9.. 19... Flashcards
Oxidation
OIL
- loss of e-
- loss of hydrogen
- gain of oxygen
- increase in oxidation number
- reducing agent is oxidised
Reduction
RIG
- gain of e-
- gain of hydrogen
- loss of oxygen
- decrease in oxidation number
- oxidising agent is reduced
how to balance redox equations
- Write the unbalanced equation and identify the atoms which change in ox. no.
- Deduce the ox.no. changes
- Balance the ox.no. changes
- Balance the charges
- Balance the atoms
what is the activity series
- metals can be ranked in order of reactivity
what metal displaces what metal according to the activity series
- metals higher in reactivity can displace less reactive metals
- more reactive metal: reducing agents
- ## less reactive metal: oxidising agent
Redox titration
- oxidizing agent is titrated against a reducing agent
- most transition metal ions naturally change color when the oxidation state changes
- sometimes indicators can be used
- two you should know: manganate (VII) titrations and iodine - thiosulfate titrations
Manganate (VII) titrations
- acididfied manganate (VII) and iron (II) ions
- manganate (VII) is strong purple color which disappears at the end point
- used for analysis of iron
Iodine - Thiosulfate Titrations
- iodine and thiosulfate ions
- light brown/yellow color of iodine turns paler as it is converted to colorless iodide ions
- starch is added to clarify the end point
- ## solution turns black/blue untitl iodine reacts, at which point the color disappears
Winkler Method
- used to measure biological oxygen demand (BOD)
- amount of oxygen used to decompose the organic matter in a sample of water over a specified time period, usually 5 days, at a specified temperature
- high BOD = greater quantity of organic waste
1 mol of O2 –> 2 mol I2 –> 4 mol S2O3-
2Mn2+(aq) + 4OH- (aq) + O2 (aq) → 2MnO2 (s) + 2H2O (l)
MnO2 (s) + 2I−(aq) + 4H+ (aq) → Mn2+(aq) + I2(aq) + 2H2O(l)
2S2O32−(aq) + I2 (aq) → S4O62−(aq) + 2I−(aq)
What is a Voltaic cell
- generate electricty from spontaneous redox reactions
- each part of the cell is called half cell
- in both half cells, there is a potential difference is set up between the rod and the solution - electrode potential
- ## chemical energy is ocnevrted into electrical energy
What does the potential difference depend on?
the nature of the ions in solution
the concentration of the ions in solution
the type of electrode used
the temperature
structure of voltaic cell
- two half cells
- salt bridge - circuit completed by allowing ions to flow from one solution to the other
- voltmeter
- positive electrode - cathode -reduction
- negative electrode - anode - oxidation
What is an electrolytic cells
- An ionic compound conducts electricity when it is molten or in solution
- The current causes the ionic compound to split up and form new substances.
uses of electrolysis
- purifying copper
- plating metals with silver and gold
- extracting reactive metals, such as aluminium
- making chlorine, hydrogen and sodium hydroxide
what is an electrolyte
- substance that the current passes through and splits up
- contains positive and negative ions
- negative ions move to anode (+) - undergo oxidation
- positive ions move to cathode (-) - undergo reduction
Comparing Voltaic and Electrolytic Cells
How to calculate Cell Potential
EMF= Eright - Eleft
standard electrode potential conditions
1.0 mol dm-3 ions concentrations
100 kPa pressure
298 K
conventional representation of cells
- cell diagram
- half cell with greates negative potential is writeen on the left of salt bridge
What is the Standard Hydrogen Electrode
- a standard electrode against which all other half-cells can be compared
- Hydrogen gas in equilibrium with H+ ions of concentration 1.00 mol dm-3 (at 100 kPa)
- 2H+ (aq) + 2e– ⇌ H2 (g)
- An inert platinum electrode that is in contact with the hydrogen gas and H+ ions
the more ___ the half cell the better ___
negative
reducing agent
3 things on which species is discharged in electrolysis of aq. solution depends on
The relative values of Eθ / position in electrochemical series
The concentration of the ions present
The identity of the electrode
how does the position in the electrochemical series affect the discharge of ions during electrolysis
- the lower the metal ion, the more readily it will gain e- (reduced)
how does the concentration of ions affect the ions discharged during electrolysis?
- ## if one ion is much more concentrated than another ion then it may be preferentially discharged
how does the nature of the electrode affect the ions discharged during electrolysis?
- safe to assume that the electrode is inert
- but if copper electrodes are used during electrolysis of a solution of copper sulfate then the positive electrode is itself oxidized to release electrons and from copper ions.
how to calculate th emass of a product at an electrode
- calculate the charge passed (Q=It, C=A x s)
- calculate the number of faradays, F (F= Q/96500)
- deduce the amount of moles, n, using mol of electrons to mol of atom
- find the. mass of product, m, using the molar mass, Mr
what is gibson’s free energy
ΔGθ
Free energy is a measure of the available energy to do useful work and takes into account the entropy change, ΔSθ, as well as the enthalpy change of a reaction, ΔHθ
spontaneous reaction
ΔGθ = -
E = +
link between ΔGθ and Eθ
ΔGθ = -nFEθ
n= number of electrons transferred
F= the Faraday constant, 96 500 C mol-1
