The change in enthalpy when 1 mole of a compound is formed from its constituent elements, measured under standard conditions.
\(\frac{1}{2}H_2 + \frac{1}{2}Cl_2 \rightarrow HCl\)
Remember: 1 mole of a compound is formed!
The change in enthalpy when 1 mole of a compound is completely combusted (burned in oxygen), measured under standard conditions.
\(C + O_2 \rightarrow CO_2\)
Note that this is the enthalpy of combustion of carbon AND the enthalpy of formation of carbon dioxide.
The change in enthalpy when 1 mole of water is formed as a result of an acid/base reaction, measured under standard conditions. It generally lies around \(-57kJ/mol\) for a reaction involving strong acids and bases. For weaker acids/bases, it is greater than \(-57kJ/mol\).
\(HCl + NaOH \rightarrow NaCl + H_2O\)
The change in enthalpy of a chemical reaction when it occurs in the molar ratios/stoichiometric quantities outlined by a given chemical equation, measured under standard conditions. The example from before works for this definition too:
\(HCl + NaOH \rightarrow NaCl + H_2O\)
The change in enthalpy when 1 mole of gaseous atoms are formed from its element, measured under standard conditions.
\(\frac{1}{2}I_2(s) \rightarrow I(g)\)
Here, the \(\Delta H_{AT}\) is the energy needed to break the \(I-I\) covalent bond + the Van der Waals forces in that molecule.
\(\frac{1}{2}Cl_2(g) \rightarrow Cl(g)\)
Here, the \(\Delta H_{AT}\) is only the energy needed to break the \(Cl-Cl\) covalent bond, since the molecule is already gaseous.
The change in enthalpy when 1 mole of a solid ionic lattice is formed from its constituent gaseous ions, measured under standard conditions.
\(2Na^+(g) + SO^{2-}_4(g) \rightarrow Na_2SO_4(s)\)
This is the polar opposite of ionization energy: the change in enthalpy when 1 mole of electrons are gained by gaseous atoms to form negatively charged ions.
Like with ionization energy, there are different definitions for each electron affinity. Take the first: the change in enthalpy when 1 mole of electrons are gained by gaseous atoms to form gaseous 1- ions. You can scale your definitions as needed.
Note that first electron affinities are always negative (exothermic).
\(Cl(g) + e^- \rightarrow Cl^- (g)\)
This article was written on 2024-08-18. If you have any thoughts, feel free to send me an email with them. Have a nice day!