Scientists Hydia, Christian, and Trynatie took 100 frogs. They kept 50 of the frogs at 35 degrees C. They kept 50 of the frogs at 25 degrees C. Then they measured how fast each frog's heart beat. What was changed or different BEFORE the experiment began?

Temperature was kept as an independent variable to observe the changes in the heart beat.

Explanation:

Independent variable can be defined as the variable which can be changed to induce change in an dependent variable which is the outcome of the experiment.

According to the given situation, the temperature in which the frogs were kept changed to observe the changes in the rate of heart functioning. So, the   temperature is the independent variable which changed before the experiment began.

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.