Achieve Week 3 & 4 #4

[Jacqueline Vargas 3L]

I'm having trouble with this question. The feedback I get is always the same but I don't know where the problem is exactly.

The highlighted options are the ones I'm unsure about. How do we determine whether energy is released or absorbed for a reaction with two types of diatomic molecules as reactants (e.g. A2 and B2) and a compound as the product (2AB) ?

For the ones that I didn't highlight, I just compared the reactants and products that were bonded and classified the reaction based on that. Is this correct?

Thank you in advance.

[Amy Xia 1J]

You're right about B2+C2 -- 2BC, but A2 +C2 --2AC would be endothermic as well since A-C bond is weaker than both A-A & C-C bond.
For determining reaction, you need to consider the bond strength for reactant and product, if the reactant bonds are stronger and product bonds are weaker, it is an endothermic reaction. vice versa. weaker reactant bonds and stronger product bonds is exothermic reaction.

[Kathleen Wijaya 3L]

If a reaction is exothermic, the energy of bonds broken are less than the energy of bonds formed; in an endothermic reaction, the energy of bonds broken is greater than the energy of bonds formed.

In the A2+C2--> 2AC reaction, A-A and C-C bonds that are broken are stronger than the A-C bond formed. Thus, this reaction would be endothermic. Hope this helps!

[Erin Chin 1L]

To determine whether a reaction is exothermic or endothermic, you need to compare the bond strength between the reactants and products. In the example from your question (A2 + B2 --> 2 AB), A-A and B-B are weaker bonds compared to A-B, therefore since there are weaker reactant bonds and stronger product bonds, it would be an exothermic reaction. You have to determine whether the energy required to break the reactant bonds is less than or greater than the energy released during the formation of the product bonds.

For the A2+ C2 --> 2AC reaction, there are stronger reactant bonds and weaker product bonds. From the table, A-A and C-C bonds are both stronger than A-C bonds, therefore, it would be an endothermic reaction.