chemistry U3 T2

Question 1

What is a redox reaction?

Answer 1

the transfer of electrons from one reactant to another

Question 2

What is oxidation and reduction?

Answer 2

OIL RIG, oxidation is loss of electrons, reduction is gain of electrons

Question 3

Oxidation number for: elements in free form? Oxygen? Hydrogen?

Answer 3

Elements = 0
Oxygen = -2 unless in peroxides (e.g., BaO2) where =-1
Hydrogen = 1 except in metal hydrides (metal w/ hydrogen) where its = -1

Question 4

Definition; oxidising agent? Reducing agent?

Answer 4

O.A. - reactant that causes another reactant to lose electrons and be oxidised and is itself reduced.
R.A. - reactant that causes another reactant to gain electrons and be reduced and is itself oxidised.

Question 5

List the reactions that can model redox (3)?

Answer 5

1. Displacement reactions - single and double
2. Combustion reactions - A + O2= B + D/ CO2 + H2O
3. Corrosion reaction the degradation of a metal to form a more stable metal oxide when exposed to gases and liquids

Question 6

How do you balance half redox equations? (3)

Answer 6

1. Set out reactant and product with oxidation no.
2. Balance no. of atoms for the element which is oxidised or reduced.
3. Calc change in oxidation no. for the elements. If there is an increase in oxidation no., add an equal no. of electrons to the right-hand side. If there is a decrease in oxidation no., add an equal no. of electrons to the left-hand side. If more than one atom is oxidised or reduced, the no. of electrons added to the half equation equals the no. of atoms of the element added to the half equation equals the no. od atoms of the element multiplied by the change in oxidation no.s

Question 7

How do you balance overall redox equations? (5)

Answer 7

1. Break the overall equation into 2 half equations, one represents the oxidation and the other reduction.
2. Balance the equations
3. Multiply each half equation by a no. chosen so that the number of electrons lost in the oxidation half equation equals the no. gained in the reduction half equation.
4. Add the two half equations resulting from the multiplications and cancel out the electrons.
5. Write a balanced equation for the reaction

Question 8

When do metals (s/aq) react with each other due to reactivity? What happens when metal reacts with metal ion (red, oxid)?

Answer 8

Metal (s) at the top of reactivity series WILL react with metal ions below them in the series.
Metal ions (aq) at the top of the reactivity series will NOT react with metal (s) below them in the series.
Metal (s) don’t react w/ each other.

When metal reacts with another metal ion it is a single displacement reactant:
1. Metal ion is reduced (less reactive)
Mg2+ + 2e- -> Mg
2. Metal is oxidised (more reactive)
2K -> 2K+ + 2e-

Question 9

Definition; Galvanic cell?

Answer 9

are an electrochemical cell in which the reduction and oxidation half-equations are separated and connected through a circuit to generate electricity

Question 10

Definition; Half-cells?

Answer 10

are half of an electrochemical cell, where either oxidation or reduction occurs

Question 11

Explain a cell (galvanic) diagram

Answer 11

Two Beakers with half equation in each, salt bridge…, anode (-) is oxidation of half cell, cathode (+) is reduction, electrongs flow from anode (-) to cathode (+)

Shorthand: Zn I Zn2+ II Cu2+ I Cu

Question 12

Ecell (electropotential (EMF) for a cell formula? Meaning of positive and negative results?

Answer 12

=E red - E oxid
Positive answer means spontaneous reactions
Negative answer means non-spontaneous and doesn’t occur without energy input

Question 13

Definition; standard electrode potentials?

Answer 13

The electrical potential that an electrode generates under standard conditions

Question 14

Definition; cell potential difference?

Answer 14

is the electrical potential difference (v) between two electrodes; the voltage that the galvanic cell can generate under standard conditions.

Question 15

Definition; Electromotive force (EMF)?

Answer 15

is the difference in potential across a cell that generates electricity

Question 16

For the standard electrode potential table, which side is the oxidised and reduced forms on? What do the numbers mean?

Answer 16

Oxid is left, red on right. The more positive the more the half equations wants to proceed, more negative is the reverse reaction.

Question 17

On the table, moving downwards the numbers become more positive meaning? moving upwards the no.s become more negative meaning?

Answer 17

down; higher potential for reduction and on the left are more easily reduced (better oxidising agents)
up; higher potential for oxidation reaction and on the right are more easily oxidised so better reducing agents)

Question 18

Definition; molten electrolysis?

Answer 18

the electrolysis of molten salts occurs at very high temps because the salts must be liquid but significant amount of energy must be used to run the cell to produce electrons to the negative cathode but also maintain the very high temperature to keep the salt molten.

Question 19

Definition; aqueous electrolysis?

Answer 19

is used when it’s impractical to produce the high temperature that are required for the electrolysis of molten salts. Water is also a reactant where water molecule might compete with the cations for the reduction at the cathode and/or the anion for the oxidation at the anode.