half-cell

Examples of half-cell in the following topics:

• The Battery

  • One half-cell includes electrolyte and the anode, or negative electrode; the other half-cell includes electrolyte and the cathode, or positive electrode.
  • Some cells use two half-cells with different electrolytes.
  • A separator between half-cells allows ions to flow, but prevents mixing of the electrolytes.
  • Each half-cell has an electromotive force (or emf), determined by its ability to drive electric current from the interior to the exterior of the cell.
  • The net emf of the cell is the difference between the emfs of its half-cells, or the difference between the reduction potentials of the half-reactions.

• Electrochemical Cell Notation

  • Cell notation is shorthand that expresses a certain reaction in an electrochemical cell.
  • The anode half-cell is described first; the cathode half-cell follows.
  • Within a given half-cell, the reactants are specified first and the products last.
  • A double vertical line ( || ) represents a salt bridge or porous membrane separating the individual half-cells.
  • A typical arrangement of half-cells linked to form a galvanic cell.

• Voltaic Cells

  • A voltaic cell is a device that produces an electric current from energy released by a spontaneous redox reaction in two half-cells.
  • This redox reaction consists of two half-reactions.
  • In a typical voltaic cell, the redox pair is copper and zinc, represented in the following half-cell reactions:
  • Each half-cell is connected by a salt bridge, which allows for the free transport of ionic species between the two cells.
  • The cell consists of two half-cells connected via a salt bridge or permeable membrane.

• Concentration of Cells

  • The standard potential of an electrochemical cell requires standard conditions for all of the reactants.
  • The change in Gibbs free energy for an electrochemical cell can be related to the cell potential.
  • The Nernst equation can be used to calculate the output voltage changes in a pair of half-cells under non-standard conditions.
  • Under standard conditions, the output of this pair of half-cells is well known.
  • Discuss the implications of the Nernst equation on the electrochemical potential of a cell

• The Nernst Equation

  • In electrochemistry, the Nernst equation can be used to determine the reduction potential of an electrochemical cell.
  • In electrochemistry, the Nernst equation can be used, in conjunction with other information, to determine the reduction potential of a half-cell in an electrochemical cell.
  • Find the cell potential of a galvanic cell based on the following reduction half-reactions where [Ni2+] = 0.030 M and [Pb2+] = 0.300 M.
  • First, find the electromotive force for the standard cell, which assumes concentrations of 1 M.
  • The added half-reactions with the adjusted E0 cell are:

• Free Energy and Cell Potential

  • The basis for an electrochemical cell, such as the galvanic cell, is always a redox reaction that can be broken down into two half-reactions: oxidation occurs at the anode, where there is a loss of electrons, and reduction occurs at the cathode, where there is a gain of electrons.
  • If E°cell > 0, then the process is spontaneous (galvanic cell)
  • If E°cell < 0, then the process is non-spontaneous (the voltage must be supplied, as in an electrolytic cell)
  • A demonstration electrochemical cell setup resembling the Daniell cell.
  • The two half-cells are linked by a salt bridge carrying ions between them.

• Gap Junctions

  • A gap junction is a specialized cell junction that directly connects the cytoplasm of two cells.
  • Each gap junction channel is made up of two half channels (hemichannels), one in each cell’s membrane. 
  • These half channels join together, bridge the extracellular space in the process, and form the entire channel that spans both cell membranes. 
  • Each of these half channels is called a connexon. 
  • For instance, when heart cells need to beat in unison, gap junctions allow for the transmission of electrical signals between the cells. 

• Exocytosis

  • Exocytosis is the process by which cells release particles from within the cell into the extracellular space.
  • Exocytosis is used continuously by plant and animal cells to excrete waste from the cells.
  • The next stage that occurs is vesicle tethering, which links the vesicle to the cell membrane by biological material at half the diameter of a vesicle.
  • In some cells, there is no priming.
  • The contents are then released to the exterior of the cell.

• Dry Cell Battery

  • The dry cell is one of many general types of electrochemical cells.
  • Unlike a wet cell, a dry cell can operate in any orientation without spilling, as it contains no free liquid.
  • A common dry-cell battery is the zinc-carbon battery, which uses a cell that is sometimes called the Leclanché cell.
  • The paste of ammonium chloride reacts according to the following half-reaction:
  • An illustration of a zinc-carbon dry cell.

• Tumor Suppressor Genes

  • Tumor-suppressor genes keep regulatory mechanisms of cell division under control and prevent abnormal cell growth.
  • Like proto-oncogenes, many of the negative cell cycle regulatory proteins were discovered in cells that had become cancerous.
  • Mutated p53 genes have been identified in more than one-half of all human tumor cells.
  • At this point, a functional p53 will deem the cell unsalvageable and trigger programmed cell death (apoptosis).
  • Cells such as these daughter cells quickly accumulate both oncogenes and non-functional tumor suppressor genes.