4. Batteries and Water Systems

What is the memory effect?

• When batteries are repeatedly recharged they begin to lose their original capacity and power. This becomes an issue with some types of cell particularly if they are recharged before half the power has been discharged

• It is said to ‘forget’ its original power capacity

What are electrolytes?

Ionic salts in solution

How is the conductivity of a solution determined?

• by measuring its electrical resistance

• The usual methos is to incorporate a conductivity cell into one arm of a Wheatstone bridge and to search for the balance point (point there is zero current)

• cations will migrate towards the negatively charged electrode and anions towards the positively charged electrode

• Ions of different valency will transport different amounts of electricity e.g. NaCl, each ion carries one unit of charge, CuSO4, each ion transports 2 units of charge

What is the equation for molar conductivity?

What are the two classes of electrolyte and how does the measure3ment of concentration dependence of molar conductivities show this?

• Strong electrolytes like KCl, they show only slight concentration dependence

• Weak electrolytes like CH₃COOH, these show that the conductivity drops sharply when the conc is increased.

• Strong electrolytes will fully ionise in solution - ionic solids and strong acids

• Weak electrolytes will not fully ionise (more likely to be carboxylic acids)

What did Kohlrausch state about molar conductivity?

• Kohlrausch law of independent migration of ions

• Each ion contributes to the molar conductivity in a purely additive manner and its contribution is proportional to its concentration

• And therefore lambda must be the sum of the limiting molar ion conductivities of the cation and anion.

How does conductivity relate to the radii of an ion? For example why is sodium’s conductivity lower than potassium’s?

• Sodium is a small highly polar ion that attracts hydration shell and the hydrodynamic radii of ions are not directly proportional to the ionic radii

What are ionic mobilities?

• when ions in solutions are subjected to a uniform electric field they quickly attain a steady speed

• The quotient speed/electric field strength is called the mobility of the ion and have the symbol μ

What are the molecular aspects of water?

• Uniquely hydrogen bonded structure

• Bonds form and break dynamically

• Only about 15% of hydrogen bonds break on melting

• Affinity of water for itself is unusually high

• Has a high specific heat capacity

What are the affects of hydrogen bonding in water?

• holds water molecules together in a unique way

• Each water molecule can form a max of 4 hydrogen bonds

• Hydrogen bonds between water molecules are eak (1/20th as strong as a covalent bond) so they form and break with great frequency

• Results in high surface tension, higher boiling point, high heat vaporisation and molecule expansion on colling

Why does ice have a lower density than water?

• when water hits zero degrees the water becomes locked in a crystalline lattice with each molecule bonded to the max of four partners

• As ice starts to melt, some H-bonds break and some water molecules pack closer together than they could int eh ice state

• due to the spaces in between molecules when they freeze to ice it cases it to be around 10% less dense than water around 4 degrees C.

• Molecules are less packed together and actually have 4 H-bonds, liquid water has an average of 3.4 H-bonds

• This also explains why ice expands when frozen and why less dense ice floats in water.

What is the structure of a water molecule?

• four electron pairs on four sp3 orbitals

• Distorted tetrahedron structure

• Two pairs are covalently linked hydrogens to the central oxygen and two are lone pairs

• Electronegativity of the oxygen atom induces a net dipole movement

• Water can be a hydrogen bond donor and acceptor which causes tetrahedral coordination.

What is the nature of Hydrogen bonds?

• strong dipole-dipole or charge-sipole interaction that arises between and acid (proton donor) and a base (proton acceptor)

• Hydrogen bonds are strongest when bonded molecules are oriented to maximise electrostatic interaction - ideally three atoms involved are in a line E.g. O-H - - - O= would be in a straight line and make a stronger H-bond than if it was at an angle.

• O and N are hydrogen acceptors that can form hydrogen bonds

What is proton Hydration?

• protons don’t exist freely in solution

• They are hydrated to form hydronium (oxonium) ions (H3O+)

• hydronium ions are solvated by nearby water molecules

• Covalent and hydrogen bonds are interchangeable which allows for fast mobility of protons in water via proton hopping

• Proton hopping leads to high ionic mobility for hydronium ion

What does the solvation of anionic solutes look like?

• ion-dipole interactions, one of the hydrogens in the water are surrounding the anionic charge

What does the solvation of cationic solutes look like?

• Ion-dipole interactions where the oxygen is facing the positive charge and and the hydrogens are facing out

How does the size of the hydration shell change?

• Size and charge of ion determines the number of bound water molecules

• smaller, more highly charged ions bind more water molecules

What does the solvation of non-polar solutes (oils) look like?

• hydrogen bonds between water molecules surrounding the oil

What is the hydrophobic effect?

• the association or folding of non-polar molecules in aqueous solution

• Main factor behind protein folding, protein-protein association, formation of lipid micelles, binding of steroid hormones to their receptors

What are the 4 Battery types and their characteristics?

• Lead-acid - used in cars - rechargeable and have no memory effect

• Alkaline - common Duracell battery - non-rechargeable

• Nickel-cadmium - phones - rechargeable, has memory effect

• Lithium-ion - laptops, phones - rechargeable, no memory effect