chemical reactions. i. physical and chemical changes ca. physical changes in matter affects only the...

CHEMICAL REACTIONS

I. Physical and Chemical Changes

A. PHYSICAL CHANGES IN MATTER Affects only the physical properties of a substance; you still have the same substance

Examples of physical properties:Size , Shape, Malleability, Texture, Solubility, Hardness, Viscosity, Phase (S,L,G)

Examples of physical changes:Freezing *CuttingCrushing *DissolvingMelting *Grinding

B. Chemical Changes in Matter The result of chemical reactions

NEW substance(s) formed

Involves the breaking of old bonds and the forming of new bonds

Examples: Burning Rusting Digesting decaying

II. Chemical Reactions

Atoms are rearranged

New substances have new chemical properties.

A. Evidence of Chemical Reactions 1. A gas is formed (Bubbles)

2. A Precipitate is formed (Turns cloudy because a new solid forms)

3. There is a energy / temperature change (Because bonds are breaking and re-forming)

4. There is a permanent color change (Because new substances are forming

5. Light is produced

B. Chemical Equations An expression that describes a chemical

reaction

REACTANTS PRODUCTS

2Mg + O2 2MgO

Chemical Equation Language

REACTANTS PRODUCTS

ReactantsOn the left of the arrowThe elements/compounds you start with

ProductsOn the right of the arrowThe elements/ compounds you finish with

Chemical Equation LanguageREACTANTS PRODUCTS

+ sign : means “and” or “added to”Arrow

“Yields” or “Results in”CoefficientThe large number in front of the element

or compoundSubscriptThe small number below and behind the

symbol

Word EquationChemical reaction written as a sentence

Example Two Hydrogen react with Oxygen to produce

two Water Molecules

2H2 + O2 2H2O

III. BALANCING CHEMICAL EQUATIONS

A. Counting Atoms Review Subscripts counted for that element only Coefficients counted for every element in

the compound

4Al2(SO4)3 Aluminum: 2 * 4 = 8 Sulfur: 1*3 = 3 * 4 = 12 Oxygen: = ?

B. Law of Conservation of Mass

Mass (matter) cannot be created or destroyed

Developed by Antoine Lavoisier in the 17th century

Mass is conserved

Therefore chemical reactions and equations MUST be balanced

C. Balanced Chemical Equations

mass of the reactants must equal the mass of the products

A BALANCED EQUATION has the same ELEMENTS & the same # OF ATOMS in BOTH the reactants and products

Balancing Chemical Equations

THIS IS NOT BALANCED!!!! Why?H2 + O2 H2O

H = 2 H = 2O = 2 O = 1

ONLY CO-EFFICIENTS MAY BE CHANGED OR ADDED TO BALANCE AN EQUATION!!!

SUBSCRIPTS MAY NOT BE CHANGED OR ADDED!!!!!

2H2 + O2 2H2OBALANCED

LET’S PRACTICE!!!KClO3 KCl + O2

1 K 1 1 Cl 1 3 O 2 not balanced

2KClO3 2KCl + 3O2

2 K 2 2 Cl 2 6 O 6 balanced

IV. Types of Chemical Reactions

Synthesis Reaction

Decomposition Reaction

Single Displacement Reaction

Double Displacement Reaction

A. Synthesis Reactions Addition reaction Two or more substances chemically combine to

form ONE product EX: Element + Element Compound

A + B AB

4Al + 3O2 2Al2O3

B. Decomposition Reactions ONE reactant breaks down into two or

more substances EX: Compound Element + Element

AB A + B

2KClO3 2KCl + 3O2

2NaH(CO3) 2Na(CO3) + H2

C. Single Displacement Reactions One ELEMENT replaces another corresponding

element in a compound EX: Element + Compound Compound + Element

A + BC AC + B

Ca + 2Ag(NO3) 2Ag + Ca(NO3)2

Fe + CuCl2 FeCl2 + Cu

D. Double Displacement Reactions TWO compounds produce two NEW COMPOUNDS The CATION of one compound replaces the

CATION of another compound Usually produces a PRECIPITATE (something not

soluble in water) & a liquid/gasEX:Compound + Compound Compound +

Compound

AB + CD AD + CB

NaCl + KBr NaBr + KCl

V. Energy in Reactions Energy is released or absorbed in ALL

chemical reactions

(When bonds are FORMED, energy is RELEASED)

(When bonds are BROKEN, energy is ABSORBED (Taken In))

A. Exothermic Reactions Energy is RELEASED

Usually occurs in the form of HEAT because BONDS ARE BROKEN

Energy will be on the PRODUCT side of the chemical equation

Temperature Rises (Feels Warmer)

Examples: Burning of Gasoline Heat Packs Explosion of Dynamite

B. Endothermic Reactions Energy is ABSORBED (Taken in)

Temperature Drops (Feels Cold)

Energy is on the REACTANT side of the chemical equation

Examples: Ice pack