Chapter #7

Chapter #7Chemical Reactions

CHAPTER #7 CONTENTS• 7-1 Grade School Volcanoes, Cars, & Detergents• 7-2 Evidence of Chemical Reactions• 7-3 The Chemical Equation• 7-4 How to Write Balanced Chemical Equations• 7-10 Classification of Chemical Reactions• 7-9 Oxidation Reduction Reactions• 7-5 Aqueous Solutions and Solubility• 7-6 Precipitation Reactions• 7-7 Aqueous Reactions• 7-8 Acid/Base and Gas Evolution Reactions

7-2 Chemical Change Evidence

Chapter 6


Chapter 6

What is a clue that a chemical reaction has occurred?

a) The color changes.

b) A solid forms.

c) Bubbles are present.

d) A flame is produced.


Chapter 6


“Colorless hydrochloric acid is added to a red solution of cobalt(II) nitrate, turning the solution blue.”

a) The color changes.

b) A solid forms.




Chapter 6


“A solid forms when a solution of sodium dichromate is added to a solution of lead nitrate.”

a) A gas forms.

b) A solid forms.



Chapter 6

7-10 Chemical Reactions• A chemical equation is an abbreviated way to

show a chemical or physical change• A chemical change alters the physical and

chemical properties of a substance• Factors that indicate a chemical change

– Change in color– Temperature change– Change in odor– Change in taste (we do not taste chemicals)

• Reactions always contain an arrow that separates the reactants from the products

Reactants Products

7-10 Types of Chemical Reactions

• Combination reaction (synthesis)– Elements for reactants– Examples:

H2 + O2 H2O

N2 + H2 NH3

Al + O2 Al2O3

The Law of Conservation of matter, states matter cannot be created nor destroyed, the means equations must be balanced.


Balance the first equation

H2 + O2 H2O

Note two oxygen atoms on the reactant side and only one on the product side, therefore place a two in front of water



H2 + O2 2H2O


The two now doubles everything in water, thus 4 hydrogen and 2 oxygen. Now place a 2 in front of hydrogen.



2H2 + O2 2H2O


The two now doubles everything in water, thus 4 hydrogen and 2 oxygen. Now place a 2 in front of hydrogen.


Now balance the second equation

N2 + H2 NH3

Note two nigrogen atoms on the reactant side and only one on the product side.

Place a 2 in front of ammonia



N2 + H2 2NH3

Note two nitrogen atoms on the reactant side and only one on the product side.

Place a 2 in front of ammonia. This makes 2 nitrogen atoms and 6 hydrogen atoms. Now place a 3 in front of hydrogen to balance hydrogen atoms.



N2 + 3 H2 2NH3

Note two nitrogen atoms on the reactant side and only one on the product side.

Place a 2 in front of ammonia. This makes 2 nitrogen atoms and 6 hydrogen atoms. Now place a 3 in front of hydrogen to balance hydrogen atoms.


• Decomposition Reaction– Compounds form simpler compounds or

elements.– Examples

H2O H2 + O2


• Decomposition Reaction– Compounds form simpler compounds or

elements.– Examples

2H2O H2 + O2

7-10 Types of Chemical Reactions• Decomposition Reaction

– Compounds form simpler compounds or elements.

– Examples

2H2O 2H2 + O2

• Notice decomposition reactions are the opposite of combination reactions


Single Replacement reactions have an element and a compound for reactants.

Example:

Zn + HCl

How do we predict the products? Trade places with the metal or nonmetal with the metal or nonmetal in the compound



Example:

Zn + HCl

How do we predict the products? Trade places with the metal or nonmetal with the metal or nonmetal in the compound



Example:

Zn + HCl ZnCl + H

Now make the products stable. Slide with Clyde



Example:

Zn + HCl ZnCl2 + H2




Example:

Zn + HCl ZnCl2 + H2


Now Balance



Example:

Zn + 2HCl ZnCl2 + H2


Now Balance



Another Example:

Cl2 + MgBr2

How do we predict the products? Trade places with the metal or nonmetal with the metal or nonmetal in the compound. In this case we are trading nonmetals



Another Example:

Cl2 + MgBr2 Br + MgCl




Another Example:

Cl2 + MgBr2 Br2 + MgCl2



Double Replacement reactions contain compounds as reactants.

HCl + Ca(OH)2 CaCl + HOH

Check formulas, and slide with Clyde when necessary



HCl + Ca(OH)2 CaCl2 + HOH




2HCl + Ca(OH)2 CaCl2 + 2HOH


Now Balance!


Combustion Reactions occur when an element or compound combine with oxygen to produce oxides of each element.

H2 + O2

CH4 + O2

What is the oxide of hydrogen?



H2 + O2

CH4 + O2

What is the oxide of hydrogen? Water



H2 + O2 H2O

CH4 + O2


And the oxide of carbon?



H2 + O2 H2O

CH4 + O2 CO2 + H2O


And the oxide of carbon? Carbon dioxide



2H2 + O2 2H2O

CH4 + O2 CO2 + H2O

Now balance



2H2 + O2 2H2O

CH4 + O2 CO2 + 2H2O

Now balance



2H2 + O2 2H2O

CH4 + 2O2 CO2 + 2H2O

Now balance


REDOX reactions where the oxidation number changes from reactants to products.

Oxidation is when the oxidation number increases, by losing of electrons.

Reduction is when the oxidation number decreases by gaining electrons.

Consider the following equation:

H2 + O2 H2O

What are the oxidation numbers of hydrogen and oxygen?


REDOX reactions where the oxidation number changes from reactants to products.

Oxidation is when the oxidation number increases, by losing of electrons.

Reduction is when the oxidation number decreases by gaining electrons.

Consider the following equation:

H2 + O2 H2O

What are the oxidation numbers of hydrogen and oxygen?

0 0

7-9 REDOX REACTIONS

H2 + O2 H2O

How about hydrogen and oxygen in water?

00 2(1+) 2- = 0

7-9 REDOX REACTIONS

H2 + O2 H2O

How about hydrogen and oxygen in water?Oxidation is caused by the oxygen molecule,

so it is referred to as the oxidizing agent (OA)

Reduction is caused by the hydrogen molecule, so it is referred to as the reducing agent (RA)

00 2(1+) 2- = 0

reducedoxidized

7-9 REDOX REACTIONSNote:

• All of the previously discussed reactions are REDOX except the double replacement reactions.

• The number of electrons lost is equal to the number of electrons gained in a reaction. Why?

• Most elements have variable oxidation numbers, except for hydrogen, oxygen, and the memorized polyatomic ions.

7-9 REDOX REACTIONSOxidation numbers for a compound must

add up to equal zero, while the oxidation numbers for a polyatomic ion must up to equal the charge of that ion.

Consider the following chlorine compounds

HClO4, HClO3, HClO2, HClO, Cl2, HCl

What is the oxidation number of chlorine in each of these compounds, assuming

H+ and oxygen is 2-

1+ 4(2-)=0





What is the oxidation number of chlorine in each of these compounds, assuming H is 1+ and oxygen is 2-

1+ 4(2-)=07+






1+ 4(2-)=07+ 5+






1+ 4(2-)=07+ 5+ 3+






1+ 4(2-)=07+ 5+ 3+ 1+






1+ 4(2-)=07+ 5+ 3+ 1+ 0






1+ 4(2-)=07+ 5+ 3+ 1+ 0 1-

7-9 REDOX REACTIONS

How about sulfur in SO3 2-

3(2-)=2-

7-9 REDOX REACTIONS


How about carbon in C6H12O6

3(2-)=2-4+

12(1+)+6(2-)=0

7-9 REDOX REACTIONS


How about carbon in C6H12O6

3(2-)=2-4+

12(1+)+6(2-)=00 +

7-5 Aqueous Ionic CompoundsMost ionic compounds dissolve in water to produce solutions that conduct electricity. The degree to which a solution will conduct electricity is used to determine if a solute is a strong or weak electrolyte. Solutions that do not conduct electricity are called nonelectrolytes. Molecular compounds that dissolve in water to conduct electricity, are acids or bases. Strong acids/bases are referred to as strong electrolytes since they are good conductors of electricity. Weak acid solutions do not conduct electricity well and are called weak electrolytes

7-5 Aqueous Ionic CompoundsMost ionic compounds dissolve in water to produce solutions that conduct electricity. The degree to which a solution will conduct electricity is used to determine if a solute is a strong or weak electrolyte. Solutions that do not conduct electricity are called nonelectrolytes. Molecular compounds that dissolve in water to conduct electricity, are acids or bases. Strong acids/bases are referred to as strong electrolytes since there solutions are good conductors of electricity. Weak acid/base solutions do not conduct electricity well and are called weak electrolytes.

How do we tell if a solution conducts electricity?

7-5 Aqueous Ionic Compounds

Strong electrolyte Weak electrolyte Nonelectrolyte

7-5 Aqueous Ionic CompoundsSolutions are homogeneous mixtures of a solute and a solvent.

• The solute is the solution component in the smallest amount while the solvent is the larger component of a solution.

• Solutes whose solutions conduct electricity are called electrolytes

• Solutes whose solutions do not conduct electricity are called nonelectrolytes

• Electrolytes are solutes that form ions when they dissolve. Ionic solutes or acids usually form solutions that conduct electricity.

There are some more specific rules that allows us to better estimate the solubility of ionic compounds.

You will be given these if you need them.

7-5 Aqueous Ionic Compounds

7-6 Precipitation ReactionsUsing the solubility rules on the previous slide asign (s) and (aq) to the formulas for the following equation

NaCl(?) + AgNO3(?) AgCl(?) + NaNO3(?)


NaCl(aq) + AgNO3(?) AgCl(?) + NaNO3(?)


NaCl(aq) + AgNO3(aq) AgCl(?) + NaNO3(?)


NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(?)


NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)


NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)This is called a formula equation


NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)This is called a formula equation

When Ionic solids and acids dissolve in water ions are formed, and the aqueous formulas really do not exist. A more realistic equation would be the ionic equation.

7-6 Precipitation ReactionsNaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)

formula equationSeparating the aqueous substances into ions produces an ionic equation.

Na+(aq) + Cl-(aq) + Ag+(aq) AgCl(s) + Na+(aq) + NO3- (aq)

ionic equation

7-6 Precipitation ReactionsNaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)

formula equationSeparating the aqueous substances into ions produces an ionic equation.

Na+(aq)+Cl-(aq)+Ag+(aq)+NO3-(aq) AgCl(s)+Na+(aq)+NO3

-(aq)

ionic equationSubstances found on both sides of the equation are called spectator ions and are boxed above. Eliminating the spectator ions generates the net ionic equation.

Ag+ (aq) + Cl- (aq) AgCl (s)

Net ionic equation

7-6 Precipitation Reactions

It is possible for all of the reactants and products to be water soluble and thus produce all spectator ions. If this is the case then all of the ions cancel out and there is no net ionic equation. When this occurs then we say that there is No Reaction, and give the label NR.

7-6 Precipitation ReactionsFive Driving Forces Favor Chemical Change

1. Formation of a solid (Precipitate formation) 2. Formation of water (acid base reactions)3. Transfer of electrons (REDOX) reaction4. Formation of a gas (many different types)5. Formation of a weak electrolyte (formation weak

acids and bases)

7-6 Precipitation ReactionsFormation of water is a normal product between acids and bases. Since acids and bases dissolve in water to make solutions that are electrolytes, then we conclude that acids and bases have some ionic character. Since water does not ionize, then when water is formed, we will also have a net ionic equation and then a chemical reaction.

7-6 Precipitation Reactions

Strong acids and bases ionize 100%!

Memorized Strong acids and bases:

Acids BasesHCl (aq)HI (aq)HBr (aq)HNO3

H2SO4

HClO4

Hydroxides of group I and II metals, except Be and Mg

Acids undergo characteristic double replacement reactions with oxides, hydroxides, carbonates and bicarbonates.

2HCl (aq) + CuO (s) CuCl2 (aq) + H2O (l)

2HCl (aq) + Ca(OH)2 (aq) CaCl2 (aq) + 2H2O (l)

2HCl (aq) + CaCO3 (aq) CaCl2 (aq) + H2O (l) + CO2 (g)

2HC l (aq) + Sr(HCO3)2 (aq) SrCl2 (aq) + 2H2O (l) + 2CO2 (g)

7-7 Aqueous Reactions

Acids undergo characteristic double replacement reactions with oxides, hydroxides, carbonates and bicarbonates.

2HCl (aq) + CuO (s) CuCl2 (aq) + H2O (l)

2HCl (aq) + Ca(OH)2 (aq) CaCl2 (aq) + 2H2O (l)

2HCl(aq) +CaCO3(aq) CaCl2 (aq)+H2O (l) +CO2 (g)

2HC l(aq)+Sr(HCO3)2(aq)SrCl2 (aq)+2H2O(l)+ 2CO2

(g)

7-8 Acid/Base Gas Evolution

Bases undergo a double replacement reaction with acids called neutralization:

NaOH (aq) + HCl (aq) H2O (l) + NaC l (aq)

In words this well known reaction is often described as: “acid plus base = salt plus water”

We previously discussed this reaction when describing types of reactions.


We have discussed the double replacement reactions and ionic equations before. Since the acids and bases undergo double replacement reactions called neutralization reactions, then they can have ionic equations too.Formula equation:

HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

Total ionic equation:

H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) Na+ (aq) + Cl- (aq) + H2O (l)

Net ionic equation:

H+ (aq) + OH- (aq) H2O (l)


Another property of acids is their reaction with certain metals to produce hydrogen gas, H2 (g).

Zn (s) + 2HC l (aq) H2 (g) + ZnCl2 (aq)

This is an example of a single replacement reaction and is a redox reaction.

Total ionic equation:

Zn (s) + 2H+ (aq) + 2Cl- (aq) H2 (g) + Zn2+ (aq) + 2Cl- (aq)

Net ionic equation:

Zn (s) + 2H+ (aq) H2 (g) + Zn2+ (aq)


The End

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