Thermochemical equations are just like other balanced equationsexcept they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol ΔH. The most common units are kilojoules, kJ. Here are two thermochemical equations:
H2 (g) + ½ O2 (g) → H2O (l); ΔH = -285.8 kJ
HgO (s) → Hg (l) + ½ O2 (g); ΔH = +90.7 kJ
When you write thermochemical equations, be sure to keep the following points in mind:
Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the ΔH when 1 mol of H2O (l) is formed from 1 mol H2 (g) and ½ mol O2.
Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct ΔH from heat of formation tables. The symbol (aq) is used for species in water (aqueous) solution.
Estimate the change in enthalpy, ΔH, for the following reaction:
H2 (g) + Cl2 (g) → 2 HCl (g)
To work this problem, think of the reaction in terms of simple steps:
Step 1 The reactant molecules, H2 and Cl2, break down into their atoms
H2(g) → 2 H(g) Cl2(g) → 2 Cl(g)
Step 2 These atoms combine to form HCl molecules
2 H (g) + 2 Cl (g) → 2 HCl (g)
In the first step, the H-H and Cl-Cl bonds are broken. In both cases, one mole of bonds is broken. When we look up the single bond energies for the H-H and Cl-Cl bonds, we find them to be +436 kJ/mol and + 243 kJ/mol, therefore for the first step of the reaction:
ΔH1 = +(436 kJ + 243 kJ) = +679 kJ
Bond breaking requires energy, so we expect the value for ΔH to be positive for this step.
A molecule is a combination of two or more atoms that are held together by chemical bonds, such as covalent bonds and ionic bonds. A molecule is the smallest unit of a compound that still displays the properties associated with that compound. Molecules may contain two atoms of the same element, such as O2and H2, or they may consist of two or more different atoms, such as CCl4 and H2O.
In the study of chemistry, molecules are usually discussed in terms of their molecular weights and moles.
Ionic compounds, such as NaCl and KBr, do not form traditional discrete molecules like those formed by covalent bonds. In their solid state, these substances form a three-dimensional array of charged particles. In such a case, molecular weight has no meaning, so the term formula weight is used instead.
Molecular Weight and Formula Weight
The molecular weight of a molecule is calculated by adding the atomic weights (in atomic mass unitsatomic weights according to its empirical formula.
or amu) of the atoms in the molecule. The formula weight of an ionic compound is calculated by adding its