Chemistry - Unit 14 - Redox Reactions

How to complete a redox reaction

1) Assign oxidation numbers. Use this to figure out what is oxidized and what is reduced.

2) Split the equation into two half equations

3) Balance equations in terms of mass

4) Balance equations in terms of charge/electrons

• Find the charge and add electrons to one side to neutralize this charge (for both half reactions)

5) Make both half reactions have the same number of electrons (added to one side) by multiplying and finding least common multiple.

6) Electrons now cancel (one on products side, one on reactants side) so half equations can be added with their new coefficents

Spectator

Ion or particle that does not lose/gain e- from reactants or products in a reaction

What is oxidation state (in terms of electrons)

Oxidation state, also known as oxidation number, describes the hypothetical charge an atom would have if all its bonds were fully ionic. In simpler terms, it represents the degree of oxidation, or the number of electrons an atom has effectively lost or gained in forming a chemical bond. A positive oxidation state indicates a loss of electrons (oxidation), while a negative oxidation state indicates a gain of electrons (reduction). 

What process involves the transfer of electrons?

The process that involves the transfer of electrons is called an oxidation-reduction reaction (also known as a redox reaction). In these reactions, one substance loses electrons (oxidation) while another substance gains electrons (reduction

The substance that is being oxidized is also the _________

The substance that is being reduced is also the ________

The substance that is being oxidized is also the reducing agent

The substance that is being reduced is also the oxidation agent

Reduction

Electrons are gained

Oxidation number decreases

Also the oxidizing agent

Oxidation

Electrons are lost

Oxidation number increases

Also the reducing agent the thing that gets oxidized is doing the reduction

If something has goes from low oxidation number to high oxidation number is it being reduced or oxidized? What about the opposite?

Low —> high = oxidized (electrons lost)

High —> low = reduction (electrons gained)

Voltaic Cell / Galvanic Cell (cathode/anode, mass)

An electrochemical cell that uses a spontaneous redox reaction to convert chemical energy to electrical energy

• Negative anode

• Positive cathode

• 2 chambers

• Salt bridge

Anode decreases in mass (electrons leaving), concentration of ions increases

• MORE active metal —> ready to loose electrons = ready to oxidize = AnOx

Cathode mass increase (electrons added), ion concentration decreases

• LESS active metal

Spontaneous Reactions (and how to tell they are spontaneous)

A reaction that does not require an external energy force in order to happen (happens on its own)

• ex: when a solid metal is more active than another metal in ionic compound (single replacement)

• How to tell —> Table J, if the metal that is replacing the other metal is higher up on the table (AKA: MORE reactive then what its replacing)

All Rules:

Element in elemental form

Monoatomic ion

Metals of group 1

Group 2

Fluorine and other Halogens

Hydrogen

Oxygen

The sum

Element in elemental form = 0

Monoatomic ion = charge

Metals of group 1 = +1

Group 2 = +2

Fluorine is always -1, the other halogens are also -1 when they are the most electronegative element in the compound

Hydrogen +1 w/ nonmetals but -1 w/ metals

Oxygen is -2 exception; with flourine and w: peroxide

The sum: sum must be zero, the sum of the oxidation number in Polyatomic ions must equal the charge of the ion

Electrolysis

When electricity is used to force a chemical reaction to occur.

—> Can be used to decompose compounds to obtain pure reactive elements

Ex: group 1,2,H2

Electrolytic Cell

Non spontaneous reaction that uses electricity to convert electrical energy to chemical energy.

• used a battery

• Anode is positive (lose electrons)

• Cathode is negative (gain electrons)

• NO salt bridge

• 1 chambers Salt

Anode (positive), looses e-

• decreases mass

Cathode (negative), gains e-

Increase mass

Electroplating

USING ELECTROLYTIC CELLS TO COAT A LAYER of metal on an object

—> object being plated is always the cathode

—> only covers the metal, nothing else

—> used in jewelry

Oxidation State:

The hypothetical charge that an atom would have in a molecule if bonds were 100% ionic 

• Used to track how many electrons are lost or gained during a reaction 

Identifying redox reactions

1) oxidation numbers for both sides, if change it is redox

Hints:

1) synthesis, decomposition, single replacement ALL REDOX

2) elemental form on one side and compound on the other side

OIL RIG

OIL RIG 

Oxidation Is Losing e

Reduction is Gaining e

THINK ABOUT THIS WHEN WRITING HALF EQUATIONS —> OXIDATION MEANS ELECTRONS ARE IN PRODUCTS AND VICE VERSA FOR REDUCTION

Oxidizing and reducing agent

Oxidizing Agent: COMPOUND that reduces another element (also the element that is being oxidized) 

• Must be on the left side (reactants) 

Reducing Agent: COMPOUND that oxidizes another element (also the element that is being reduced) 

• Must be on the left side (reactants)

Half reactions, what are they? (difference)

Every redox reaction is made up of two half-reactions: in one, electrons are lost (oxidation) and in the other, those electrons are gained (reduction process) 

LOST ELECTRONS = Oxidation Half = electrons are in products 

GAIN ELECTRONS = Reduction Half = electrons are in reactants

Redox Half Reaction Steps 

Process of Balancing charge in Redox reactions.

1 - Assign oxidation numbers. What is oxidized? What is reduced?

2- Split the whole equation into two half equations: red half + ox half

• ox half = electrons as products

• red half = electrons as reactants

3- Balance equations in terms of mass. Balance in terms of charge using electrons.

4 - Find least common multiple for electrons and multiple each half reaction so electrons equal least common multiple

5 - add the equations (electrons cancel but coefficents stay)

Half reactions Location of electrons

Oxidation:

Reduction :

Oxidation: electrons are a reactant

Reduction: electrons are a product

What metals are easiest to oxidize and why

Easiest to oxidize: Metals in group 1 and group 2 because they are easily giving away electrons, making them strong reducing agents. If they are good reducing agents they are good at oxidizing.   

Spectator

Spectator: Ion or particle that does not loose/gain electrons from reactant to products in a reaction 

• Can be omitted from half reactions and net equation 

Electrochemical cell:

Electrochemical cell: involves a chemical reaction (redox, single replacement) and a flow of electrons. Parts of an electrochemical cell include: two metal electrodes (anode and cathode), ion solutions, wires connecting the electrodes.

Voltaic Cell/Galvanic Cell

Voltaic Cell/Galvanic Cell

An electrochemical cell that uses a spontaneous redox reaction to convert chemical energy to electrical energy 

• Has a salt bridge, positive electrode is a cathode, negative electrode is anode, NO BATTERY because its spontaneous reaction

• Forces electrons through a specific path (wire) to use the flow of electrons as electricity  

AnOx RedCat (applies to both)

AnOx (applies to both) 

Anode = Oxidation = Losing electrons = oxidation state is more positive (losing electrons) 

RedCat (applies to both) 

Reduction = Cathode = Gaining electrons = oxidation state is more negative (gaining electrons) 

What is the purpose of the salt bridge

Provide mobile ions that can move between two half cells to keep the charge of the solution neutral (Ex: NaCl can split into ions Na and Cl, when and ion travels to the other half cell then on of the ions will replace it so its neutral)

The electrons always flow from: 

ANODE → CATHODE

Anode = oxidation half cell

Cathode = reduction half cell

In Voltaic/Galvanic… Why two half cells: 

In a negative electrode you have the oxidation half equation, the products are the ion and the electrons, these electrons are pulled to the cathode because…

• The product is the oxidized element but with a charge (lost electrons), the concentration of this thus increases 

• As oxidized the mass of metal anode decreases because its dissolving (the strip) into electrons and ions

In positive electrode you have the the reduction half equation which have electrons as a reactant (thus why the electrons are pulled to cathode) 

• The ions (what is being reduced) are being used (reactants) so the concentration of them decreases 

• As reduction happens the mass of the cathode increases because gaining of electrons means it turn into a solid metal, adhering to the cathode, increasing mass

Electrolytic

Electrolytic Cell: nonspontaneous reaction that uses electricity (uses battery b/c not spontaneous)  to convert electrical energy to chemical energy 

Electroplating:

Electroplating: using electrolytic cell to coat a layer of metal on an object using an electrolytic cell, object being plated is always the cathode 

• Battery pushes electrons to the cathode where they attach to

What is the battery’s purpose?

What is the battery’s purpose?

• Pulling electrons from anode to cathode

• This makes the neutral metal particles on the anode now ions which can dissolve in water

  • Neutral → ion 

• Electrons are being pushed into the cathode. This side has electrons so the floating ions that came from the anode (metal we are plating that lost an electron to the battery who stole it) want electrons to become neutral, they go over to the cathode, take an electron, and now they are neutral and solid.

What is added to the water to facilitate electroplating?

• Chemicals that dissipate into positive and negative ions

• Balance 

• Work as electrolytes → make the water conductive so the circuit is completed and the battery works.

Cathode and Anode (electroyltic) —> talk about mass

Cathode: RedCat = reduction at cathode, mass increases cathode 

• Negative electrode = Cathode 

• Where the metal we are plating goes (going to be covered in the other metal)

  • Mass increase 

Anode: AnOx = oxidation at anode, mass decreases

• Positive electrode = Anode

• Where the used metal is (in electroplating what we are transferring to random metal to plate) 

  • Mass decreases

Cathode and anode in voltaic v electrolytic

Voltaic cell - Anode is negative, cathode positive

Electrolytic cell - Anode is positive, cathode negative

NEUTRAL ATOMS MAKE UP SOLID METAL. METAL IONS USUALLY DISSOLVE IN WATER

THUS —> Electolytic cell works and elctorplating works

Electrolysis:

When electricity is used to force a chemical reaction to occur. Can be used to decompose compounds to obtain pure elements. 

• On table J the replacement metal is lower in reactivity then what it is replacing 

• Electroplating is a type of electrolysis 

Difference between Galvanic cell and electrolytic cell: 

• Galvanic cell is spontaneous and thus doesn't need a battery

• Electrolytic is non spontaneous and thus needs a battery 

• Galvanic cell has positive electrode of the cathode and negative electrode of the anode

• Electrolytic Cell has positive electrode as the anode and the negative electrode as the cathode

• Biggest difference: Galvanic = Chemical → electrical, Electrolytic = Electrical → Chemical 

• Salt bridge and two different chambers in voltaic V

• No salt bridge and connection to battery with one chamber for electrolytic