AP Chem Unit 6.1: Endothermic and Exothermic Processes

For a classroom demonstration, a chemistry teacher puts samples of two different pure solid powders in a beaker. The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker. After a short time the students observe that the bottom of the beaker is frozen to the wood surface. The teacher asks the students to make a claim about the observation and to justify their claims. Which of the following is the best claim and justification based on the students' observation?

An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.

Which of the following best helps to explain why the value of ΔH° for the dissolving of CaF2 in water is positive?

Ca2+ ions have very strong ion-ion interactions with F- ions in the crystal lattice.

In the spring, blossoms on cherry trees can be damaged when temperatures fall below −2°C−2°C. When the forecast calls for air temperatures to be below −5°C−5°C for a few hours one night, a farmer sprays his blossoming cherry trees with water, claiming that the blossoms will be protected by the water as it freezes. Which of the following is a correct scientific justification for spraying water on the blossoms to protect them from temperatures below −2°C−2°C?

The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C−2°C.

K(s) + Cl2(g) → KCl(s) ΔH° = − 437 kJ/molrxn The elements K and Cl react directly to form the compound KCl according to the equation above. Refer to the information above and the table below to answer the questions that follow Which of the values of ΔH° for a process in the table is (are) less than zero (i.e., indicate(s) an exothermic process) ?

y and z only

K(s)+ ½ Cl2(g) → KCl(s) ΔH° = -437kJ/molrxn The elements K and Cl react directly to form the compound KCl according to the equation above. Refer to the information above and the table below to answer the questions that follow. Which of the values of ∆Ho for a process in the table is (are) less than zero (i.e., indicate(s) an exothermic process)?

y and z only