In chemistry, calculus has direct practical applications concerning, e.g.

- Determining the rates of chemical reactions.

- Finding out some necessary information of radioactive decay reactions.

My advice is to show the students the value of the science, they are learning, in their own lives in the first two meetings. Challenging questions can help in raising interests. Brain storming approach will start with few students & then the circle is widened.

The use of social media for puzzles in maths is far better than using them for empty chats. 

Calculus is definitely one of the more complex things to learn, so I’m not sure any example would make it easy.  Science has the most ready-made examples of applications of calculus.  As Nizar mentions, chemistry has many examples like chemical reactions (or essentially end other example of rates of change).  Newton invented calculus to solve physics problems, so physics also provides many examples.  I feel Physics probably provides the easiest examples of calculus for somebody just learning calculus.  In high school physics, teachers typically help students derive the basic kinematics equations with algebra.  It takes many steps.  But if you start with a single equation like: X(t) = 1/2 a t2 + Vi t + Xi and you take the 1st and 2nd derivatives with respect to time, you practically instantly derive all the kinematics formulas.  The ease of deriving formulas hopefully shows the value of calculus.  It’s also a good example because most people already have an intuitive sense of velocity and acceleration, so the rates of change in calculus become more intuitive.  Applications like these help students think more flexibly about calculus because they often get so used to y as a function of x, that they don’t realize the broad applicability (like position as a function of time).  There are many other examples too, like the biology of ecosystems (e.g., birth and death rates), economics (e.g., marginal profit), and statistics (e.g., integrating over probability distribution).  Best wishes with your calculus classes, Kinley. ~ Kevin

Emphasize the fun of working out a challenging problem.  My students were more motivated by the working out of problems then by my telling them neat ways calculus is handy for physics and chemistry.  I did invent problems with science and economics applications in each topics but basically working problems and comparing solutions was the best stimulation.    A. Hankla

You might find something useful in the book from a project that looked at developing ways of teaching calculus with technology. The book is free to download at the link below.

https://eprints.soton.ac.uk/372030/

Calculus is used to solve many heat transfer problems for example to get a solution for temperature distribution for fins, solution in heat exchangers such as boilers, condensers etc.