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 temperatureChange in colorSmellFormation of a precipitateFormation of bubbles

Diatomic Molecules

Always occur in 2’s when aloneBr 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

Description SymbolYields Reversible reaction Solid (s)Liquid (l)Gas (g)Aqueous-dissolved in water (aq)Reactants are heatedPressure is 2 atm Pressure is higher than normalTemp is 25 degrees C 25 C Catalyst MnO2 was used

Balancing Equations

Law of Conservation of Mass must be satisfied Equal # of atoms on both sides

Balance by adjusting coefficientsDo not alter subscriptsBalance O’s and H’s last

Counting Atoms

Distribute coefficient to every atom2 H2O - 4 H’s and 2 O’s3 Ca (NO3)2 – 3 Ca’s, 6 N’s, 18 O’s

Example

__ Al2O3 ___ Al + ___ O21. Count atoms- 2 Al’s on left and 1 on right2. Balance by placing 2 in front of Al on right

__ Al2O3 2 Al + ___ O23. Count atoms- 3 O’s on left and 2 O’s on

right4. 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 anotherReactants: element + compoundA + 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 reaction2 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 + XBA, X= cationsY, B= anions

Reaction Types: Combustion

Reaction of O2 gas with anything, normally a hydrocarbon (compound with H + C)

CxHy + O2 CO2 + H2O