Topic 9: Redox Processes

Question 1

What is oxidation?

Answer 1

Addition of oxygen to a substance/the loss of one or more electrons from a substance.

Question 2

What happens when something is oxidized?

Answer 2

Reduction - a gain of one or more electrons to a subtances

Question 3

How to write half equations

Answer 3

Separate oxidation (add electrons on product side to balance) and reduction (add electrons on reactant side to balance) reactions

Question 4

What does increase in oxidation state mean

Answer 4

Substance was oxidized

Question 5

Decrease in oxidation state

Answer 5

Substance was reduced

Question 6

Sum of all oxidation states in a compound = ?

Answer 6

0

Question 7

sum of all ox states in an ion = ?

Answer 7

the charge of the ion

Question 8

Lone elements oxidation state

Answer 8

0

Question 9

Oxygen oxidation state

Answer 9

-2 except when in H2O2 it is -1

Question 10

Hydrogen oxidation state

Answer 10

+1 except in metal hydrides (NaH) it is -1

Question 11

oxidizing agent

Answer 11

oxidizes another substance, is reduced itself, is usually in its highest oxidation state (oxidation state lowers when reduced)

Question 12

Reducing agents

Answer 12

readily reduces another substance (donates electrons), is oxidized itself

Question 13

Example of oxidizing agents

Answer 13

O2, Cl2, F2, SO3, (SO4 2- in solution), MnO4- and Cr2O7 2 -

Question 14

Example of reducing agents

Answer 14

H2, Na, C, CO, and SO2 (SO3 2- in solution)

Question 15

How to identify redox reaction?

Answer 15

Change in oxidation state - Note: reactions where an element is uncombined on one side of the equation and combined on the other side must be redox
reactions since there must be a change in oxidation state

Question 16

Balancing redox reactions for no change in oxygen

Answer 16

1. split into half equations
2. multiple so that both equations have equal amounts of electrons (so they can cancel out when added together)
3. put the two half equations together

Question 17

Balance redox reaction for a change in number of oxygen atoms

Answer 17

1. split into half equations
2. balance out oxygen with water on the other side, then use H+ to balance out the hydrogens on the other side
3. add up charges on both sides
4. add electrons to balance the charges
5. multiply both sides so that the electrons are equal on both sides
6. put together two half equations
7. simplify by reducing what is on both sides

(to make an equation basic add OH that will combine with H+ to get rid of them)

Question 18

How to deduce activity series for reducing agents? (more ready to lose?)

Answer 18

considering the reactivity of metal with water and acids, and the reaction of metals with the ions of other metals. The more readily a metal loses its outer electrons makes it more reactive.

Question 19

Why can only metals above hydrogen on the activity series react with acids?

Answer 19

Because they are able to displace the hydrogen ions.

Question 20

Activity series for oxidizing agents?

Answer 20

The more readily they gain electrons, the more reactive (the more electronegative the more reactive of an oxidizing agent it is)

Question 21

What is an application of a redox process

Answer 21

winklers method - used to measure BOD which is biological oxygen demand

Question 22

What is BOD

Answer 22

biological oxygen demand - measure of the amount of dissolved oxygen (ppm) needed to decompose organic matter in water biologically over a set time period (usually five days)

Question 23

What does a high BOD in polluted water mean?

Answer 23

Polluted water unable to replenish oxygen but also have a high BOD will not be able to sustain life.

Question 24

Describe winklers method

Answer 24

using ratios of sodium thiosulfate to iodide to mangenese (4) oxide to manganese (2) salt to oxygen –> find the concentration of dissolved oxygen in ppm

Question 25

what is ppm

Answer 25

parts per million = [mass of solute (g)/mass of solution (g)]*10^6

Question 26

What are the ratios in winklers method

Answer 26

1 O2 : 2 MnO2 and 1 MnO2 : 1 I2 and 1 I2 : 2 S2O3 2- and so 1 O2 : 4 S2O3 2-

Question 27

What is in a voltaic cell?

Answer 27

Two half cells - metals in contact with an aqueous solution of its own ions, half cells connected by a salt bridge (free movement of ions) and external wire, electrons flow from negative anode (oxidation) to positive cathode (reduction). Metal higher on the activity series will be oxidized (reducing agent)

Question 28

What is the convention for writing cells in voltaic cell?

Answer 28

Cell undergoing oxidation is on left, reduction on right metal undergoing oxidation/aqueous ion solution || aqueous ion solution/metal undergoing reduction

Question 29

What is anode

Answer 29

where oxidation occurs

Question 30

cathode

Answer 30

where reduction occurs

Question 31

Electrolytic cell

Answer 31

Electrolytic cells cause non-spontaneous redox reactions to occur by providing energy in the form of electricity from an external source (the opposite of a voltaic cell which is from spontaneous redox reactions) - positive anode and negative cathode - Occurs in the same tub of electrolytes (electrolytes conduct electricity when molten - chemically decomposed electrolytes - battery pumps electrons given in the molten decomposed electrolytes from the anode to the cathode

Question 32

Describe how current is conducted in an electrolytic cell

Answer 32

current is conducted by electrons in the wire and within, the ions are completing the circuit and allowing the current to flow.

Question 33

In a voltaic cell what happens to the mass of the anode electrode

Answer 33

the mass decreases because its undergoing oxidation and the oxidized ions join the solution

Question 34

In a voltaic cell what happens to the mass of the cathode electrode

Answer 34

The mass of the copper increases because the ions are being reduced into the atom.

Question 35

Is a higher or lower metal on the activity series oxidized

Answer 35

higher

Question 36

what does the salt bridge do in a voltaic cell, give examples of salts

Answer 36

KNO3, KCl, completes the circuit and keeps the half cell electrically neutral (giving them the ions they are giving away..?), negative ions migrate to anode and positive migrate to cathode, salt bridge is chosen so it doesnt interfere with electrolytes in half cells