protonation

Examples of protonation in the following topics:

• Polyprotic Acid Titrations

  • Polyprotic acid are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule.
  • Certain types of polyprotic acids have more specific names, such as diprotic acid (two potential protons to donate) and triprotic acid (three potential protons to donate).
  • Oxalic acid is an example of an acid able to enter into a reaction with two available protons, having different Ka values for the dissociation (ionization) of each proton.
  • The diprotic acid has two associated values of Ka, one for each proton.
  • Triprotic acids can make three distinct proton donations, each with a unique Ka.

• Electric Charge in the Atom

  • Both protons and electrons have charge that is quantized.
  • Protons are found in the center of the atom; they, with neutrons, make up the nucleus.
  • The number of protons in an atom defines the identity of the element (an atom with 1 proton is hydrogen, for example, and an atom with two protons is helium).
  • They are much smaller than protons; their mass is $\frac{1}{1836}$ amu.
  • Small electrons orbit the large and relatively fixed nucleus of protons and neutrons.

• Atomic Number and Mass Number

  • The atomic number is the number of protons in an element, while the mass number is the number of protons plus the number of neutrons.
  • Neutral atoms of an element contain an equal number of protons and electrons.
  • For example, carbon's atomic number (Z) is 6 because it has 6 protons.
  • Protons and neutrons both weigh about one atomic mass unit or amu.
  • For example, a lithium atom (Z=3, A=7 amu) contains three protons (found from Z), three electrons (as the number of protons is equal to the number of electrons in an atom), and four neutrons (7 – 3 = 4).

• Nuclear Stability

  • The protons, which are both positively charged, repel one another through electrostatic force.
  • This force is offset by the nuclear force, which attracts protons and neutrons.
  • This is because, for any constant number of protons, the difference between nuclear force and electrostatic repulsion of protons increases with increasing neutron count.
  • An example is nickel-48, which has 28 protons and 20 neutrons, both of which are magic numbers.
  • Stability of isotopes is shown as a function of proton and neutron numbers.

• Brønsted Acids and Bases

  • A Brønsted acid is any species capable of donating a proton; a Brønsted base is any capable of accepting a proton.
  • To that end, if a compound is to behave as an acid by donating a proton, there must be a base to accept that proton; the Brønsted-Lowry concept is therefore defined by the reaction:
  • The conjugate base is the ion or molecule that remains after the acid has donated its proton, and the conjugate acid is the species created after the base accepts the proton.
  • The reaction can proceed either forward backward; in each case, the acid donates a proton to the base.
  • Here, H2O donates a proton to NH3.

• The Brønsted-Lowry Definition of Acids and Bases

  • A Brønsted-Lowry acid is any species capable of donating a proton; a Brønsted-Lowry base is any species capable of accepting a proton.
  • This is because if a compound is to behave as an acid, donating its proton, then there must necessarily be a base present to accept that proton.
  • Here, a conjugate base is the species that is left over after the Brønsted acid donates its proton.
  • The conjugate acid is the species that is formed when the Brønsted base accepts a proton from the Brønsted acid.
  • For instance, in the presence of ammonia, water will donate a proton and act as a Brønsted-Lowry acid:

• Proton Reduction

  • Anaerobic respiration utilizes highly reduced species - such as a proton gradient - to establish electrochemical membrane gradients.
  • In the mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force.
  • The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria, and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential or proton motive force.
  • A proton motive force or pmf drives protons down the gradient (across the membrane) through the proton channel of ATP synthase.
  • Proton reduction is important for setting up electrochemical gradients for anaerobic respiration.

• Diprotic and Polyprotic Acids

  • As their name suggests, polyprotic acids contain more than one acidic proton.
  • Two common examples are carbonic acid (H2CO3, which has two acidic protons and is therefore a diprotic acid) and phosphoric acid (H3PO4, which has three acidic protons and is therefore a triprotic acid).
  • With any polyprotic acid, the first amd most strongly acidic proton dissociates completely before the second-most acidic proton even begins to dissociate.
  • Ka1 > Ka2); this is because the first proton to dissociate is always the most strongly acidic, followed in order by the next most strongly acidic proton.
  • For example, sulfuric acid (H2SO4) can donate two protons in solution:

• Salts that Produce Acidic Solutions

  • Salts with acidic protons in the cation are most commonly ammonium salts, or organic compounds that contain a protonated amine group.
  • Acid salts can also contain an acidic proton in the anion.
  • Examples of anions with an acidic proton include:
  • Each of these anions contains a proton that will weakly dissociate in water.
  • The NH3+ group contains an acidic proton capable of dissociating in solution; therefore, a solution of anilinium chloride in pure water will have a pH less than 7.

• Bacteriorhodopsin

  • It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell.
  • The resulting proton gradient is subsequently converted into chemical energy.
  • The resulting proton gradient is subsequently converted into chemical energy.
  • This results in a second proton being released to the EC side.
  • The releases the proton from its "holding site," where a new cycle may begin .