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