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Estimate the  change in enthalpy , ΔH, for the following reaction: H 2  (g) + Cl 2  (g) → 2 HCl (g) Solution To work this problem, think of the reaction in terms of simple steps: Step 1  The reactant molecules, H 2  and Cl 2 , break down into their atoms H 2 (g) → 2 H(g) Cl 2 (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 O 2 and H 2 , or they may consist of two or more different atoms, such as CCl 4  and H 2 O. 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 ...