Electrode potentials Flashcards
How do batteries convert chemicals energy into electrical energy?
- Using redox reactions
- The oxidation reaction and reduction reaction but occur in isolation in half cells
- The two half cells are joined by an external circuit (form electron donor to electron acceptor) to make a complete electrochemical cells and this achieved by a salt bridge
What does a half cell contain?
The chemical species present in a redox half equation
What would happen if the chemicals in the two half cells were not kept apart?
Electrons would flow in an uncontrolled way and what energy would be related rather than electrical energy
What is the simplest half cell (metal/metal ion)?
- A metal rod dipped into solution of its aqueous metal ion, represented by Zn2+(aq)I Zn (s)
- At the phase boundary where the metal is in contact with its ions, an equilibrium is set up
Which way is the forward reaction?
reduction
What is an ion/ion half cell?
-Ions of same element in different oxidation state, with inert metal electrode made of platinum
What happens in an isolated half cell? What does the direction of electron flow depends on?
- No transfer of electrons either into or out of the metal
- When two half cells are connected, the direction of electron flow depends upon the relative tendency of each electrode to release electrons (to be reduced), or how far to the right the position of equilibrium of each half-cell lies, this is a relative scale, with each half-cell having a standard electrode potential value
What happens in a cell with two metal/metal ion half-cells connected?
- The more reactive metal releases electrons more readily and is oxidised
1. The electrode with more reactive metal loses electrons and is oxidised and this is the negative electrode
2. The electrode with the less reactive metal gains electrons and is reduced and this is the positive electrode
What sit ehe starry electrode potential?
The electromotive force of a half cell compared with a standard hydrogen half cell, measured at 298L with solution concentrations of 1moldm^3 and a gas pressure of 100kPa
What happens if the standard electrode potential is positive?
The half cell has a greater tendency to gain electrons than the hydrogen cell and the metal becomes the positive electrode
What happens if the standard electrode potential is negative?
The half cell has a lower tendency to gain electrons than the hydrogen cell and the meta becomes the negative electrode
How do you measure standard electrode potential? What is a salt bridge?
- Half cell connected to s standard hydrogen electrode ((solution of H+(aq) ions and H2(g) and an inert platinum used to allow electrons into and out of half cell)
1. The two electrodes are connected by a wire to allow a controlled flow of electrons
2. The two solutions are connected with a salt bridge which allows ions to flow. The salt bridge typically contains a contracted solutions on an electrolyte that does not react with either oesltuion, e/g/ strip of filter paper soaked in aqueous potassium nitrate
What do you need to remember in diagrams?
- Label standard conditions
2. Salt bridges
How can you calculate standard cell potential directly from standard electrode potentials?
E cell = E(positive electrode) - E (negative electrode)
-Standard symbols on E’s
What can standard electrode potentials be used for?
- Predicting the feasibility of a chemical reaction
- A reaction is energetically feasible is the dleatEstandard is positive (more positive than the redox system of reducing agent)
