (l)- Gas (g)

In reactions, compounds can also be AQUEOUS (aq)-This means that the compound dissolves in water

Law of Conservation of Mass

In a chemical reaction, the total mass of the reactants is equal to the total mass of the products

– This means that we will have the same number of atoms of each element of reactants, as products

Balancing Chemical EquationsIn order to create a balanced chemical equation, you must know how to determine the total number of atoms in a compound using coefficients.

What is the total number of atoms of each element?a) 2 NaClb) 6 H2Oc) 2NaNO3

d) (NH4)2SO4

Writing Chemical Equations

1. Word Equations

hydrogen + oxygen water

Reactants Product

“Reacts with” “yields OR reacts to produce”

Writing Chemical Equations

2. Skeleton Equation

H2(g) + O2(g) H2O(l)

What is wrong with this chemical equation?

It must be balanced!!!

Writing Chemical Equations

H2(g) + O2(g) H2O(l)

Determine the number of atoms for each element in the reactant.

Determine the number of atoms for each element in the product.

H =

O =

H =

O =

2

2

2

1

Balance by adding coefficients to the unequal element

Writing Chemical Equations

H2 + O2 2 H2O

Determine the number of atoms for each element in the reactant.

Determine the number of atoms for each element in the product.

H =

O =

H =

O =

2

2

4

2

Continue balancing by adding coefficients to the unequal element

Writing Chemical Equations

2 H2(g) + O2(g) 2 H2O(l)

Determine the number of atoms for each element in the reactant.

Determine the number of atoms for each element in the product.

H =

O =

H =

O =

4

2

4

2

Writing Chemical Equations

2. Balanced Skeleton Equation

2 H2(g) + O2(g) 2 H2O(l)

PracticeBalance each chemical equation:

A) Mg (s) + O2 (g) MgO (s)

B) Li (s) + Br2 (g) LiBr(s)

C) Al (s) + CuO (s) Al2O3 (s) + Cu (s)

Homework

Textbook: • Read Section 4.3

Balancing Equations Worksheets

Chapter 4 Quiz Wednesday!