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Chemical Reactions

+Chemical Reactions

n  Officially: n  The changing of substances into different substances by the

breaking of bonds in the reactants and the formation of bonds to form products

n  We represent chemical reactions with equations n  Basic format for equations in Chemistry:

Reactants à Products

A + B à C + D

+The importance of an equation

n  Chemical reactions are summarized with the chemical equation

n  Combination of formulas, symbols and other notations explain how a reaction will proceed.

n  The equation must be balanced!

n  To be balanced, it must obey the law of conservation of mass n  Simply states that “mass cannot be created nor destroyed”

2H2 + O2 à 2H2O REACTANTS PRODUCTS

+Why use an equation?

n  Again, a chemical equation is a representation of a chemical reaction

n  The formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right)

2H2 + O2 à 2H2O

+Coefficients

n  Coefficients indicate the number of each substance in a reaction

C6H12O6 + 6O2 à 6CO2 + 6H2O

Coefficients

n  ONLY coefficients are changed to balance an equation!!!

+Subscripts

n  Subscripts are used to write the formula for a substance.

C6H12O6 + 6O2 à 6CO2 + 6H2O

Subscripts

n  If you change a subscript, a new substance is produced (you have

changed a reaction). You CAN’T change subscripts when balancing an equation!!!

+Catalysts

n  A catalyst is a substance that speeds up the reaction but is not used up in the reaction.

Without Catalyst With Catalyst

+Skeleton Equation

n  Does not indicate the relative amounts of the reactants and products involved in the reaction n  It is UNBALANCED

n  For example, the chemical equation for iron rusting is

Fe + O2 à Fe2O3

n  The amount of iron and oxygen are not equal on both sides.

n  This violates the Law of Conservation of Mass n  Mass of products MUST equal the mass of the reactants.

n  This is why this is a skeleton equation, because it is bare bones!

+Word Equations

n  A description of a chemical reaction

n  Uses the names of the reactants and products

n  Example: n  Rusting is the chemical reaction of iron and oxygen to form iron (III)

oxide n  Iron + Oxygen à Iron (III) Oxide n  Fe + O2 à Fe2O3

n  When methane is burned (reacts with oxygen) it produces carbon dioxide and water n  Methane + Oxygen à Carbon Dioxide + Water n  CH4 + O2 à CO2 + H2O

+Diatomic Elements

n  Remember these from Covalent Bonding?!

n  Diatomic Elements: n  Seven elements that, when in their pure form will bond in pairs. n  More stable in pairs than alone

n  When you read one of the diatomic elements in word equations, you must remember that they exist as a pair!

n  Example: n  Rusting is the chemical reaction of iron and oxygen to form iron (III)

oxide n  Iron + Oxygen à Iron (III) Oxide n  Fe + O2 à Fe2O3

+Skeleton Equation Examples:

n  Sodium metal reacts with water to produce aqueous sodium hydroxide and hydrogen gas

Na(s) + H2O(l) à NaOH(aq) + H2(g)

n  Aqueous hydrochloric acid will react with solid sodium carbonate to form carbon dioxide, water, and aqueous sodium chloride

HCl(aq) + NaHCO3(s) à CO2(g) + H2O(l) + NaCl(aq)

+Balanced Chemical Equations

n  Represents a chemical reaction that rearranges atoms but does not create or destroy them

C6H12O6 + 6O2 à 6CO2 + 6H2O

n  For each element, the number of atoms on the reactant side must equal the number of atoms on the product side

C H O

Reactant 6 12 18

Product 6 12 18

+Steps to Balance Equations

1.  Write out the skeleton equation n  Do NOT change the chemical formulas in the equation… EVER

2.  Insert coefficients to change numbers of atoms on each side through multiplication

3.  Maintain a list of elements on each side to check the number of atoms as you adjust

4.  Double check your work when finished

+Balance the following…

n  Pb(NO3)2 + Na à NaNO3 + Pb Reactants Products

Pb

NO3

Na

n  KClO3 à KCl + O2

Reactants Products K

Cl

O

+Balance the Following …

n  KNO3 à KNO2 + O2

Reactants Products

K

N

O