Electrode potentials

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  • Created by: Hindleyc
  • Created on: 11-04-19 22:13
What is oxidation
Loss of electrons or an increase in oxidation number
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What is reduction
Gain of electrons or a decrease in oxidation number
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Any element on its own
Oxidation state =0
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A simple ion
Oxidation state = the charge on the ion
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A group 1,2 or 3 element in a compound (metals)
Oxidation state = charge = to group number always +ve
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Oxygen in a compound
Oxidation state = usually -2
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Group 7 element in a compound
Oxidation state =-1
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H bonded to a non metal
+1
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Compound
Oxidation state =sum of oxidation number of all atoms and ions in a compound is 0
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Complex ion
Sum of the oxidation number of all atoms and ions in anion is equal to the charge
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How do you balance a redox eon using half equations
Write Half ionic equation first, balance for central ion/atom (not O or H), Balance for O by adding water, Balance H by adding H+ ions and balance charges by adding electrons
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Then
Make no. of e's gained In one of the half equations equal to the number of electrons lost by the other half-eqn (multiply) by certain factor then combine 2 equations together cancelling out the e's&any other species that appear on both side of eqn
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How to balance redox eqn using oxidation numbers 1
try balance atoms in eqn as normal if successful move onto step 2 if not go to step 3
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Step 2
Check the charges are balanced on both sides of eqn add up to the same number and if they do=done if not step 3
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3
Work out Oxidation number for all elements in reaction
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4
Work out change in oxidation number of the element that is oxidised and the element that is reduced
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5
find numbers need to multiply by that would make the changes in oxidation number the same
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6
Add these numbers into balanced equation
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7
Balance rest of eqn as normal
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How do you balance in alkaline conditions given previous were in acidic contiditions using H+
1 Work out electron half equation for the conversion of ion to atom
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2
balance half eqn for other ion and produce electron half eqn which would apply if the reaction was done under acidic conditions just like previous method
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3
Convert this into alkaline conditions by adding enough hydroxide ions to each side to neutralise all the hydrogen ions present producing water molecules - simplify by looking for h2O that occur on both sides of eqn
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4
combine with eqn from 1 to give ionic eqn
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Simplify steps
1=central atom, 2=O 3=H 4=E- 5=OH-to each side 6=Cancel
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What is Alkali
Soluble base that release OH- ions in solution
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Oxidation eqn
Electrons right
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Reduction eqn
Electrons left
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What do redox reactions involve
Transfer of electrons
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What does an electrochemical cell do
Controls this electron transfer to produce electrical energy
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What are electrochemical cells made up of
2 half cells
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where
One half cell released electrons (oxidation) and one half cell gains electrons (reduction)
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What is a half cell made of
An element in 2 oxidation states eg a ***** of metal placed in a solution of metal ion
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What can ions do in solution
move whereas in solid can't move
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At surface of the ***** of Cu metal what exists
A chemical equilibrium Cu2+ +2e- = Cu(s)
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How are all half eqns written
With e's on the left
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How is an electrochemical cell made
By connecting 2 half cells together to make a complete circuit
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What does this do
produces a voltage which can be measured
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What does the voltage tell us
How likely something is to be oxidised /reduced
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How is electrode potential always given
as standard electrode potential which are relative to the H electrode H=0
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How is it measured
With a voltmeter
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how is EQN for standard electrode potential written
As a reduction
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Electrode potential-
Voltage produced when we connect 2 half cells together relative to standard H
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The more +ve a standard electrode potential value is
the easier it is to reduce the species (gain electrons )
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What does it act as
Oxidising agent
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The more -ve an Electrode potential value is
The harder it is to reduce the species - easy to oxidise so reducing agent
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What is the OFFical definition of standard electrode potential
The electromagnetic force of a half cell compared to standard hydrogen electrode under standard conditions
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What are standard conditions
298k, gases at 1atm pressure, solution at 1 mol per dm3
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Where are electrons
always on left (reduction )
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What is the standard H electrode
reference electrode
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What happens
Hydrogen gas is bubbled in at the top and slowly bubbles out of the bulb at the bottom allowing contact between the platinum electrode, hydrogen gas and H+ ions in solution (Cant be metal)
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Why is a platinum electron used
Ensures good contact between hydrogen gas and H+(aq) to allow electrode reactions to take place quickly and it is an inert unreactive metal
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How is it set out
Acid solution containing 1 mol/dm3 (aq) with Pt electrode which is unreactive, Glass tube with holes in to allow bubbles of H2 gas to escape , H2 gas at 298k 1atm connected to another half cell
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What causes the electron transfer
2 diff forms touch
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What is the equilibrium for the half cell
2H+ +2e-=H2
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What 2 possible reactions may occur in the half cell
2H+ +2E- =H2 (RED), H2=2H+ +2E- (OX)
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The one that occurs depends on what
The species in the other half cell
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What if it is more likely to reduce than Hydrogen
Then it is reduced and hydrogen is oxidised
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How to measure standard electrode potentials
half cell made using different forms of same element in different oxidation states, this half cell connected up to another half cell usually standard H electrode to complete circuit then standard H electrode is used as reference point
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How
All other electrode potentials are relative to hydrogen
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What does connecting 2 half cells produce
a complete circuit known as electrochemical cell so voltage can be measured
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Other cards in this set

Card 2

Front

What is reduction

Back

Gain of electrons or a decrease in oxidation number

Card 3

Front

Any element on its own

Back

Preview of the front of card 3

Card 4

Front

A simple ion

Back

Preview of the front of card 4

Card 5

Front

A group 1,2 or 3 element in a compound (metals)

Back

Preview of the front of card 5
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