ICP I Unit 6  Reactions

Reactions

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

 

® Means “yields”

 

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

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

            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

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

n            Balance atoms by using coefficients

n            Check work by counting atoms of each element

K + Br2 ® KBr

 

Before:  1 K and 2 Br

After:       1 K and 1 Br

 

2K + Br2 ® 2KBr

 

Before:  2 K and 2 Br

After:      2 K and 2 Br

Hints for Balancing Equations

n         Check for Diatomic molecules

n        H2            O2            N2            Cl2            Br2            I2            F2

n         Balance

n        Polyatomic ions    (if same poly. Ion on both sides ® balance as a chunk)

n        Metals

n        Nonmetals

n        “O” & “H”

n         Recheck your count – it can take several steps!

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

Practice:

n          Balance the following Equations

 

n             4Fe + 3O2 ® 2Fe2O3

 

n          C2H5OH + 3O2 ® 2CO2 + 3H2O

Classifying Chemical Reactions

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

      A + B ®  AB

n          S + O2 ®  SO2

n          CO2  + H2O ®  H2CO3

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

       AB ®  A  +  B

n        2H2O ®  2H2   + O2

n          CaCO3  ®  CaO + CO2

 

 

Classifying Chemical Reactions

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

      A + BX ®  AX + B

n        Mg + CuSO4 ®  MgSO4   + Cu

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

      AX + BY ®  AY + BX

n          CaCO3 + 2HCl ®  CaCl2 + H2CO3

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

Energy

n       Most chemical reactions involve changes in energy

n        Exothermic reactions:  release heat (Temperture ­)

n        Endothermic reactions:  absorb heat (Temperature ¯)

n       Most endo and exothermic reactions require and initial input of energy to get started.  This energy is called the Activation Energy. 

 

Acids                         Bases

Produce H+ ions in water (hydronium)

Formula starts with H

Sour taste

Break down metals

pH less than 7

Poisonous and corrosive to skin

 

 

Produce OH- ions in water (hydroxide)

Formula ends with OH

Bitter taste, slippery feel

Break down fats & oils

pH greater than 7

Poisonous and corrosive to skin

pH Scale

n      Numeric scale that shows the acidity or basicity (alkalinity) of substances.

 

0                                 7                       14

Strong Acid        Weak Acid Neutral          Weak Base        Strong Base

pH Indicators

n   Chemical substance that changes color in an acid or a base.

n   pH paper; paper strips that turn from red to green to purple as pH increases

n   Litmus paper:  starts red or blue

n   Base:      Blue stays blue and red turns blue

n   Acid:      Blue turns red and red stays red

n   Neutral:   Red stays red and blue stays blue