7.     For which of the following reactions does Kc equal Kp?*A. Sn(s) + 2H2O(g) <---> SnO2(s) + 2H2(g)B. 2C2H6(g) + 7O2(g) <----> 4CO2(g) + 6H2O(g)C. NH4Cl(s) <---> NH3(g) + HCl(g)D. N2(g) + 3H2(g) <----> 2NH3(g)

The equilibrium constant Kc equals Kp for reactions where the number of moles of gaseous reactants is equal to the number of moles of gaseous products. This is because the relationship between Kc and Kp is given by the equation Kp = Kc(RT)^(Δn), where Δn is the change in moles of gas (moles of gaseous products - moles of gaseous reactants), R is the ideal gas constant, and T is the temperature in Kelvin. If Δn is zero, then Kp = Kc.

Let's examine each reaction:

A. Sn(s) + 2H2O(g) <---> SnO2(s) + 2H2(g) The number of moles of gaseous reactants (2) is not equal to the number of moles of gaseous products (2). However, since the solid reactant and product do not count towards the total moles of gas, Δn = 0. Therefore, Kc equals Kp for this reaction.

B. 2C2H6(g) + 7O2(g) <----> 4CO2(g) + 6H2O(g) The number of moles of gaseous reactants (2+7=9) is not equal to the number of moles of gaseous products (4+6=10). Therefore, Kc does not equal Kp for this reaction.

C. NH4Cl(s) <---> NH3(g) + HCl(g) The number of moles of gaseous reactants (0, as NH4Cl is a solid) is not equal to the number of moles of gaseous products (1+1=2). Therefore, Kc does not equal Kp for this reaction.

D. N2(g) + 3H2(g) <----> 2NH3(g) The number of moles of gaseous reactants (1+3=4) is not equal to the number of moles of gaseous products (2). Therefore, Kc does not equal Kp for this reaction.

So, the correct answer is A. Sn(s) + 2H2O(g) <---> SnO2(s) + 2H2(g).