Flashcards
23.4 Electrode potentials
Flashcards do conjunto (34)
- 1What does a voltaic cell do?Converts chemical energy into electrical energy
- 2What do you need to make a voltaic cell?As electrical energy results from movement of electrons, you need chemical reactions that transfer electrons from one species to another. These are redox reactions.
- 3What does a half-cell contain?A half cell contains the chemical species present in a redox half equation
- 4How is a voltaic cell made from half cells?A voltaic cell can be made by connecting together different half-cells, which then allows electrons to flow
- 5In the cell, the chemicals in the two half-cells must be kept apart. Why?If allowed to mix electrons would flow in an uncontrolled way and heat energy would be released rather than electrical energy
- 6Metal/Metal ion half-cells - How does this look and how do you represent this?Metal rod dipped into a solution of its aqueous metal ion
Represented using a vertical line for the phase boundary between the aqueous solution and the metal e.g. Zn2+|Zn(s) and Cu2+|Cu(s) half-cells - 7Metal/Metal ion half-cells - What is set up?At the phase boundary where the metal is in contact with its ions, an equilibrium will be set up.
By convention, the equilibrium in a half-cell is written so that the forward reaction shows reduction and the reverse reaction shows oxidation - 8Metal/Metal ion half-cells - What about in an isolated half-cell?There is no net transfer of electrions either into or out of the metal
- 9Metal/Metal ion half-cells - What happens when the two half cells are connected?The direction of electron flow depends upon the relative tendency of each electrode to release electrons
- 10Ion/Ion half-cells - What does this contain?An ion-ion half cells contains ions of the same element in different oxidation states
e.g.
Fe3+(aq) + E- → Fe2+(aq) - 11Ion/ion half cell - What metal is used?There is no metal to transport electrons either into or out of the half cell so an inert metal electrode made out of platinum is used
- 12Electrode potentials - How do you know which electrode has a greater tendency to gain or lose electrons?In a cell with two metal/metal ion half-cells connected, the more reactive metal releases electrons more readily and is oxidised
- 13In an operating cell what happens at the negative electrode?The electrode with more reactive metal loses electrons and is oxidised - this is the negative electrode
- 14In an operating cell what happens at the positive electrode?The electrode with the less reactive metal gains electrons and is reduced - this is the positive electrode
- 15Define standard electrode potential E⦵The tendency to be reduced and gain electrons is measured as a standard electrode potential
The standard electrode potential is the e.m.f of a half-cell connected to a standard hydrogen half-cell under standard conditions of 298K, solution concentrations of 1moldm-3 and a pressure of 100kPa - 16What is the standard chosen?Half-cell containing hydrogen gas, H2(g) and a solution containing H+(aq) ions
An inert platinum electrode is used to allow electrons into and out of the half-cell - 17By definition, what are the standard conditions used?Solutions have a concentration of exactly 1moldm-3
The temperature is 298K (25degrees C)
The pressure is 100kPa (1 bar) - 18By definition what is the standard electrode potential of a standard hydrogen electrode?O V
- 19What does the sign of a standard electrode potential represent?Shows the sign of the half-cell connected to the standard hydrogen electrode and shows the relative tendency to gain electrons compared with the hydrogen half-cell
- 20To measure a standard electrode potential, the half-cell is connected to a standard hydrogen electrode. How are they connected?The two electrodes are connected by a wire to allow a controlled flow of electrons
The two solutions are connected with a salt bridge which allows ions to flow. - 21What is a salt bridge and how do you prepare it?A salt bridge allows ions to flow
The salt bridge typically contains a concentrated solution of an electrolyte that does not react with either solution. An example of a salt bridge is a stip of flter paper soaked in aqueous potassium nitrate, KNO3(aq) - 22How do you write the half equation?Forward reaction shows reduction and reverse reaction shows oxidation
- 23The more negative the E⦵ value...The greater the tendency to lose electrons and undergo oxidation
The less the tendency to gain electrons and undergo reduction - 24The more positive the E⦵ value....The greater the tendency to gain electrons and undergo reduction
The less the tendency to lose electrons and undergo oxidation - 25Difference between E⦵ values between metal and non-metals?Metals tend to have negative E⦵ values and lose electrons
Non-metals tend to have positive E⦵ values and gain electrons - 26In general - standard electrode potentials in terms of reactivityThe more negative the E⦵ value, the greater the reactivity of a metal in losing electrons
The more positive the E⦵ value, the greater the reactivity of a non-metal in gaining electrons - 27Measuring standard cell potentials - 1. ConditionsPrepare two-standard half-cells
For a metal/metal ion half-cell, the metal ion must have a concentration of 1 moldm-3
For an ion/ion half cell, both metal ions present in the solution must have the same concentration. There must be an inert electrode, usually platinum.
For a half-cell containing gases (e.g. a hydrogen half-cell), the gas must be 100kPa pressure, in contact with a solution with an ionic concentration of 1 moldm-3. There must be an inert electrode usually platinum.
For all half-cells, the temperature must be 298K - 28Measuring standard cell potentials - 2. What do you connect the metal electrodes to?Connect the metal electrodes of the half-cells to a voltmeter using wire.
- 29Measuring standard cell potentials - 3. How do you prepare the salt bridge?Prepare a salt bridge by soaking a stip of filter paper in a saturated aqueous solution of potassium nitrate, KNO3
- 30Measuring standard cell potentials - 4. What do you do with the salt bridge?Connect the two solutions of the half-cells with a salt bridge
- 31Measuring standard cell potentials - 5. What do you use to record the standard cell potential?Record the standard cell potential from the voltmeter
- 32What happens in the zinc-copper cell - what electrode is positive and which one is negative and why?The copper half-cell has the more positive E⦵ and a greater tendency to undergo reduction and gain electrons
The zinc half-cell has the more negative E⦵ and a greater tendency to undergo oxidation and lose electrons
Electrons flow along the wire from the more negative zinc half-cell to the less negative copper half-cell
The zinc electrode is negative and copper electrode is positive - 33How can you write an overall cell equation from combining: Cu2+(aq) + 2e- → Cu(s) Zn2+(aq) + 2e- → Zn(s)The copper equilibrium has the more positive E⦵ and undergoes reduction, gaining electrons. The reduction half equation is the same way round as the equilibrium, with the electrons on the left-hand side.
The zinc equilibrium has the more negative E⦵ and undergoes oxidation, losing electrons. So this equilibrium is reversed to give the oxidation half equation with the electrons on the right-hand side - SO FLIP ITS EQUATION
The overall cell equation is obtained by combining the reduction and oxidation half-equations
Cu2+(aq) + 2e- → Cu(s)
Zn(s) → Zn2+(aq) + 2e-
Overall: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) - 34What is the equation to calculate standard cell potential from the standard electrode potentials?E⦵cell = E⦵(positive electrode) - E⦵(negative electrode)
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- A level Chemistry (Module 5: Physical chemistry and transition elements) FlashCards sobre 23.4 Electrode potentials, criado por Urvi B em 24-03-2019.
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- Urvi B
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