Chemistry SM-1232Week 10 Lesson 3

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademySpring 2008

Class Today• No class Friday• Tests are graded• Quizes are not yet graded. I’ll have them sometime this week.• Only 1 wiki even started!!! GET ON IT! At least start the page.• Quote• Chapter 16- just two days, today and Wednesday• Quiz on the Wiki on Friday• Start Chapter 18 on Monday, Test next Wednesday

Sea Fever• I must down to the seas again, to the lonely sea and the sky,

And all I ask is a tall ship and a star to steer her by,And the wheel's kick and the wind's song and the white sail's shaking,And a grey mist on the sea's face, and a grey dawn breaking.

I must down to the seas again, for the call of the running tideIs a wild call and a clear call that may not be denied;And all I ask is a windy day with the white clouds flying,And the flung spray and the blown spume, and the sea-gulls crying.

I must down to the seas again, to the vagrant gypsy life,To the gull's way and the whale's way where the wind's like a whetted knife;And all I ask is a merry yarn from a laughing fellow-roverAnd quiet sleep and a sweet dream when the long trick's over.

By John Masefield (1878-1967).(English Poet Laureate, 1930-1967.)

Oxidation

• 2 H2 + O2 2H2O

• 4 Fe(s) + 3O2 2 Fe2O3

• CH4 + O2 CO2 + H2O

• Each chemical reacting with oxygen ends up with oxygen in the formula. Since they gain oxygen we call it an oxidation or an oxidizing reaction.

Reduction

• 2 H2O 2H2 + O2

• 2 Fe2O3(s) 4Fe + 3O2

• CO2 + H2O CH4 + 2 O2

• The reaction produces oxygen in each case. When the reaction loses oxygen it’s called a reduction.

Oxydation does not need oxygen

• 4 Na + O2 2Na2O

• 2 Na + Cl2 2 NaCl

• Chemists consider both to be oxidation reactions

A closer look

• 4 Na + O2 2Na2O

• Each Na is losing an electron• 4Na 4Na+ + 4e-

• Oxygen is gaining two electrons• O2 + 4e- 2O2-

similarly

• 2 Na + Cl2 2 NaCl

• Each Na is losing an electron• 2Na 2Na+ + 2e-

• Chlorine is gaining one electrons• Cl2 + 2e- 2Cl-

You try

• Mg + Cl2 MgCl2

• 2Ca + O2 CaO

Simpler definition

• LeoGer• Aka Leo the Lion goes Ger• Lose electrons = oxidation• Gain Electrons = reduction

Redox reactions happen simultaneously

• As an oxidation occurs to lose electrons a reduction is happen to gain electrons.

• The substance that is oxidized is called the reducing agent because it causes the reduction of the other substance.

• The substance that is reduced is called the oxidizing agent because it causes the oxidation of the other substance.

Labels Applied

• 2H2 + O2 2H2O

• H2 is the reducing agent because it’s reducing oxygen.

• O2 is called the oxidizing agent because it’s oxidizing H2.

You try

• Write the reactions and label which is the oxidizing agent and which is the reducing agent.

• 2Mg + O2 MgO

• Fe + Cl2 FeCl2

• Zn(s) + Fe+2 Zn+2 + Fe(s)

Oxydation State: keeping track of electrons

• C + 2S CS2

• Let’s figure out where the electrons are!

Rules!• 1. Pure elements have an oxydation state of 0• 2. Any charged ion has an oxydation state equal to its

charge• 3. If a compound is neutral the sum of all oxydation

states equals 0• 4. If a compound is charged the sum of all oxydation

states equals the charge• 5. Group 1,2,3 will always be +1,+2,+3• 6. Non metals get oxidation numbers on the next

slide.

More Rules: Oxidation numbers

• This is list is also in order of precedence! • Fluorine oxidation is always -1• Hydrogen is +1• Oxygen is -2• Group 7 is -1• Group 6 is -2• Group 5 is -3

Examples

• Br2

• K+• LiF• CO2

• SO42-

• Na2O2

Back to Carbon and Sulfur

• C + 2S CS2

• What are the oxidation states?• What gets oxidized?• What gets reduced?• Which is the oxidizing agent?• Which is the reducing agent?

Balancing Redox Equations

• Al(s) + Ag+(s) Al3(aq)+ + Ag(s)• What are the oxidation numbers of each one

of these?• Let’s break this down into the oxidation and

the reduction!

Oxidation

• Al Al+3

• What’s the oxidation state of Al? Al+3

• This isn’t balanced!• We’re missing the electrons!

• Al Al+3 + 3e-

• Now we are balanced!

Reduction

• Ag+ Ag(s)• What’s the oxidation state of Ag(s)? Ag+?• This isn’t balanced!• We’re missing the electrons!

• Ag + + e- Ag(s)

Let’s look at them together

• Al(s) Al+3 + 3e-

• 1 e- + Ag+ Ag(s)

• How many times must Ag+ be reduced in order for 1 Al(s) be oxidized?

3 Times!

• Al(S) Al+3 + 3 e-

• 3(1e- + Ag+ Ag(s))

• Al(s) Al+3 + 3e-

• 3e- + 3 Ag+ 3 Ag(s)• If we add them the electrons can cancel out.

• Al(s) + 3 Ag+ 3 Ag(s) + Al+3

You Try

• Al(s) + Cu+2 Al+3 + Cu(s)

• We’re looking to have charge and atom balanced!

Still Time?

• I- + Cr2O7-2 Cr3+ + I2(s)

Read all of chapter 16

• Work on your wikis• Work on your homework• Homework due Monday• Take Home Quiz on Friday Too• Review Monday- Start Chapter 18• Quiz on Wednesday.