2018 AP Chemistry exam answers and explanations
The free response questions to the 2018 AP Chemistry can be founder here: https://apcentral.collegeboard.org/pdf/ap18-frq-chemistry.pdf?course=ap-chemistry
Let's now understand the questions and how to answer them.
1. A student performs an experiment to determine the value of the enthalpy change, DH, for the
oxidation-reduction reaction represented by the balanced equation above.
(a) Determine the oxidation number of Cl in NaOCl.
To determine the oxidation number, we first get all other oxidation numbers which are +1 for Na (group 1), and -2 for O (group 16). All of the charges must add up to zero in a neutral molecule. Therefore, we can make an equation of +1+x-2 = 0 x=+1, thus Cl has a -1 charge.
(b) Calculate the number of grams of Na2S2O3 needed to prepare 100.00 mL of 0.500 M Na2S2O3(aq).
Molarity= moles/L. Since we are given both ml and M, we can calculate the number of moles. We will first convert ml to L by diving by 1000. Thus, 100ml = .1L. Moles = .5M*.1L = 0.05moles. We can now convert moles to grams by multiply them by molar mass of Na2S2O3. 0.05moles *158.11 g/mol = 7.9055 grams, or 7.91 grams.
(c) Using the balanced equation for the oxidation-reduction reaction and the information in the table above, determine which reactant is the limiting reactant. Justify your answer.
The limiting reactants is the one that creates the least product. Therefore, in the chart we see that all of the reactants start out with the same molarity and therefore same number of moles. Since the ratio of reactants is 1 to 4 to 2, we can see that NaOCl will run out first since it has the largest coefficient of 4, and will produce the least moles of product.
(d) According to the graph, what is the temperature change of the reaction mixture?
We can see on the graph that the temperature changes from 20 to 32.5, which results in a change of 12.5 degrees.
(e) The mass of the reaction mixture inside the calorimeter is 15.21 g.
(i) Calculate the magnitude of the heat energy, in joules, that is released during the reaction. Assume that the specific heat of the reaction mixture is 3.94 J/(g·°C) and that the heat absorbed by the calorimeter is negligible.
heat = mass *specific heat capacity* change in temperature. Heat = 15.21g *3.94 J/gC * 12.5 = 749.0925 joules = 749 Joules