Chemical Reactions Lecture Notes

CHEMICAL REACTIONS

MOST ESSENTIAL LEARNING COMPETENCY

 Apply the principles of conservation of mass to chemical reactions;
 Explain how the factors affecting rates of chemical reactions are applied in food preservation and materials
production, control of fire, pollution, and corrosion.

DESCRIBING CHEMICAL REACTIONS:

PHYSICAL CHANGE

 retain original properties, characteristics, etc.
 appearance may change but not the properties

CHEMICAL CHANGE

 produces one or more new substances with new properties, characteristics, etc.
 the new substance can no longer be returned to its original form

PHYSICAL CHANGE

 Melting ice
 Tearing paper
 Splitting wood
 Mixing cake ingredients

CHEMICAL CHANGE

 Rusting iron
 Burning paper, wood, etc…
 Cooking an egg
 Baking a cake

CHEMICAL REACTION

 is a process in which one or more substances are converted into new substances with different physical and
chemical properties.

CLUES TO A CHEMICAL REACTION

 Color change
 Formation of a solid (precipitate)
 Formation of a gas (bubbles)
 Heat and/or flame is produced or absorbed

 Exothermic: refers to a reaction where energy (as heat) flows from the system

 Endothermic: refers to a reaction where energy (as heat) flows into system

DURING CHEMICAL REACTIONS:

 Atoms have the opportunity to obtain a complete set of valence electrons and become more stable. Old bonds
are broken, new bonds are formed and atoms are REARRANGED.
 Involves a rearrangement of the ways in which atoms are grouped. The atoms form bonds to become stable
(set of 8). New molecules usually result.
 This process always involves energy.

Chemical reactions are represented by sentences known as chemical equations. A chemical equation describes exactly

, Ex) calcium + oxygen  calcium oxide

2. Formula Equations (Chemical Equations): uses chemical symbols and formulas

Ex) 2Ca + O2  2CaO

PHYSICAL STATES AND SYMBOLS IN A REACTION

 In an equation the physical states of the reactants and the products are indicated by using the following
symbols:

Symbol Meaning

(s) Solid

(l) Liquid

(g) Gas

 In an equation the physical states of the reactants and the products are indicated by using the following
symbols:

Symbol Meaning

(aq) Dissolved in water (aqueous solution)

 yields

↔ reversible reaction

Δ, heat reactants are heated

Catalyst added to speed up a reaction

BALANCING CHEMICAL EQUATIONS

 Notice that in a chemical reaction, ATOMS ARE NEITHER CREATED NOR DESTROYED, THEY ARE
CONSERVED (Law of Conservation of Matter). The atoms are just rearranged and connected differently. All
atoms present in the reactants must be accounted for among the products. In other words, there must be the
same number of each type of atom on the product side as on the reactant side of the arrow.
 This is called BALANCING THE CHEMICAL EQUATION for the reaction. When balancing an equation
the identities (formulas) of the compounds must never be changed. In other words, the subscripts in the
formula cannot be changed, nor can atoms be added or subtracted from a formula. The correct way to balance
is to use COEFFICIENTS: the number in front of a chemical formula (must be simplest integers).

HINTS

 Balance by inspection
 Simplest whole number ratio of atoms
 Save hydrogen and oxygen for last
 Balance using polyatomic ions to save time
 Odd-Even Technique (Use fractions to balance then multiply by 2).
 7 diatomic elements ONLY WHEN ALONE (H2, N2, O2, F2, Cl2, Br2, I2)

Examples:

2K(s) + 2H2O(l)  H2(g) +2KOH(aq)

K= 2 K= 2

H= 4 H= 4


O= 2 O= 2