By convention, when heat flows OUT of the system into the surroundings, q is:

Answer: Negative (exothermic) Sign convention: q < 0 means the system loses heat (exothermic); q 0 means it gains heat.

A reaction with ΔH 0 is

Answer: Endothermic Positive ΔH = heat absorbed by the system from surroundings.

Based on the result, NH4NO3 is the active ingredient in instant cold packs. Why?

Answer: Its dissolution is endothermic, so the solution becomes cold by absorbing heat from surroundings An endothermic dissolution absorbs heat from the surroundings (including a person's body), producing a cooling effect.

AP Chemistry · Topic 6.1

Endothermic and Exothermic Processes Practice

Q: A reaction with ΔH 0 is

Answer: Endothermic Positive ΔH = heat absorbed by the system from surroundings.

Q: Based on the result, NH4NO3 is the active ingredient in instant cold packs. Why?

Answer: Its dissolution is endothermic, so the solution becomes cold by absorbing heat from surroundings An endothermic dissolution absorbs heat from the surroundings (including a person's body), producing a cooling effect.

Part of Thermodynamics.(ENE-2.A)

Practice questions

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Sample questions

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Sample 1difficulty 1/5
$By convention, when heat flows OUT of the system into the surroundings, q is:$
- A
  $Equal to ΔS$
- B
  $Positive (endothermic)$
- C
  $Negative (exothermic)$
  check_circle
- D
  $Always zero$
Why
$Sign convention: q < 0 means the system loses heat (exothermic); q > 0 means it gains heat.$
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Sample 2difficulty 1/5
$A reaction with ΔH > 0 is$
- A
  $Endothermic$
  check_circle
- B
  $Spontaneous$
- C
  $Catalyzed$
- D
  $Exothermic$
Why
$Positive ΔH = heat absorbed by the system from surroundings.$
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Sample 3difficulty 1/5
$A student dissolves 5.00 g of NH4NO3 (M = 80.04 g/mol) in 100.0 g of water in a coffee-cup calorimeter. Initial water temperature = 22.0 C. After dissolution and complete mixing, the minimum temperature is 18.6 C. Specific heat of solution = 4.18 J/(g*C). Total mass of solution = 105.0 g. Calorimeter heat capacity is negligible.$
$Based on the result, NH4NO3 is the active ingredient in instant cold packs. Why?$
- A
  $It is exothermic, releasing heat to keep the pack cold$
- B
  $It evaporates instantly$
- C
  $Its dissolution is endothermic, so the solution becomes cold by absorbing heat from surroundings$
  check_circle
- D
  $It chemically reacts with hands to feel cool$
Why
$An endothermic dissolution absorbs heat from the surroundings (including a person's body), producing a cooling effect.$
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Sample 4difficulty 2/5
$In an endothermic process, q for the system is$
- A
  $Zero$
- B
  $Negative$
- C
  $Positive$
  check_circle
- D
  $Undefined$
Why
$Heat flows into system \to q_sys > 0.$
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Sample 5difficulty 2/5
$A hand warmer (oxidation of iron) is$
- A
  $Exothermic$
  check_circle
- B
  $Always non-spontaneous$
- C
  $Athermal$
- D
  $Endothermic$
Why
$Oxidation of Fe releases heat; exothermic, ΔH < 0.$
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