What is the first step in measuring energy transfer using the provided method?

The first step in measuring energy transfer when using a polystyrene cup is to fill it with the reaction mixture. This is crucial because the reaction mixture is what will undergo the chemical reaction that generates or absorbs energy. By starting with the reaction mixture, you ensure that you are measuring the temperature change that directly results from the reaction taking place.

Filling the cup with water would not be appropriate at this stage, as it would not provide the necessary conditions for the chemical reaction to occur. Similarly, while measuring temperature and placing a lid on the cup are important steps in the overall procedure, they come after the reaction mixture has been prepared. Therefore, beginning with the reaction mixture is essential to accurately measure the energy transfer involved in the process.