Chemical Reactions

Chemical Equations Reactants are the substances entering a reaction  Written on left Products are the result  Written on right A chemical equation uses symbols and formulas to represent identities and amounts of products and reactants in a chemical equation

Indicators of Chemical Reaction Change in temperature Change in color Smell Formation of a precipitate Formation of bubbles

Diatomic Molecules Always occur in 2’s when alone Br I N Cl H O F

Equations Word equations: use words and symbols to describe a reaction Formula equations: use formulas and symbols to describe a reaction Write a word & formula equation for the following: during electrolysis, water decomposes into hydrogen and oxygen gas

Symbols DescriptionSymbol Yields  Reversible reaction  Solid(s) Liquid(l) Gas(g) Aqueous-dissolved in water(aq) Reactants are heated Pressure is 2 atm Pressure is higher than normal Temp is 25 degrees C25 C  Catalyst MnO2 was used

Balancing Equations Law of Conservation of Mass must be satisfied  Equal # of atoms on both sides Balance by adjusting coefficients Do not alter subscripts Balance O’s and H’s last

Counting Atoms Distribute coefficient to every atom 2 H 2 O - 4 H’s and 2 O’s 3 Ca (NO 3 ) 2 – 3 Ca’s, 6 N’s, 18 O’s

Example __ Al2O3  ___ Al + ___ O2 1. Count atoms- 2 Al’s on left and 1 on right 2. Balance by placing 2 in front of Al on right __ Al2O3  2 Al + ___ O2 3. Count atoms- 3 O’s on left and 2 O’s on right 4. Balance by placing 2 in front of compound on left and 3 in front of the O’s on the right to make 6 O’s on each side

Example 2 Al2O3  2 Al + 3 O2 5. Now Al’s are not balanced. Make the one on right a 4 2 Al2O3  4 Al + 3 O2 Equation is now balanced!

Reaction Energy 1. A catalyst is used to speed up or slow down a reaction, but it does not react 2. Activation energy is the amount of energy needed for a reaction to start 3. ∆H is the symbol used to represent a change in energy  + ∆ = exothermic (heat is released)  - ∆ = endothermic (heat is absorbed)

Energy Diagrams Exothermic- energy is released and is lower than starting  *catalyst lowers the hump (activation energy) Endothermic- energy is absorbed and is higher than starting

Reaction Types: Single Replacement One element replaces another Reactants: element + compound A + BX  AX + B

Reaction Types: Decomposition One compound splits into 2 pieces  Can be elements or simpler compounds AB  A + B

Reaction Types: Synthesis Reverse of decomposition reaction 2 pieces join together to produce one, more complex compound  Pieces can be elements or simpler compounds A + B  AB

Reaction Types: Double Replacement Cations and anions of 2 different compounds switch places AB + XY  AY + XB A, X= cations Y, B= anions

Reaction Types: Combustion Reaction of O2 gas with anything, normally a hydrocarbon (compound with H + C) C x H y + O 2  CO 2 + H 2 O