U6.1 · Endothermic and Exothermic ProcessesPractice · 吸热与放热过程
Lessons covered:U6.1 Endothermic and Exothermic ProcessesU6.2 Energy Diagrams
- 1Multiple choice
A baking soda () solution is mixed with a citric acid solution. It fizzes, and the temperature of the solution drops from 23.5 °C to 18.2 °C. Which conclusion is correct?
ExplanationThe thermometer reads the water in the solution, which is the surroundings. The water cooling means it lost heat, and that heat flowed into the reacting substances (the system), so the reaction is endothermic. When the system absorbs heat, the surroundings get colder.
- 2Select all
Which of these processes are exothermic? Select all that apply.
More than one option may be correct. Choose all that apply, then press Submit.
ExplanationCombustion and the slow oxidation of iron are exothermic chemical changes. A liquid freezing is an exothermic physical change: the particles attract each other and settle into place, releasing heat. Evaporation and sublimation must overcome intermolecular attractions, so they absorb heat (evaporating alcohol cools your skin), and electrolysis needs a steady electric current, so it is endothermic. Both physical and chemical changes can release or absorb heat.
- 3Multiple choice
A reusable hand warmer contains a supersaturated solution of sodium acetate. Flexing a metal disc inside makes crystals form rapidly, and the pack gets hot. Which statement is correct?
ExplanationThe crystals are still sodium acetate, so no new substance forms: it is a physical change. Energy is released as the particles come together and attract each other in the crystal, so the pack warms. Heating the pack in boiling water redissolves the crystals so it can be used again. Releasing heat does not prove a chemical change.
- 4Multiple choice
An endothermic reaction runs in a flask that sits in a bath of water inside an insulated container. What happens to the temperature of the water bath as the reaction proceeds?
ExplanationThe heat the reaction (the system) absorbs comes from its surroundings, the flask and the water, so the water cools. The insulation only stops heat exchange with the outside air; heat still flows between the system and the water.
- 5Multiple choice
A student mixes two solutions, and the thermometer reading rises from 21.0 °C to 26.5 °C. The student says, “The solution absorbed heat, so the reaction is endothermic.” Which evaluation is correct?
ExplanationSeparate the system from the surroundings first: the system is the reacting particles, and the surroundings are the solvent water and everything around it. The water absorbed heat and warmed, and that heat came from the reaction, so the reaction is exothermic (ΔH < 0). An endothermic reaction would cool the water.
- 6Multiple choice
At 0.5 min, two salts, P and Q, are each added to 100 g of water. The graph shows the temperature of each solution against time. Which statement about dissolving salt P is correct?
ExplanationP’s solution drops from 22.0 °C to about 16.3 °C. Take the dissolving salt as the system and the water as the surroundings: the water cools, so energy flowed from the water into the system, and dissolving is endothermic (ΔH > 0). Q is the opposite: it warms to about 32.2 °C, so it is exothermic. There is no “cold” to release, and a temperature change alone doesn’t say which way energy moved; its direction does. Forming bonds (or ion–dipole attractions) releases energy.
Review: U6.1 Endothermic and Exothermic ProcessesU6.2 Energy Diagrams
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