AP Chemistry Unit 4: Chemical Reactions

Explore AP Chemistry Unit 4 questions on chemical and physical changes, balanced equations, particulate models, stoichiometry, titrations, acid-base reactions, precipitation, and…

Covered syllabus points

4.1 Introduction for Reactions 4.2 Net Ionic Equations 4.3 Representations of Reactions 4.4 Physical and Chemical Changes 4.5 Stoichiometry 4.6 Introduction to Titration 4.7 Types of Chemical Reactions 4.8 Introduction to Acid-Base Reactions 4.9 Oxidation-Reduction (Redox) Reactions

[Maximum number: 8]

A student is given the task of determining the $\mathrm{I}^{-}$ content of tablets that contain KI and an inert, water-soluble sugar as a filler. A tablet is dissolved in 50.0 mL of distilled water, and an excess of $0.20 M \mathrm{~Pb}\left(\mathrm{NO}_{3}\right)_{2}(a q)$ is added to the solution. A yellow precipitate forms, which is then filtered, washed, and dried. The data from the experiment are shown in the table above.

(a)

For the chemical reaction that occurs when the precipitate forms,

[ 2 ]

(i)

write a balanced, net-ionic equation for the reaction, and

[ 1 ]

(ii)

explain why the reaction is best represented by a net-ionic equation.

[ 1 ]

(b)

Explain the purpose of drying and weighing the filter paper with the precipitate three times.

[ 1 ]

(c)

In the filtrate solution, is $\left[\mathrm{K}^{+}\right]$ greater than, less than, or equal to $\left[\mathrm{NO}_{3}{ }^{-}\right]$ ? Justify your answer.

[ 1 ]

(d)

Calculate the mass percent of $\mathrm{I}^{-}$ in the tablet.

[ 2 ]

(e)

In another trial, the student dissolves a tablet in 55.0 mL of water instead of 50.0 mL of water. Predict whether the experimentally determined mass percent of $\mathrm{I}^{-}$ will be greater than, less than, or equal to the amount calculated in part (e). Justify your answer.

[ 1 ]

(f)

A student in another lab also wants to determine the $\mathrm{I}^{-}$ content of a KI tablet but does not have access to $\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}$ . However, the student does have access to $0.20 M \mathrm{AgNO}_{3}$ , which reacts with $\mathrm{I}^{-}(a q)$ to produce $\operatorname{AgI}(s)$ . The value of $K_{s p}$ for AgI is $8.5 \times 10^{-17}$ .

[ 1 ]

(i)

Will the substitution of $\mathrm{AgNO}_{3}$ for $\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}$ result in the precipitation of the I- ion from solution? Justify your answer.

[ 1 ]

[Maximum number: 3]

Answer the following questions about an experiment in which $\mathrm{CaCO}_{3}(s)$ is combined with HCl(a q), represented by the following balanced equation.

\mathrm{CaCO}_{3}(s)+2 \mathrm{HCl}(a q) \rightarrow \mathrm{CaCl}_{2}(a q)+\mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l)

(a)

Write the balanced net ionic equation for the reaction.

A student performs an investigation to study factors that affect the rate of the reaction. In each trial the student combines 50.0 mL of HCl(a q) at 21.2°C with 1.00 g of $\mathrm{CaCO}_{3}(s)$ and measures the time required for the reaction to go to completion. The data are given in the following table.

[ 1 ]

(b)

The HCl(a q) was present in excess in all trials of the experiment. Determine the molarity of the HCl(a q) in the beaker after the reaction is complete in trial 2. Assume that the volume of the mixture remains constant at 50.0 mL throughout the trial. (The molar mass of $\mathrm{CaCO}_{3}$ is $100.09 \mathrm{~g} / \mathrm{mol}$ .)

\mathrm{CaCO}_{3}(s)+2 \mathrm{HCl}(a q) \rightarrow \mathrm{CaCl}_{2}(a q)+\mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l)

In order to measure the enthalpy of the reaction shown, the student repeats trial 1 by mixing 50.0 mL of HCl(a q) with 1.00 g of $\mathrm{CaCO}_{3}(s)$ using a coffee cup calorimeter. The student records the temperature of the system every 20 seconds. The data are given in the following table.

[ 2 ]

For the correct calculated moles of HCl reacted (may be implicit):
$\begin{aligned} 1.00 \mathrm{~g} \mathrm{CaCO}_{3} \times \frac{1 \mathrm{~mol}}{100.09 \mathrm{~g}}=0.00999 \mathrm{~mol} \mathrm{CaCO}_{3} 0.00999 \mathrm{~mol} \mathrm{CaCO}_{3} \times \frac{2 \mathrm{~mol} \mathrm{HCl}}{1 \mathrm{~mol} \mathrm{CaCO}_{3}}=0.0200 \mathrm{~mol} \mathrm{HCl} \mathrm{reacted} \end{aligned}$

1 point

For the correct calculated [HCl] remaining, consistent with the number of moles reacted:
$\begin{aligned} 0.0500 \mathrm{~L} \times \frac{1.00 \mathrm{~mol} \mathrm{HCl}}{1 \mathrm{~L}}=0.0500 \mathrm{~mol} \mathrm{HCl} \text { initially present } 0.0500 \mathrm{~mol}-0.0200 \mathrm{~mol}=0.0300 \mathrm{~mol} \text { remaining } \frac{0.0300 \mathrm{~mol}}{0.0500 \mathrm{~L}}=0.600 \mathrm{M} \mathrm{HCl} \text { remaining } \end{aligned}$

1 point

Total for part (e)

2 points

[Maximum number: 3]

Answer the following questions relating to the element aluminum, Al.

The following diagram gives an incomplete particulate representation of the reaction. The beaker on the left represents the system before the mixture reacts. Complete the drawing on the right to represent the system after the reaction has occurred. Be sure to include 1) the correct type and number of particles based on the number shown on the left and 2) the relative spacing to depict the appropriate phases.

Figure for Question 4 Chemical Reactions question 3 — AP Chemistry

\begin{array}{ll} \text { Al }=\mathrm{Al} \text { atom } & \text { Ag }=\mathrm{Ag} \text { atom } \\ \mathrm{Al}^{3+}=\mathrm{Al}^{3+} \text { ion } & \mathrm{Ag}^{+}=\mathrm{Ag}^{+} \text {ion } \end{array}

The student finds the standard reduction potentials given in the table, which are related to the reaction that occurs.

Table for Question 4 Chemical Reactions question 3 — AP Chemistry

For a drawing that shows product formation and indicates the conservation of matter: 1 point
4 Al and 8 Ag particles in the beaker on right (see sample drawing below)
For a drawing that shows product formation and conservation of charge: 1 point
$2 \mathrm{Ag}^{+}$ ions and $2 \mathrm{Al}^{3+}$ ions in the beaker on the right (see sample drawing below) 1 point
6 Ag atoms that are solid and $2 \mathrm{Al}^{3+}$ ions that are aqueous in the beaker on the right

Figure for Answer to Question 4 Chemical Reactions question 3 — AP Chemistry

Total for part (d) 3 points

[Maximum number: 1]

Questions 14-17 refer to the following information.

Equimolar samples of $\mathrm{CH}_{3} \mathrm{OH}(l)$ and $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(l)$ are placed in separate, previously evacuated, rigid 2.0 L vessels. Each vessel is attached to a pressure gauge, and the temperatures are kept at 300 K. In both vessels, liquid is observed to remain present at the bottom of the container at all times. The change in pressure inside the vessel containing $\mathrm{CH}_{3} \mathrm{OH}(l)$ is shown below.

Figure for Question 4 Chemical Reactions question 4 — AP Chemistry

14. Which of the following best describes the change that takes place immediately after the $\mathrm{CH}_{3} \mathrm{OH}(l)$ is introduced into the previously evacuated vessel?

A chemical change takes place because covalent bonds are broken.

A chemical change takes place because intermolecular attractions are overcome.

A physical change takes place because covalent bonds are broken.

A physical change takes place because intermolecular attractions are overcome.

All question bank results loaded

Covered syllabus points

4 Chemical Reactions question 1

Question (a)

Question (i)

Question (ii)

Question (b)

Question (c)

Question (d)

Question (e)

Question (f)

Question (i)

4 Chemical Reactions question 2

Question (a)

Question (b)

4 Chemical Reactions question 3

4 Chemical Reactions question 4