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# How to solve an unsolvable problem?

By backing up what a question is asking for, often you can solve most problems in your AP Chemistry exam. Especially conceptual ones.

I have a video of a question that has a great example of when you can back up to an answer and when you simply need to know the answer. WATCH HERE.

For example, look at this question:

The normal boiling point of $\inline&space;N_{2}H_{4}$ is $\inline&space;114^{\circ}C$, whereas the normal boiling point of $\inline&space;C_{2}H_{6}$ is $\inline&space;-89^{\circ}C$. Explain, in terms of the inter-molecular forces present in each liquid, why the boiling point of $\inline&space;N_{2}H_{4}$ is so much higher than $\inline&space;C_{2}H_{6}$.”

How do we back this up? Maybe you know the answer to this question already. But play along…

Clearly, the boiling point of $\inline&space;N_{2}H_{4}$ is greater. What does that mean in terms of intermolecular forces. Even if you didn’t know, it has to mean that they are stronger, holding the molecules together, not allowing them to evaporate, right? So you know what to prove, you need to show that $\inline&space;N_{2}H_{4}$ has stronger intermolecular forces (IMFs going forward). But how?

Well, why would anything have stronger IMFs? Polarity, that’s right. We know that H-bonds are the strongest IMFs around. And we know these form between H and N/O/F, so there you go. We also know that poor old $\inline&space;C_{2}H_{6}$ is non-polar as are ALL C-H compounds, giving it only weak London forces.

And we are done. Stronger forces = high boiling point.

Watch this video too now!