When the aim is to get the highest output of teaching secondary chemistry (i.e. that most of the students understand the taught topics), then there is a huge challenge in front of the teacher.

My answer, to your second question, tackles the opposite direction: the most easy to teach (and the most enjoyable) is practical chemistry because the students will learn what they see (or vice versa) on the spot. Chemistry ought not be abstract depending mostly on imagination.

The topic in chemistry, that is least learnt in many chemistry departments in many countries, is industrial chemistry. There are universities which do not have more than a single industrial chemistry course & there are universities which have ignored it focusing upon other topics. The reason for this, in my opinion, comes from the classical division of chemistry into 4 branches: analytic, inorganic, organic, and physical. Therefore, courses are tailored to serve these branches.

Chemistry concepts are abstract and naïve understanding is most of the time deeply entrenched. So, to eradicate naive understanding of chemistry concepts is challenging. In my research, I advocate "cognitive perturbation" to help learners move from a naïve understanding level to a more scientific level.