Left Pane
Generally speaking and at the most fundamental level, the Enthalpy of a reaction is the sum of the products (enthalpys) minus the sum of the reactants.
At a fundamental practical beginning level of chemistry, the sums products and reactants use their heats of formation (which is usually given to you in table form) to calculate the differences.
So the answer must involve a difference making A and B wrong. Though badly worded, the answer is C. The activation energy must be overcome in order for the reaction to take place at all. But it does not represent the Energy (Enthalpy) of the reaction making D wrong.
Answer C
Middle Pane Periodic table.
Your periodic table just isn't complete enough to give you the right answer. You must give me the number of grams in a mole of Cl. I can give you the method of doing the problem, but it will be a little off because of what I use for Chlorine. According to Wiki, chlorine has an atomic mass of 35.453 grams per mole. If you accept that, then here is the rest of the question.
Step One
Find the molecular mass of chlorine
Cl2 = 2 * 35.453 = 70.906
Step Two
Find the number of moles of Cl2
n = given mass / molar mass
given mass = 85.3
molar mass = 70.906
n = 85.3 / 70.906
n = 1.2030 moles of Cl2
Step Three
Find deltaH
3 moles of Cl2 produces 464 kJ / 3 moles Cl2 [That comes from the balanced equation.]
1.2030 moles of Cl2 is x kJ
3/464 = 1.2030 / x Cross multiply
3x = 464 * 1.203
3x = 558.19 Divide by 3
x = 558.19 / 3
x = 186.1 kJ
You want this to go to 3 sig digs so the answer should be 186 kJ. Since the question is worded as deltaH = etc. then your answer is plus 186 kJ to 3 places.
Right Pane
All you do is read deltaH. It is always on the reactants side (left). If it is plus (as written in the question), it is endothermic.
If it is minus, it is exothermic. Memorize.
Top Left DeltaH is < 0 Exothermic
Top Right DeltaH is > 0 Endothermic
Bottom Left DeltaH is < 0 Exothermic
Bottom Right DeltaH is > 0 Enothermic.
