Ions & Electrolytes
Define electrolysis.
The decomposition (breakdown) of an ionic compound, molten or in aqueous solution, by the passage of an electric current.
What two products form when molten lead bromide is electrolysed?
Lead forms at the cathode; bromine forms at the anode.
Name the two extra ions present in an aqueous electrolyte, in addition to the ions from the dissolved salt..
H+ (from water) and OH- ions
State the three subatomic particles found in an atom.
Protons, neutrons and electrons
What is an ion?
An atom (or group of atoms) that has gained or lost electrons, giving it an overall electric charge.
What is an electrolyte?
A liquid or solution that conducts electricity and is chemically decomposed in the process, because it contains free-moving ions.
At which electrode is lead deposited during electrolysis of molten lead bromide, and why?
The cathode — because Pb2+ ions are positively charged and are attracted to the negative electrode.
What gas is usually produced at the cathode when a dilute aqueous solution is electrolysed (with a metal more reactive than hydrogen)?
Hydrogen gas.
State the relative charges of a proton, a neutron and an electron.
Proton +1, neutron 0, electron −1.
How does a metal atom typically form a positive ion (cation)?
By losing one or more electrons from its outer shell.
Why can't a solid ionic compound conduct electricity, even though it contains ions?
In the solid, the ions are held in fixed positions in the lattice and cannot move to carry charge.
Write the half-equation for lead forming at the cathode.
Pb2+ + 2e- → Pb
hen dilute sodium chloride solution is electrolysed using inert electrodes, what product forms at the anode?
Oxygen gas.
Define atomic number (proton number).
The number of protons in the nucleus of an atom, which equals the number of electrons in a neutral atom.
How does a non-metal atom typically form a negative ion (anion)?
By gaining one or more electrons into its outer shell.
Name the electrode connected to the negative terminal of the power supply, and state which type of ion moves towards it.
The cathode — cations (positive ions) move towards it.
Write the half-equation for bromine forming at the anode.
2Br- → Br2 + 2e-
When concentrated sodium chloride solution (brine) is electrolysed, what forms at the anode instead of oxygen — and why?
Chlorine gas — because the concentration of chloride ions is much higher than hydroxide ions, so chloride is discharged preferentially.
Define mass number, and explain how you would calculate the number of neutrons in an atom.
Mass number = number of protons + number of neutrons; so neutrons = mass number − atomic number.
Describe how an ionic bond forms between a metal and a non-metal, e.g. sodium and chlorine.
Electrons are transferred from the metal atom to the non-metal atom; oppositely charged ions form and are held together by strong electrostatic forces of attraction.
Explain, in terms of particles, why molten lead bromide conducts electricity but solid lead bromide does not.
In the solid the ions are locked in fixed positions in the lattice; when molten the lattice breaks down and the ions are free to move, so they can carry charge to the electrodes.
In the electrolysis of a molten binary ionic compound, which ion is oxidised and which is reduced?
The anion is oxidised (loses electrons) at the anode; the cation is reduced (gains electrons) at the cathode.
State the general rule used to predict which ion is discharged at each electrode during aqueous electrolysis.
At the cathode, hydrogen is discharged unless the metal is less reactive than hydrogen, in which case the metal is deposited instead. At the anode, a halide ion is discharged in preference to hydroxide; if no halide is present (or it's dilute), oxygen forms from hydroxide ions.
Explain what an isotope is. State one way isotopes of the same element differ, and one way they are the same.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (different mass numbers). They have the same chemical properties but different physical properties, such as mass.
Explain why ionic compounds have high melting and boiling points, in terms of structure and bonding.
Ionic compounds form a giant lattice structure with strong electrostatic forces of attraction between oppositely charged ions acting in all directions; a large amount of energy is needed to overcome these many strong bonds.