Electrical conductivity

Electrical conductivity

Lessons

In this lesson, we will learn:
  • A definition of electrical conductivity.
  • Why some compounds are conductors of electricity and others are not.
  • How solutions affect electrical conductivity.
  • How to recognize conductors of electricity by their chemical formula.

Notes:
  • Electrical conductivity is the rate of flow of electric charge. It is the ability of any substance to allow electric charge to flow throughout its structure.
    "Electric charge" is just moving electrons – if electrons can flow freely through a substance as its structure allows them to, then the substance can conduct electricity.

  • This is important for chemists because solutions can have very different properties to the individual solvent or solutes they are made of – ionic compounds can't conduct in the solid state but they can when dissolved or melted.

  • Ionic compounds are able to conduct electricity in solution because when ionic compounds dissolve, they break up into their separate ions (called dissociation). These charged particles allow any electric charge (read: electrons) free movement. For example when sodium chloride is dissolved (NaCl):

  • NaCl → Na+(aq) + Cl-(aq)

    Remember that when dissolved in water, we say a chemical is in the aqueous phase. If a compound is insoluble in water it should not be referred to as being aqueous.

  • As with any chemical property, different compounds dissociate in solution more or less than others – some might only show 1% of molecules dissociating, others over 90%. This will affect how well electricity is conducted as the molecules must be in dissociated ionic form for charge to be transferred.
    • Experiments can be performed to show this, where circuits are made using electrodes in a solution with light bulbs in the circuit:
      • One or more bulbs of different power consumption (watts) are put in the electric circuit and show different brightness with different solutions tested, which have different conductivity depending on the solute/solvent mixture.
      • The brighter the bulb(s) glow, the better the solution conducts electricity.

  • Phase is important to electrical conductivity – both aqueous and the liquid phase allow free flowing particles including electric charge. Liquid and aqueous states allow the molecules to dissociate into ions and for those ions to have the energy to move freely!
    • Experiments like those above with solutions can be done with different phases to find this out:
      • Crystals and solid ionic compounds do not conduct electricity and the bulbs do not glow.
      • Once dissolved or heated to a molten state the bulbs begin glowing.

  • Another issue affecting conductivity is concentration. The more concentrated a solution is, the greater the conductivity measured (if the solute is a conductor in solution). This is because the higher the concentration of the solute, the more ions will be available to carry electric charge.

  • Generally speaking, the following rules can be followed to determine if a substance will conduct electricity:
  • Substance (state)

    Conducts electricity?

    Metal (solid)

    Yes

    Metal (melted)

    Yes

    Acid or base/alkali (solid)

    No

    Acid or base/alkali (solution)

    Yes

    Ionic compound (solid)

    No

    Ionic compound (melted)

    Yes

    Ionic compound (solution)

    Yes

    Organic acids (solution)

    Yes

    None of the above

    No


  • Using the table above and knowledge of ions helps us think about what chemicals and substances are conductors of electricity – look for:
    • Metals (their structures contain delocalized electrons themselves)
    • Mineral acids and bases (inorganic compounds that have H or OH groups)
    • Other compounds that will dissociate into ions when dissolved (organic acids and their metal salts)
    What these all have in common is that charged particles are present or generated in all of them.

  • If we need solutions where charged, ion-forming compounds are soluble, what effect could this have on the solvents you can use to make conducting solutions? Conducting solutions are made with polar solvents – a nonpolar solvent won't dissolve polar, ion-forming solutes!
  • Introduction
    Conducting electricity
    a)
    What is electrical conductivity?

    b)
    Factors affecting electrical conductivity.

    c)
    Why are solvents important in conducting electricity?

    d)
    How to tell if something conducts electricity.


  • 1.
    Recall the factors that affect electrical conductivity.
    A student sets up a circuit containing a large lightbulb, using electrodes placed in a solution of different compounds dissolved in water. The solutions are measured to be all be the same concentration. The circuit was switched on and the state of the lightbulb was recorded.
    a)
    Some results shows the lightbulb not glowing at all. What does this tell us about the solution?

    b)
    In one solution, the bulb glowed weakly. The student then decided to increase the concentration tenfold by adding ten times more solute (the solution is still unsaturated). Describe and explain what happened to the lightbulb after doing this.

    c)
    In one experiment, the student did not use a solution, but instead crystals of an ionic compound. The lightbulb did not glow at all. Using standard laboratory equipment, suggest two things that the student could do to make the light bulb glow.


  • 2.
    Recall the types of chemical substances that can conduct electricity.
    Read the list of chemical substances below and highlight the substances that would conduct electricity, and in what situation(s).
    i) CH4
    ii) NaCl
    iii) Mg
    iv) CO2
    v) H2O
    vi) CH3COOH