Consider the gas-phase transfer of an electron from a sodium atom to a chlorine atom:
Na(g)+Cl(g)-->Na^(+)(g)+Cl^(-)(g)
I was able to write the reaction that relates to an ionization energy and that relates to an electron affinity.
I broke it up into:
Na(g)--> Na^(+)+e^(-)
Cl(g)+ e^(-)--> Cl^(-)(g)
Which was correct, but I'm having trouble with how I'm supposed to use these equations to calculate the enthalpy of the above reaction. I have to use Hess's Law and then some given data but I don't know which.
How should I approach this problem? Explanation would be great, thanks.
You don't tell us what the problem is. Post the problem as is.
Part B: Use the result from part A, data in this chapter (Chapter 7:Periodic Properties of the Elements), and Hess's law to calculate the enthalpy of the above reaction.
Part A is:
Na(g)--> Na^(+)+e^(-)
Cl(g)+ e^(-)--> Cl^(-)(g)
You want to use the ionization potential (in joules) + electron affinity (in joules). Add them together to arrive at dHrxn. Usually the electron affinity is given for the Cl2 molecule; you will need to take 1/2 that. Usually that's a negative number while the IP is a positive dH.
To calculate the enthalpy of the reaction using Hess's Law, you need to use the concept of enthalpy change (ΔH) and the known enthalpy changes of related reactions.
Hess's Law states that if a reaction can be expressed as the sum of two or more other reactions, then the enthalpy change of the overall reaction is equal to the sum of the enthalpy changes for the individual reactions.
In this case, we can consider the reaction you provided as the sum of two steps:
Step 1: Na(g) → Na+(g) + e^−
Step 2: Cl(g) + e^− → Cl−(g)
To calculate the enthalpy change (ΔH) for the overall reaction, you need to find the enthalpy changes for these two steps. Here's how you can approach it:
1. Find the known enthalpy change of Step 1 (Na(g) → Na+(g) + e^−):
You can reference the ionization energy for sodium (Na), which represents the enthalpy change when an electron is removed from an atom in the gas phase.
Na(g) → Na+(g) + e^− ΔH = Ionization energy of Na
2. Find the known enthalpy change of Step 2 (Cl(g) + e^− → Cl−(g)):
You can reference the electron affinity for chlorine (Cl), which represents the enthalpy change when an electron is added to a chlorine atom in the gas phase.
Cl(g) + e^− → Cl−(g) ΔH = Electron affinity of Cl
3. Add the enthalpy changes together to obtain the enthalpy change for the overall reaction:
ΔH overall = ΔH Step 1 + ΔH Step 2
By using these known values from the ionization energy of sodium and the electron affinity of chlorine, you can calculate the enthalpy change of the overall reaction. Make sure to pay attention to the signs associated with the enthalpy changes, as they indicate if the reactions are exothermic (−ΔH) or endothermic (+ΔH).
Remember to use the proper units for the enthalpy changes, typically in kilojoules per mole (kJ/mol), and confirm that the enthalpy changes you're using are for the same conditions (e.g., gas phase).