Unit 4 Electrochemistry Day 1
Electrochemistry
- the interconnection between ________________ and _________________ reactions
Oxidation – Reduction Reactions
Oxidation – a process in which chemical entities _______________________ ex. ___________________________________
Reduction – a process in which chemical entities ________________________ ex. ___________________________________
Memory Aids
LEO the lion say GER
Losing Electrons: Oxidation Gaining Electrons: Reduction OIL RIG
Oxidation Is Loss Reduction Is Gain
Redox Reaction – a reaction in which one reactant is ______________ and the other reactant is ________________
_________________________________________________________________ In this reaction Zn ____________ electrons ∴ ____________________
Cu ___________ electrons ______________∴
To identify the oxidized and reduced reactants in a redox reaction: 1. write the total ionic equation
2. write the net ionic equation
3. assign “0” charge to uncombined elements
4. compare charges to identify loss/gain of electrons or
1. if reaction is between two elements (no ions), assign charge of “0” to elements 2. determine ion charges in compound
ex. Identify the oxidized and reduced reactants in the following reaction
Recall that the aqueous reactants and products are ions in solution. Step 1 – total ionic equation
Step 2 – net ionic equation
Step 3 – assign charges
Step 4 – compare charges
ex. Identify the oxidized and reduced reactants in the following reaction
No ions formed so use second method
Step 1 – assign charges to elements
Step 2 – assign charges in compound
Step 3 – compare charges
Homework p.376 a,c,e,g
Practice Oxidation/Reduction Use p.374-376 to help
p.377 #1,2,3a,c,e,g,5
Day 2
The redox reactions studied up to this point all involve a _____________with a
___________________, also known as ___________compounds. Many redox reactions do involve non-metals that ______________electrons
ex. sulfur dioxide can produce acid rain if released into the atmosphere ______________________________________________________
Oxidation Numbers
Non-metals will _____________electrons but often it is an _________________sharing because one element is more ______________________ (stronger pull on electrons) than the other.
Consider carbon dioxide – CO2
ex. #1 Determine the oxidation number of carbon and oxygen in CO2
- according to rule #4, oxygen is always ___________ - there is _____ carbon and ____ oxygens
- the sum of the oxidation numbers must equal ________ for a compound
the oxidation number for carbon is ________ and oxygen is
∴ _______
ex. #2 Determine the oxidation number of the nitrogen atom in lithium nitrate, LiNO3.
the oxidation number for nitrogen is __________ ∴
Identifying Redox Reactions Using Oxidation Numbers Day 3
Perhaps one of the most useful applications of oxidation numbers is to determine if a redox reaction has occurred or not. In order for a redox reaction to have occurred, there must have been…
• an oxidation number ___________ (reduction) • an oxidation number ___________(oxidation)
ex. #1 – Use oxidation numbers to show that the reaction of zinc metal with sulfur is a redox reaction. The chemical equation is…
__________________________________________ Write all known oxidation numbers
zinc oxidation number changes from _________________________ sulfur oxidation number changes from _______________________
there _____________________ in oxidation numbers so this
∴ is a ____________________
ex. #2 – Is the following reaction of sulfur trioxide with water a redox reaction? ______________________________________________________
Write all known oxidation numbers
Solve for unknown S
there is ______________________ in oxidation numbers for any of the elements so this is ∴
__________________________________________
Which of the following equations represent redox reactions? Which do not represent redox reactions? Prove your answer with oxidation numbers.
a. H2(g) + Cl2(g) 2HCl(g).
b. CaCO3(s) CaO(s) + CO2(g)
c. 2H2O(l) 2H2(g) + O2(g)
d. 2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)
e. Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)
The Activity Series Day 4
The _______________________________________
- a list of ____________ arranged in order of their tendency to react (become _______________), based on observations gathered from _______________________________ reactions Recall that a single displacement reaction involves one metal
_____________________ another metal.
The _________________________________ (the most easily oxidized) are at the top of the list, the ___________________________ are at the bottom
ex. the metals at the top are so reactive that they even react with
relatively unreactive substances like _______________, where metals like _________ _____________ _________________are unreactive even in the strongest acids
Predicting Chemical Reactions
- if a metal is found __________ in the activity series it will _______________ a metal that is ___________
- the lower metal is always left as a pure metal
ex. Use the activity series to predict whether a chemical reaction will occur and write the balanced chemical equation
a) aluminum foil is placed in a solution of silver nitrate (AgNO3)
b) copper is placed in a solution of iron (II) sulfate (FeSO4)
c) zinc metal is placed in a solution of hydrochloric acid (HCl)
Homework
Galvanic Cells Day 5
How do batteries work?
- next couple lessons will cover this
Converting Chemical Energy to Electrical Energy Consider the redox reaction
Zn becomes __________, so zinc __________________ (_____________________)
___________ becomes ___________, so copper ____________________ (_____________)
- by separating the ___________and _____________ ions by placing a metal wire between them, electrons that are ____________ by the __________ are forced to travel through the metal wire to the _______________ solution
- moving electrons have _______________________
- __________________ can be used to power a device such as a radio or a clock
Galvanic Cell
- a ___________________ converts _______________energy from __________ reactions into __________________energy
- batteries contain ______________________cells
- these reactions are _____________________________, meaning they require no outside assistance or ___________________________
- the __________________________ of one metal and the ____________________ of another metal are in separate compartments called _______________________ - each half–cell contains a solid conductor called an ______________________
- the ____________________ consists of a strip of zinc in a zinc solution
- the ____________________ consists of a strip of copper in a copper solution - the half–cells are connected using a _____________and a _________________
- a salt bridge is a tube that contains a concentrated solution of an ________________ that replaces _______________ in solution (salt bridges discussed more later)
Galvanic cell song:
http://chemistsk027.blogspot.ca/2010_12_01_archive.html
Cell Reactions
Consider the zinc/copper galvanic cell
- zinc is higher on the activity series so it is oxidized
Anode half–reaction: ___________________________________________ Cathode half–reaction: ___________________________________________
- atoms from the ____________ electrode _______ electrons and dissolve into solution as _________
as the cell operates, the mass of zinc electrode
∴ _________________, and the mass of copper electrode ______________________
The overall reaction for the zinc/copper galvanic cell is the ______________________ of the two half reactions
Anode half–reaction: Cathode half–reaction:
Overall cell reaction
ex. Write the anode, cathode, and overall cell reactions that occur when each pair of half– reactions is combined to form a galvanic cell
#1 – A _____________ strip in a solution of copper (I) nitrate, _____________ (aq), and a tin
strip in a solution of _______________________, SnCl2 (aq)
- tin is ____________ on the activity series so it is _____________________
Anode half–reaction: Cathode half–reaction:
Overall cell reaction
#2 – An aluminum strip is a solution of aluminum nitrate, ________________ (aq), and a silver
strip in a solution of ____________________________, AgNO3 (aq)
- __________________ is higher on the activity series so it is _______________
Anode half–reaction: Cathode half–reaction:
* Balance number of electrons * Anode half–reaction:
Cathode half–reaction: Overall cell reaction
Purpose of the Salt Bridge
- the salt bridge provides ___________ to prevent charge buildup from occurring around the ________________
- basically it will replace ions being removed from the solution to maintain a ____________________________________________
- ∴ removing the salt bridge will stop the flow of electrons and has the same effect as ______________________________________
Cell Potential (Voltage)
- a measure of the ______________________ across the _______________________ - it is measured in ______________ ex. 9V battery has 9V of difference between the
electrodes
- as the galvanic cell operates, the potential difference gradually ______________until it becomes ____, meaning the electrons are no longer ____________and the cell is ‘dead’
Batteries Day 6
Many products today require mobile, long-lasting, and lightweight power sources ex. ipod, cell phone, watches, remote controls, key fobs
Battery
- a battery is a group of two or more galvanic cells connected in series to each other - many different types and sizes available now
- becoming more specialized
- never touch a leaking battery, they are very corrosive to your skin
How they are made: Outside
- outside casing protects us from harmful chemicals and provides information about the type of battery it is and the voltage it can produce
- both the positive and negative terminals of the battery are exposed
- the size of the battery determines the amount of chemicals it can hold and how long it will last ex. D batteries will last longer than AA batteries
Inside
- 3 basic parts, two electrodes (an anode and a cathode), and an electrolyte paste - in the center is a brass pin surrounded by an anode of powdered zinc
- the cathode chemicals of manganese dioxide MnO2 and carbon surround the anode
- a thin fabric separates the two and the electrolyte paste allows ions to move back and forth between the two
Cell Reactions
- electrons only moving when switch is on - batteries can last up to 5 years and still work
- electrons from oxidation of zinc travel through pin, out negative terminal of battery, through electrical device and back into positive terminal of battery
Primary and Secondary Cells
Primary cells – cannot be recharged, generally short life span ex. all alkaline dry cell batteries, lithium cells in cell phones
Secondary cells – can be recharged, difficult to dispose of because metals are very toxic ex. nickel-cadmium cells
Corrosion Day 7
Corrosion
- the deterioration of a metal as a result of slow oxidation
- without protection from corrosion, most metals are oxidized by their environment - some metals protect themselves by forming a protective layer of corroded metal on
the surface but the metal below remains intact
ex. the copper roof on the parliament forms a layer of corroded copper that is green in colour, as opposed to the normal reddish brown colour of copper. This layer of oxidized copper protects the other layers of copper below and can last well over 50 years
zinc also forms a protective layer, which explains why galvanized (meaning zinc covered) substances, like fences last much longer than iron fences
aluminum is another metal that forms a protective layer, Al2O3, one of the hardest
compounds known which gives aluminum products incredibly long lifespans Rusting
- the reddish-brown flaky material that is produced when iron-containing metals, like steel corrode
- rust is a mixture of oxides and hydroxides
- rust does not stick to the layer of metal below it and flakes off exposing the layer below it
- rust is a redox reaction where oxidation and reduction are happening on the same surface of metal
- the anode is located where the metal has been dented or scratched - the cathode can be almost anywhere
- the presence of moisture (water) and oxygen help iron releases 2 electrons - rusting is just like a galvanic cell
Factors that Affect Rusting Moisture
- a relative humidity of at least 40% is required for corrosion to take place Electrolytes
- salts do not cause corrosion but they help
- they provide a conductive solution to move the electrons and act like a salt bridge to prevent the buildup of ions at the cathode and anode
Contact with Less Reactive Metals
- corrosion can occur when 2 different metals come in contact with each other
- more reactive metals (higher on activity series) lose electrons to less reactive metals which starts the corrosion process
- this is why it is important to make sure the proper nails, rivets, or screws are used Mechanic Stresses
- bending, shaping, or cutting metals can introduce stresses in the metal that could become corrosion sites
Homework p. 416 #1-6
Preventing Corrosion
- sometimes corrosion of a metal surface is fine ex. a fence
- other times it can be disastrous ex. oil pipeline, bridge, or artificial bone replacement
Methods to Prevent Corrosion Protective Coatings
- cover the metal with a rust-inhibiting paint
- simplest, and one of the cheapest ways to prevent corrosion
- some problems are that it only works for aboveground structures and the whole surface has to be covered completely or a chip or scratch in the coating could be a location for corrosion to begin and could extend under the paint without even knowing
- galvanize the metal by coating it with zinc
- this can be done by dipping the steel in molten zinc or by electroplating (will look at tomorrow)
Corrosion-Resistant Metals
- removing undesirable impurities and adding corrosion-resistant elements that usually makes the metal stronger
- these mixtures of metals are called alloys and have specific desirable properties like strength, and unreactivity
- by making alloys with aluminum and magnesium, they can form protective oxide layers and be corrosion-resistant
Cathodic Protection
- forces the metal object you wish to protect into the cathode of a corrosion cell - 2 ways
Sacrificial Anode
- involves covering the surface of a metal with another one that is more likely to corrode, like zinc, so even if the metal becomes scratched, it is only the zinc that corrodes not the iron underneath
- electrons are sent to the iron and it remains a cathode and does not corrode - gas stations do this by connecting their gas tanks to magnesium blocks Impressed Current
- involves pumping electrons into the metal object to turn it into a cathode, this is done by connecting it to a power source
