Study GuidesExam QuestionsPast PapersMock Exams
AP®ChemistryCollege BoardExam QuestionsUnit 4: Chemical ReactionsChemical Changes

Chemical Changes (College Board AP® Chemistry): Exam Questions

41 mins15 questions

Select a download format for Chemical Changes

Scroll for more

Select an answer set to view for
Chemical Changes

Medium
Hard
1a1 mark

A student investigates different types of changes by performing four experiments.

The student dissolves NaCl in water and observes that the salt disappears. Classify this change as physical or chemical, and justify your answer in terms of molecular interactions.

How did you do?

1b1 mark

The student places a strip of magnesium in hydrochloric acid, and bubbling is observed. Identify the gas produced and explain why this is evidence of a chemical change.

How did you do?

1c1 mark

The student melts solid iodine (I2) into a purple liquid. Explain why this is a physical change at the molecular level.

How did you do?

1d1 mark

The student burns ethanol and collects the products in a cold test tube, where condensation forms. Write a balanced chemical equation for this combustion reaction and identify the substance in the condensation.

How did you do?

Did this page help you?

2a1 mark

A student investigates a precipitation reaction between aqueous lead(II) nitrate [Pb(NO3)2] and aqueous potassium iodide (KI), forming a yellow precipitate of PbI2.

Write the balanced molecular equation for this reaction.

How did you do?

2b1 mark

The student is asked to write the complete ionic equation and the net ionic equation for this reaction. Explain the difference between these two forms.

How did you do?

2c1 mark

The student stirs the precipitate for 10 minutes, and it does not dissolve. Explain why PbI2 remains as a solid rather than dissolving.

How did you do?

2d1 mark

This reaction follows the law of conservation of mass. Explain how the particulate-level changes in this reaction demonstrate mass conservation.

How did you do?

Did this page help you?

3a1 mark

A student is given a sample of solid sodium bicarbonate (NaHCO3) and adds hydrochloric acid (HCl). The reaction produces carbon dioxide gas, water, and a dissolved ionic compound.

Write the balanced molecular equation for this reaction.

How did you do?

3b2 marks

Draw a particulate diagram that represents the species present before and after the reaction in solution.

How did you do?

3c1 mark

Explain why this is a chemical change using evidence from molecular rearrangement. Justify your answer.

How did you do?

Did this page help you?

4a3 marks

A student investigates several changes that occur when substances interact in the lab. Each process is analyzed at the macroscopic and molecular level.

A small amount of solid NaHCO3 is added to vinegar (a dilute solution of CH3COOH). Bubbling is observed, and a colorless gas is released.

Explain why this reaction is classified as a chemical process. In your answer, refer to both observable evidence and molecular-level changes.

How did you do?

4b1 mark

Write a balanced molecular equation for the reaction in part (a).

How did you do?

4c1 mark

Write the corresponding complete net ionic equation for the reaction in part (b).

How did you do?

4d2 marks

Explain why the reaction between NaHCO3 and CH3COOH proceeds to completion, even though both are weak electrolytes.

How did you do?

4e2 marks

The student performs a similar experiment dissolving the soluble salt in water, and observes that no gas forms.

Explain why dissolving NaCl in water is classified as a physical change, even though ionic bonds are broken in the process.

How did you do?

4f1 mark

The student notices that the reaction mixture feels cooler after NaHCO3 is added to CH3COOH.

What does this temperature change suggest about the reaction?

How did you do?

Did this page help you?

1a2 marks

Mass of KI tablet

0.425 g

Mass of thoroughly dried filter paper

1.462 g

Mass of filter paper + precipitate after first drying

1.775 g

Mass of filter paper + precipitate after second drying

1.699 g

Mass of filter paper + precipitate after third drying

1.698 g

A student is given the task of determining the 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 Pb(NO3)2 (aq) 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.

For the chemical reaction that occurs when the precipitate forms,

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

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

How did you do?

1b1 mark

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

How did you do?

1c1 mark

In the filtrate solution, is [K+] greater than, less than, or equal to [NO3−] ? Justify your answer.

How did you do?

1d1 mark

Calculate the number of moles of precipitate that is produced in the experiment.

How did you do?

1e1 mark

Calculate the mass percent of I− in the tablet.

How did you do?

1f1 mark

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 I− will be greater than, less than, or equal to the amount calculated in part (e). Justify your answer.

How did you do?

1g2 marks

A student in another lab also wants to determine the I− content of a KI tablet but does not have access to Pb(NO3)2 . However, the student does have access to 0.20 M AgNO3 , which reacts with I−(aq) to produce AgI (s). The value of Ksp for AgI is 8.5 × 10−17.

i) Will the substitution of AgNO3 for Pb(NO3)2 result in the precipitation of the I− ion from solution? Justify your answer.

ii) The student only has access to one KI tablet and a balance that can measure to the nearest 0.01 g. Will the student be able to determine the mass of AgI produced to three significant figures? Justify your answer.

How did you do?

Did this page help you?