Consider the formation of formation of chlorine and water vapor, H2O(g), by the
ID: 738967 • Letter: C
Question
Consider the formation of formation of chlorine and water vapor, H2O(g), by the reaction of hydrogen chloride with oxygen:HCl(g) + 1/4 O2(g) 1/2 Cl2(g) + H2O(g) H = -28.6 kJ
(a) What is the enthalpy change for the reverse reaction?
Hreverse = kJ/mol.
(b) Balance the forward reaction with whole-number coefficients. What is H for the reaction represented by this equation?
H = kJ/mol.
(c) Which is more likely to be thermodynamically-favored, the forward reaction or the reverse reaction?
(d) If the (forward) reaction were written to produce liquid water, H2O(l), instead of water vapor, would you expect the magnitude of H to increase (become more positive), decrease (become more negative), or stay the same?
H should .
Explanation / Answer
enthalpy (what is shown as Hrxn) being a thermodynamic state function - its current value depends on the current state of the system but not by the route that state was reached. the principle of conservation of energy: This means you can treat chemical equations and associated enthalpy changes arithmetically -- adding/subtracting, inverting, scaling. You can treat an overall reaction as being the result of intermediate steps, and you can exploit this to find thermodynamic data for intermediate reactions which, in practice, you may not be able to measure. therefore, the enthalpy change for the reverse reaction would simply be the negetive of the enthalpy change for the forward reaction (i.e) Hreverse=-4(-28.6 kJ) = 114.4kJ the balanced forward reaction: 4HCl(g) + O2(g) -> 2 Cl2(g) + 4 H2O(g)