What is Electrochemistry?

What is Electrochemistry?

So in Chem 20 we looked at individual elements and drew electron dot diagrams for them.

Metal Non-Metal

Last we learned that when metals form compounds they lose electrons and when non-metals form compound they gain electrons.

The study of the transfer of electrons during chemical reactions is known as Electrochemistry.

UNIT B: ELECTROCHEMICAL CHANGECHAPTER 13 (P.556 – 609)

CHEM 30

Electrochemical (Electron Transfer) reactions are the most common type of reactions in both living and non-living systems.

Electrochemical reactions were discovered way before their was science to explain it.

What do you call chemical reactions before there is science to explain them?

Magic!

No, I’m kidding… Its sad, but true.

But seriously…I’m actually not.The Alchemists main goal was to turn common metals into gold and create a magical potion so they could live forever….

Alchemists and Early Science

So I know so far I have made the Alchemists out to be sort of…..nut jobs, and they sort of were….

But!

Even though the alchemists and other early scientists had somewhat…

“unusual” ideas, their contribution to our current understanding of how things work was important. Their empirical knowledge (things learned through mixing things and experimenting) helped our current understanding of chemistry.

“Technology Drove Science”

So in prehistoric times when people were learning how to turn metal ores (mixtures of different metals) into metals they could make things out of, they actually discovered (by accident) the science of metallurgy.

Metallurgy is the science of extracting pure metals from their naturally occurring compounds (ores) and adapting these metals for

Useful Purposes.

Tools Weapons

Primitive's….

http://www.youtube.com/watch?v=twcK2T6aeXY

MetallurgyEarly people learned that if they heated a pile of raw ore (mixture of different metals) in a fire it would reduce (get smaller) into a small amount of pure metal when it cooled.

Metal Ore

Heated In A Fire

Pure Metal

This is where the term Reduction comes from.

ReductionEarly people didn’t understand that the metal ores were reacting with something (usually gases) which caused them to be reduced.

Carbon MonoxideCharcoal (Carbon)

Hydrogen Gas

Understanding Oxidation and Reduction

Oxidation and Reduction were happening during early smelting and metallurgy, the people didn’t know the science, but that didn’t stop them from doing it.

It wasn’t until the 1700’s….more than 6500 years after the first copper was produced…that the science was around to explain oxygen’s role in burning and corrosion (rust).

6500 Years

OxidationIt was only now that scientists understood that when things interacted with oxygen they did the opposite of being reduced to pure, useable metals.

In general scientists noticed that combustion and corrosion were similar processes (both broke down substances, Both reacted with oxygen)

Through experimentation scientists found other gases that caused similar reactions. So they referred to all reactions that caused pure metals to turn back into compounds Oxidation.

OxidationOxidizing Agent: Any substance that causes a metal to oxidize and turn into a metal compound.

OxygenChlorine

Bromine

Oxidation and ReductionElectron Transfer Theory

This modern theory says that all chemical reactions can be broken in two pieces….2 ½ reactions.

When reactants combine to form products, one reactant gains electrons and one reactant loses electrons.

Electrons are transferred from one reactant to the other in equal amounts.

Lets Try One.

For example, when zinc metal is dropped in hydrochloric acid…what happens?

All those bubbles are hydrogen gas escaping. (Hydrogen’s flammable right…….)

So lets break this reaction into two pieces…..First, lets look at the zinc.

Second, look at the hydrogen.Zn(s) Zn2+(aq)

2H+(ag) H2(g)

+2e- Oxidation

Reduction

http://www.youtube.com/watch?v=HxiW2iPUZiw

Practice

Zn(s) Zn2+(aq)+ 2e- Oxidation

Pb2+(aq) Pb(s) Reduction

Metal Compound

Compound Metal

Its Just That Easy….

Redox Reactions Take Place In (aq)

Oxidation/Reduction(Redox) reactions take place in aqueous (liquid) environments.

Because reactions happen in water we have to include water in our balanced ½ reaction.

AND because water can easily be acidic or basic (just a few ions one way or the other and water isn’t neutral anymore) we balance the equation using the acid or basic ions.

In fact most chemical reactions need water to happen…there are very few chemical that can be put together as solids and react.

So what is the acidic ion? H3O+(ag)

Basic Ion? OH-(aq)

Hydronium

Hydroxide

Balancing Half Reactions In Acids & Bases

1HNO2(aq) NO(g)

2

O 2 1

+ H2O(l)

=

3 Balance hydrogen ions by adding (H+(aq)) to the

opposite side.

H+(aq) +


HNO2(aq) NO(g) O+ H2O(l)H+(aq) +

AKA….add e- to the same side as you added the H+

(aq).

e- +

But what if it happened in a Basic solution!??


HNO2(aq) NO(g) + H2O(l)H+(aq) +e- +

a basic

*** First 4 steps are exactly the same***

Add e- to the same side as you added the H+(aq).

Balance hydrogen ions by adding (H+(aq)) to the

opposite side.


HNO2(aq) NO(g) + H2O(l)H+(aq) +e- +

a basic

Add OH-(aq) to both sides equal to the number

of H+(aq) you added back in step 3.

OH-(aq) + /

Write This Down

Practice (Acidic)

Practice (Basic)

So we have seen so far when we looked at the Electron Transfer Theory that Redox reaction involve the transfer of electrons.

But we have also seen that when elements form compounds or ions they do it so they can be more stable…….So they don’t want to transfer electrons and undergo redox reactions.

So then why do they do it? Any ideas?

Tug of War For Electrons

The two players in a redox reaction compete with each other seeing who is strong enough to take the other players electrons.

If one of the players is strong enough to win the competition and take the others electrons a redox reaction occurs….if not, nothing happens.

Example:

Example:

Zn Cu

****Reduced: Gains electrons and its charge decreases (Less -) as a result.

Oxidized: Loses electrons and its charge increases (more +) as a result.****

So which one is oxidized and which one is reduced?

Oxidized

Oxidizing and Reducing Agents (RA & OA)

Reducing Agent (RA): The reactant that causes the other reactant to be reduced. (It actually gets oxidized)

Oxidizing Agent (OA): The reactant that causes the other reactant to be oxidized. (It gets reduced in the process)

RA & OA

Zn Cu

RA OA

Zinc gets oxidized from Zn(s) to Zn2+

(aq)….it causes Cu2+

(aq) to be reduced to Cu(s)……therefore it’s the RA.

Copper gets reduced from Cu2+(aq) to Cu(s)….it causes Zn(s) to be oxidized to Zn2+(aq)…therefore it’s the OA.

Recap of Oxidation/ReductionSo far we have learned that when a reaction occurs, one reactant gets their valence electrons taken away and gets oxidized. ANDThe other reactant takes the electrons and thus becomes more negatively charge and gets reduced.

The reactant that gets oxidized and thus causes the other reactant to become reduced is referred to as the Reducing Agent (RA). ANDThe other reactant that gets reduced and thus causes the other reactant to become oxidized is referred to as the Oxidizing Agent (OA).

So….

You put two reactants….one solid and one not then one will reduce and one will oxidized?

Copper(II) Nitrate….lots of copper ions.

ERRRRR. Wrong again…..Reactions are NOT always spontaneous.

So there’s the question. How do we know if the reaction will happen or not?

To Be or Not To Be…….

So to find out what combinations of solid reactants will react spontaneous (right away as soon as their mixed) and which will not…

We could just randomly mix different combinations of metals and solutions and see which ones happen spontaneously.

It would take forever….and why? Really, who has the time for that? And besides, somebody already did it for us…..so lets look at the Redox table in your text book and learn how to use it. That way we can predict without having to do all that work.And Remember! Its not guessing…I prefer to call it scientific guestimation…..sounds more scientific.

Spontaneous

Redox Table and Ranking RA’s and OA’s Ranked According to StrengthP. 569

So in this experiment scientists mixed a bunch of metals and metal ions together to see which combinations would be spontaneous and which would not.They just tried every possible combination and listed them according to which reactant work and caused a reaction the most times when combined with another reactant. (The same way as you rank a team in a round robin tournament in sports.

Metals/Ions

Ag+(aq) Cu2+

(aq) Pb2+(aq) Zn2+

(aq)

Ag(s)

Cu(s)

Pb(s)

Zn(s)

3

Redox Table and Ranking RA’s and OA’s Ranked According to StrengthP. 569

Metals/Ions

Ag+(aq) Cu2+

(aq) Pb2+(aq) Zn2+

(aq)

Ag(s)

Cu(s)

Pb(s)

Zn(s)

3

Strongest Oxidizing

Agent

Weakest Oxidizing

Agent

Strongest Reducing

Agent

Weakest Reducing

Agent

Redox Table and Ranking RA’s and OA’s Ranked According to StrengthTextbook Pg. 828 or Data Booklet Pg. 7

“Table of Selected Standard Electrode Potentials*”The table has all the possible redox reactions listed by relative strength.

Along the right hand side all the Electrical Potentials E° (V) are listed for each reaction as well…..

DON’T WORRY ABOUT THE E° (V), its used for something else….they just list them their because its convenient.

You will also notice that on the bottom “*For 1.0mol/L solutions at 298.15K (25.00 °C) and a pressure of 101.325 kPa”

Nice that they put it there…..BUT I HAVE NEVER USED IT FOR ANYTHING.

It actually give ½ STP and ½ SATP….How useless.

Decreasing

Strength

OA

Dec

reas

ing

Stre

ngth

RA

If OA is above RA on the table

SPONTANEOUS

SPONTANEOUS!

NOT SPONTANEOUS!

Predicting Redox Reactions

The first step to predicting a redox reaction is to list all the species (Atoms/Ions/H+/OH-) that are present (reactants only).

***REMEMBER to add H2O(l) as a species that’s present because it’s a solution…ALWAYS!

***Keep an eye out to see if the example is acidic or basic….if it is make sure to also include H+ or OH-.


Example

Au(s) HNO3(aq)

H+(aq) NO3-

(aq)

H+(aq)Au(s) NO3-

(aq)

H2O(l)

H2O(l)

Did we get everything?


Example

H+(aq)Au(s) NO3-

(aq) H2O(l)

Next, you use the Redox table to assign all of the entities as either an RA or an OA.

***Look for the entities in the table EXACTLY as they are in your question…SAME STATE.

RA

OA

RA


Example

H+(aq)Au(s) NO3-

(aq) H2O(l)

RA

OA

RA

Third, from your choice (SA/RA), choose the strongest SA and OA.

SOA

SRAFourth, write out the ½ reactions for the SOA and SRA exactly as they appear in the Redox table.

*****Remember to write the SRA Right To Left (Backwards)***

Fifth, balance the number of electrons so both ½ reactions have the same amount.You balance by multiplying the whole ½ reaction by a number in front.

2 X

Sixth, you rewrite the two equations using the balancing amounts and add the two equations together.

2H2O(l) O2(g) + 4H+(aq) +4e-


Example

2 X

4 NO3-(aq)+ 8H+

(aq)+ 4e- 2N2O4(g)+ 4H2O(l)

2H2O(l) O2(g) + 4H+(aq) +4e-

2H2O(l) O2(g) + 4H+(aq) +4e-

2H2O(l) O2(g) + 4H+(aq) +4e-

+4 NO3-

(aq) +8H+(aq) +2H2O(l) + 4e- 2N2O4(g) + 4H2O(l)+O2(g) + 4H+

(aq) +4e-

Last, we cancel out anything that shows up on both sides.

/ // /2

4 NO3-(aq) +8H+

(aq) 2N2O4(g) + 2H2O(l)+O2(g) + 4H+

(aq)


Practice


Practice

SOA

SRA


Practice

5 X1 X


Practice

Can A Reactant React/Redox With Itself?

If you have looked closely at your Redox table and practiced assigning RA and OA to species in a redox question...

Anyway, there ARE species that can act as either an OA or an RA, and in fact, react with themselves.

This is called Disproportionation.***If you run into a question where you find the SRA AND SOA are the same species, its ok...your not wrong!***Just treat it like any other Redox and follow the same 5 steps.

Which you all have….Right?

So lets set the stage for all this …

So when elements join together to form compounds electrons are transferred. e-

This transfer of electrons is what gives elements their + or – charge.

(p. 583)

Oxidation StatesThese + and – charges we have talked about

so far are known as FORMAL CHARGES.

Oxidation States are similar to this with assigning + and – charges, but that’s about where the similarity ends.

The + and – charges we assign for oxidation states ARE NOT the charge of the element from the periodic table!

Rules For Assigning Oxidation Numbers

Oxidation Numbers

*****Elements by themselves not combined with anything have an oxidation number of 0!!!!

Lets Try It

HClO4

+1

Periodic Table

0

Doesn’t Apply

-2

Exception! Because the halogen (Cl) is the last one left rule 5 doesn’t apply, because the rules are a hierarchy, meaning first come first serve, the first 4 rules cancel out number 5

Doesn’t Apply

So HClO4 is neutral (0) overall, so we add up what we have and Cl get the whatever oxidation number will work to make HClO4 neutral like its suppose to be

So 1 + charge, 8 – charges (-2)(4)…therefore Cl has to have a number +7 to make HClO4 neutral overall.

+7

Practice

CH4

MnO4-

Sodium Sulfate Na2SO4

***** Watch out for the overall charge of -1 here…

+1-4

-2+7

-2+1 +6

Oxidation and Reduction Reactions

Oxidation: When the charge of an element increases during a reaction….its charge becomes more positive (+).

+3-----+5

-3 ------ -1

-2 ------ +2

Reduction: When the charge of an element decreases during a reaction…its charge becomes more negative (-)

+4 ------ +2

+3 ----- -1

-2 ------ -5

(P. 559)

Oxidizing Agent: The element in a chemical reaction that gets reduced, causes the other element to be oxidized.

Reducing Agent: The element in a chemical reaction that gets oxidized, causes the other element to be reduced.

-2+1 0 -2

+1

H: +1 --- 0

Reduction

Oxidation

Reduction

Reduction

Oxidation

0 +2Yes0 -1

Yes

No

Yes

-2+1+3

+1

+5

+5

Balancing ½ Reactions In Acids or Bases

We’ve been assigning oxidation numbers and putting in e- to balance them so far.

Remember, For any chemical reaction you can do TWO ½ reactions….one for the Oxidation (becomes more +) and one for the Reduction (more -).

Lets Refresh Our Memories

What Ion is present in an acidic solution? What is the acidic Ion

What Ion is present in a basic solution? What is the basic Ion

H+ or H3O+

OH-

Cr2O72- Cr3+

-2+6 3+

7 OxygensSo add 7 H2O to the opposite side.

7 x 2 = 14So add 14 H+ to this side.WHAT!? Where did these H+ come from?

What!? Where did these H2O come from?

Acidic Solution

So the first FOUR steps to balance all ½ reactions is the same 4 steps. SO, try it out, try using the rules to balance these first four reactions.There’s two of each type (acid and base). So if you do the four steps for an all the solutions, the acid ones are done, but there are 2 more steps to add on to the base one, so just leave some room on these to do 2 more steps later.

Now Balancing in Basic Solutions

So like I said the first 4 steps are the same for acidic and basic solutions.

So in your notes there is an example of a just basic solution balancing. (All 6 steps)

Step 5: Add OH- to both sides of the equation to equal the H+ you added in step 3.

Step 6: H+ + OH- = H2O So you cancel out as many as possible if H2O is on both sides you cancel it out.

Lets Complete One We already Started.

Step 1: Use Coefficients to balance everything but H and O.

Cu2O(aq) Cu(s)2Step 2: Balance O by adding H2O to opposite side.

+H2O

Step 3: Balance H by adding H+ to opposite side.

2H++

Step 4: Add e- to more positive side to even out the charge on both sides.

2e-+

Lets Complete One We already Started.

Cu2O(aq) Cu(s)2 +H2O2H++2e-+Step 5: Add OH- to both sides…as many as there is H+

2OH-+ +2OH-

Step 6: Combine H+ and OH- to make waters, cancel out if on both sides.

2H2O/

/

Cu2O(aq)+2e-+H20 (l) 2Cu(s)+2OH-(aq)

BACK

So far we have used stoichiometry ½ a dozen ways to find out amounts of everything from a mass (g) to a concentration of ions.

Well, its sort of like Lord Voldemort.

Its Back!

But luckily, it still works exactly the same…still uses (R/G).

But just like before, You need a balanced chemical equation!

Make sure you have a balanced chemical equation first!Make sure, you have a balanced chemical equation!

Write the information you know under the species in the balanced chemical equation.

A little Stoichiometry Review/A Few Reminders.

VnC

Mmn

A potassium permanganate solution is titrated with a solution of acidified iron(II) sulfate solution, It takes 16.7ml of potassium permanganate before the 10ml sample of acidified iron(II) sulfate is neutralized. What is the concentration of the potassium permanganate solution? What is the mass of the potassium dissolved? HINT: K=MnO4

The hard part is making the balanced ionic equation.

So follow the 5 step method and do all the work listing ions and SOA’s and ROA’s and balancing, and kabam!

A potassium permanganate solution is titrated with a solution of acidified iron(II) sulfate solution, It takes 16.7ml of potassium permanganate before the 10ml sample of 0.450mol/L acidified iron(II) sulfate is neutralized. What is the concentration of the potassium permanganate solution? What is the mass of the potassium permanganate dissolved?

V= 0.016L V= 0.010LC= 0.450mol/Ln=?C=?

n=?Mols X (R/G)/(1/5)0.28125mol/L

~ 0.28mol/L

Documents

What is Electrochemistry?