5.2. Energy: Module 5: Physical Chemistry & Transition Elements: Chemistry OCR A A Level

Lattice Enthalpy

Lattice enthalpies are used for ionic substances. Lattice enthalpy is a measure of the strength of ionic bonding in a giant ionic lattice.

Lattice Dissociation Enthalpy

The enthalpy change when one mole of a solid ionic compound is completely dissociated into its gaseous constituent ions under standard conditions.

Lattice Formation Enthalpy

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous constituent ions under standard conditions.

Atomisation Enthalpy

The energy required for the formation of one mole of gaseous atoms from an element under standard conditions.

Enthalpy of Electron Affinity

The enthalpy change when one mole of electrons is added to a mole of gaseous atoms to form one mole of gaseous 1- ions under standard conditions.

Born-Haber Cycles

Born-Haber cycles are similar to the cycles used with Hess’s Law.

Enthalpy of Solution

The enthalpy change when one mole of ionic solid is dissolved in water to infinite dilution so that the ions no longer interact under standard conditions.

Enthalpy of Hydration

The enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions under standard conditions.

Method for Balancing Half Equations

1. Balance all atoms except for oxygen and hydrogen.

2. Add H2O to balance oxygens (if needed).

3. Add H+ ions to balance hydrogens (if needed).

4. Add e- to balance charges (if needed).

Electrochemical Cells

Electrochemical cells use redox reactions since the electron transfer between products creates a flow of electrons. This flow of charged particles is an electrical current that flows between electrodes in the cell. A potential difference is produced between the two electrodes which can be measured using a voltmeter.

Standard Hydrogen Electrode (SHE)

The standard hydrogen electrode is the measuring standard for half-cell potentials. It has a cell potential of 0.00V, measured under standard conditions. These conditions are:

● Solutions of 1.0 mol dm^-3 concentration

● A temperature of 298K

● 100 kPa pressure

Limitations

There are limitations to both calculating a standard cell potential using the SHE and using the calculated value to determine reaction feasibility.

Commercial Cells

Electrochemical cells can be a useful source of energy for commercial use. They can be produced to be non rechargeable, rechargeable or fuel cells.

Fuel Cells

Fuel cells are a type of electrochemical cell which continuously produces a voltage when supplied with a fuel and oxygen. The fuel donates electrons at one electrode and oxygen gains electrons at the other electrode. The energy released is used to generate a voltage.

Lithium ion Batteries

Lithium ion batteries are a commonly used example of an electrochemical cell. They are rechargeable, however, their benefits are counteracted by risks of toxicity and fire.