Chemical Reactions

Lesson 1. The Law of Conservation of Mass

In the late eighteenth century, Antoine Lavoisier, a French chemist, recognized the importance of accurate

measurements. He extensively studied and explained the nature of

combustion. He found out that combustion involved

reaction with oxygen.

Law of Conservation of Mass

Total mass of reactants

= Total mass of

products

Antoine Lavoisier

Mass is neither created nor

destroyed during chemical or

physical reactions.

Reactants are the starting material in a chemical reaction.

Products are the substance formed as a result of a chemical

reaction.

In a chemical equation, reactants are found on the left side and the products are on the right side. A chemical reaction can therefore be summarized as:

Reactants → Products

CHEMICAL REACTION

sodium metal + chlorine gas ---- table salt

(sodium chloride)

The Law of conservation of mass states that matter cannot be created or destroyed in any

chemical reaction The atoms in the reactants are

rearranged to form new compounds, but none of the atoms disappear, and no new atoms are

formed.H

H

HH O O

Reactants Products

The same number of each kind of atom must be on the left side of the arrow as are on

the right side when an equation is balanced.

Al Al

Al Al

O O

O OO O

Al AlOO O

Al AlOO O

Oxygen, as a product, must be multiplied by 2. A coefficient is placed in front of the compounds that contains O.

Oxygen atoms are =When a coefficient is added you must

multiply all atoms in the compound by this

number. Redo the inventory

After redoing the inventory, add coefficients to balanced other atoms. With each additional coefficient the inventory must be updated.

Oxygen atoms are≠

Hydogen atoms are =

2 H atoms 2 H atoms

2 O atoms 1 O atom

Hydogen atoms are ≠Oxygen atoms are =

First: Inventory the atoms on both sides of the yield arrow

Balancing an equation: H2 + O2 H2O

HH

HO O O

H

Second: Determine which atoms are not equal and add coefficients to balance these atoms.

2

4 H atoms

2 O atoms

2

4 H atoms

2 O atoms

Hydogen atoms are =

This is now a balanced equation

So: Chemical equations are balanced, when the numbers and kinds of atoms on each

side of the reaction arrow are equal.

HH

HH O O

Reactants Products

HH

HH

OO

4 Hydrogen 4 Hydrogen

2 Oxygen 2 Oxygen

Because atoms are conserved, the mass of the reactants in the chemical reaction is equal to the mass of the products.

H2 + F2 2 HF

2g + 38g = 2( 20g) 40g = 40g

40 g 40 g

Reactants Products

H H F F H F H F

The Law of Conservation of Mass allows the calculation of the reactants

or products.

If 27.0 g of mercuric oxide (the red solid compound) is heated and

completely decomposes to give the elements oxygen and mercury. 2.0 g of oxygen are produced. How

many grams of mercury are produced?

mercuric oxide mercury + oxygenHgO Hg + O2

Reactants Products

27.0 g HgO

? g Hg

2.0 g O2

27 g HgO = 2 g O2 + ? Hg =25 g Hg

Which of the following chemical reactions indicate that mass in conserved?

A. Mg + Cl2 2 MgClB. 2 Ca + O2 2 CaOC. Zn + S 2 ZnSD. C + O2 2 CO

The correct answer is B

CaCa

OO

OOCa

Ca

Because atoms must be equal on each side of the equation:

Mg + HCl → MgCl2 + H2

When the above equation is balanced, the coefficient for magnesium is —

A 0B 1C 2D 4

2 Mg + 2 HCl → MgCl2 + H2

The correct answer is C

Suppose 100 g of iron metal rusts. We weigh the rust and find that the rust has a mass of 143 g. What mass of oxygen reacted with the iron?

A. 243 g B. 57 g C. 100 gD 43 g

43 g Oxygen Choice D

 Iron + Oxygen Rust 100 g + ?g 143g What mass of oxygen is needed?