Predict whether each of the following reactions would be exothermic or endothermic.
a. CO(g) --> C(g) + O(g)
b. 2H(g) + O(g) --> H20(g)
c. Na+(g) + Cl-(g) --> NaCl(s)
What is the sign of DH in each of these reactions?
Please explain. I am very confused. Thanks
Look up delta H formation and plug into
dHrxn = (n*dHf products) - (n*dHf reactants). If the sign is - it is exothermic; endothermis is + sign.
To predict whether a reaction is exothermic or endothermic, we need to look at the change in enthalpy (ΔH) of the reaction. Enthalpy is a measure of the heat energy of a system at constant pressure. If ΔH is negative, the reaction is exothermic (it releases heat), and if ΔH is positive, the reaction is endothermic (it absorbs heat).
Let's look at each reaction and determine the sign of ΔH:
a. CO(g) → C(g) + O(g)
In this reaction, we are breaking the CO bond and forming the C and O bonds. Breaking a bond requires energy, and forming a bond releases energy. Since the energy released when forming the C-O bond is greater than the energy required to break the CO bond, the overall reaction is exothermic. Therefore, the sign of ΔH is negative.
b. 2H(g) + O(g) → H2O(g)
Here, we are breaking the H-H and O=O bonds and forming the O-H bonds. Again, breaking bonds requires energy, and forming bonds releases energy. In this case, the energy released when forming the O-H bonds is greater than the energy required to break the H-H and O=O bonds. Thus, the overall reaction is exothermic, and ΔH is negative.
c. Na+(g) + Cl-(g) → NaCl(s)
In this reaction, we have the gaseous Na+ and Cl- ions coming together to form the solid NaCl. Forming an ionic solid like NaCl involves the release of energy as the positive and negative ions combine to form a lattice. Therefore, the reaction is exothermic, and ΔH is negative.
In summary:
a. ΔH is negative (exothermic).
b. ΔH is negative (exothermic).
c. ΔH is negative (exothermic).
Remember, the above predictions are based on typical energy considerations for these types of reactions, but it's essential to note that to determine the sign of ΔH accurately, thermodynamic data or calorimetry experiments are required.