Chemical Reactions

n       Chemical Reaction: a process in which one or more substances are converted into new substances with different physical and chemical properties

n       Reactant:  a substance that enters into a chemical reaction

n       Product:  A substance produces by a chemical reaction

n       Chemical Reactions occur so that atoms can obtain a full set of valence electrons and become more stable

Chemical Equations

n      Chemical Equations:  are used to describe what happens in a chemical reaction

n       Identifies the reactants  and the products

 

     Reactants  ®  Products

 

Types of Chemical Equations

n        Word Equations:  give the names of the reactants and products

      Magnesium + Nitrogen ® Magnesium Nitride

n        Formula Equations or Chemical Equations:  use chemical symbols and formulas instead of names

n         Coefficients:  precede the symbol or formula and indicate the relative number of particles

3 Mg + N2 ®  Mg3N2

      3 atoms of magnesium react with one molecule of nitrogen to yield one particle of magnesium nitride

Practice

Convert word equations below into formula equations

Hint:  H O N and the halogens all exist as diatomic molecules

        H2 = hydrogen    O2 = Oxygen        N2 = Nitrogen

        Cl2 = chlorine    Br2 = Bromine       F2 = Fluorine, etc.

 

n            sulfur + oxygen ®  sulfur dioxide

 

n            carbon dioxide + water ®  carbonic acid

 

n            iron + copper (II) sulfate ® iron (II) sulfate + copper

Answer

 

n            sulfur + oxygen ®  sulfur dioxide

            S + O2 ® SO2

n            carbon dioxide + water ®  carbonic acid

            CO2 + H2O ® H2CO3

n            iron + copper (II) sulfate ® iron (II) sulfate + copper

            Fe + CuSO4 ® FeSO4 + Cu

 

n       Balanced Chemical Equation:  The Law of Conservation of Mass has been observed

n        Matter can neither be gained nor lost through a chemical reaction

n        For mass to remain constant the number of atoms of each element must be the same before and after a chemical reaction (Atoms Before = Atoms After)

Example:        3 Mg + N2 ®  Mg3N2

 

Element                Mg      N

Atoms Before       3      2

Atoms Afterward      3      2

 

Balancing Equations

n            Write a formula equation with the correct symbols and formulas  

K + Br2 ® KBr

n            Count the number of atoms of each element on each side of the arrow

Before:  1 K and 2 Br            After:    1 K and 1 Br

n            Balance atoms by using coefficients

2K + Br2 ® 2KBr

n            Check work by counting atoms of each element

Before:  2 K and 2 Br       After:      2 K and 2 Br

 

Hints for Balancing Equations

n      Use a Pencil!

n      Begin with atoms of elements that occur only once on each side of the equation

n      It will often take multiple steps to get the right answer

n      Never forget step 4

 

Balancing Example

                             CH4 + O2 ® CO2 + H2O

Balance C                    1C                  1C

Balance H                    4H                                 2H

Add Coefficient              CH4 + O2 ® CO2 + 2H2O

Balance H             4H                                 4H

Balance O                        2O      2O + 2O = 4O

Add Coefficient              CH4 + 2O2 ® CO2 + 2H2O

Check all atoms                    C   H      O

                                    Before:    1    4      4

                                    After        1    4      4

Practice:

n          Balance the following Equations

n             Fe + O2 ® Fe2O3

n          C2H5OH + O2 ® CO2 + H2O

 

Answers:

n             4Fe + 3O2 ® 2Fe2O3

n          C2H5OH + 3O2 ® 2CO2 + 3H2O

 

Writing Complete Chemical Reactions

n      Complete Chemical Reactions:  include the physical state of each reactant and product

n       Written after the formula in parentheses:

n  (g) = gas

n  (l) = liquid

n  (s) = solid

n  (aq) = aqueous (dissolved in water)

n      CH4(g) + 2O2(g) ® CO2(g) + 2H2O(l)

n         1 molecule of Methane gas reacts with two molecules of oxygen gas to form 1 molecule carbon dioxide and 2 molecules of water

 

Classifying Chemical Reactions

n       Direct Combination (or synthesis) reactions:  2 or more simple reactants come together to form a single more complex product

n        General form of reaction: 

      A + B ®  AB

n        Reactants can be elements or compound

n          S + O2 ®  SO2

n          CO2  + H2O ®  H2CO3

 

Classifying Chemical Reactions

n       Decomposition reactions:  a reaction in which a single compound is broken down into two or more smaller compounds or elements

n        General form of reaction: 

       AB ®  A  +  B

n        Products can be elements or compound

n        2H2O ®  2H2   + O2

n          CaCO3  ®  CaO + CO2

 

 

n      Single Replacement Reactions:  An uncombined element displaces an element that is part of a compound

n       General form of reaction: 

     A + BX ®  AX + B

n       A more reactive element replaces a less reactive element

n       Mg + CuSO4 ®  MgSO4   + Cu

 

n      Double Replacement Reactions:  atoms or ions from two different compounds replace each other

n       General form of reaction: 

     AX + BY ®  AY + BX

n       Usually require that compounds be dissolved in water

n         CaCO3 + 2HCl ®  CaCl2 + H2CO3

n       AgNO3  + CaCl2 ® Ca(NO3)2  + AgCl


 

n       Combustion reactions: reaction in which something is burned (reacted with oxygen)

n        Many direct combination reactions are also combustion reactions:

    4 Fe + 3 O2 ®  2 Fe2O3

n       General form of reaction: 

      CxHyOz + O2 ®  CO2 + H2O

n          When compounds containing carbon and hydrogen (can also have oxygen) burn the products always include carbon dioxide and water

n        CH4 + O2 ® CO2  + 2H2O

n        C2H5OH+ 3O2 ®2 CO2  + 3H2O

 

n       Neutralization reactions:  are a special case of double replacement reaction.

n        One of the reactants is an acid (starts with H), on is a base (ends with OH).  The products are always a salt and water.  All ionic compounds are salts.

n        General form of reaction: 

      AOH + HX ®  AX + HOH

n        Neutralization reactions occur in water

n          Ca(OH)2 + 2 HCl ®  CaCl2 + 2 H2O

n        AgOH  + HNO3 ® AgNO3  + H2O

 

The Unique Bonding of Carbon

n       Carbon’s half filled valence level and relatively small size give it unique bonding properties

n        It is one of the few elements that forms 4 bonds and the only one that does so in such a large variety of combinations:

n   Carbon bonds with itself to form long chains or ring structures

n   These bonds are strong, short and covalent so the structures are very stable

n   Each Carbon in the chain or ring can form 4 single bonds, 2 double bonds, 1 double & 2 single bonds, 1 single and 1 triple bond.  Thus a HUGE variety of compounds containing carbon exist

n   These compounds provide the framework for most of the molecules that living organisms make or use.  This is why in general carbon containing compounds are called Organic compounds.

n   Hydrocarbons:  are compounds made of hydrogen and carbon

Predicting Reaction Products

n       In a single replacement reaction metals replace metals or hydrogen and nonmetals replace nonmetals. 

n          Example:  Cl2 + 2KI  ® 2KCl + I2

n        The activity series can be used to predict whether one metal will replace another

n        An activity series of metals is a listing that ranks metals according to their relative reactivity

n        A metal can replace only those metals below it on the list

 

Predicting Products

n      Double replacement reactions are likely to proceed if at least one of the products is a molecular compound, a precipitate, or a gas.

n       Solubility rules can be used to help determine whether a product will be a precipitate 

 

Solubility Rules:  for solubility in water

Compounds that contain these ions are generally soluble

n        Alkali metals and ammonium ions

n        Acetate ion (C2H3O2-)

n        Nitrate ion (NO3-)

n          Halide ions (X), except AgX, Hg2X2 and PbX2

n          Sulfate ion except SrSO4, BaSO4, and PbSO4

 

Compounds that contain these ions are generally insoluble

n          Carbonate ion (CO3-2) except with rule 1 ions

n          Chromate ion (CrO4-2) except with rule 1 ions

n          Phosphate ion (PO4 -2) except with rule 1 ions

n          Sulfide ion (S2-) (CaS, SrS, BaS and rule 1 exceptions are soluble)

n          Hydroxide ion (OH -2) (Ca(OH)2, Sr(OH)2, and Ba(OH)2 and rule 1 exceptions are soluble

Reaction Rate

n       Many reactions are reversible – once there are enough products, the products can change back into reactants

n       Chemical Equilibrium is the state in which the concentration of the reactants and products remain constant with time because the rate at which they are formed in each reaction equals the rate at which they are consumed in the opposite reaction (forward rate = reverse rate)

n        Le Chatelier’s Principle: Conditions affect equilibrium.  Thus different conditions will affect the relative amount of products that are formed.

Conditions that affect Rate

n       Concentration:  the amount of a substance

n        Adding a substance to a system at equilibrium causes the system to consume that substance

n   Add a reactant – increase forward rate

n   Add a product – increase reverse rate

n       Pressure: affects some gaseous system

n        Increase pressure – the reaction will shift in the direction that produces fewer molecules of gas

n        Example:  In the reaction below fewer gas molecules are produced by the forward reaction.  An increase in pressure will increase the rate of the forward reaction

                   2 NO2 (g)             N2O4 (g)            

Conditions that Affect Rate

n       Temperature:  to determine the affect of temperature you must know whether the reaction is endothermic or exothermic:

n        Exothermic reactions – give off heat

n        Endothermic reactions – absorb heat

n        If the forward reaction is exothermic the reverses is endothermic and vice versa

n   If the reaction is exothermic:  adding heat will drive the reverse reaction

n   If the reaction is endothermic, adding heat will drive the forward