Calorimetry and chemical energy
แนะนำใหม่| English | ความหมาย |
|---|---|
| enthalpy change/enˈθælpi tʃeɪndʒ/ | การเปลี่ยนแปลงเอนทาลปี |
| exothermic/eɡzəˈðɜːmɪk/ | การคายความร้อน |
What would explain this observation?
- A cup warms when two solutions react. The temperature rise measures energy transferred to the surroundings; it does not directly equal the enthalpy change 焓变.
Build the model
- Exothermic 放热的 reactions transfer energy to surroundings.
- Endothermic reactions take energy from surroundings.
- Bond breaking requires energy; bond formation releases energy.
- exothermic: Transferring energy to the surroundings.
- enthalpy change: Heat change at constant pressure for a stated process.
Which process requires energy?
ความยาก 1/5 · 100 เหรียญ + โบนัสเล็กน้อยในการทำครั้งแรกวันนี้
Breaking a bond always requires energy input to separate the atoms; forming a bond releases energy. A reaction is exothermic overall only when the energy released in bond formation exceeds the energy taken in for bond breaking.
Match each technical term to its precise meaning.
ความยาก 2/5 · 140 เหรียญ + โบนัสเล็กน้อยในการทำครั้งแรกวันนี้
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Use energy transferred = mass × specific heat capacity × temperature change.
- Convert joules to kilojoules before dividing by reaction amount.
- An exothermic molar enthalpy change has a negative sign.
In the lab: Use insulation and a lid, measure starting temperatures consistently, stir, and record a temperature-time series. Estimate the reaction temperature from an appropriate extrapolation rather than ignoring cooling during measurement.
Which two habits make the investigation or model in this case more defensible?
ความยาก 2/5 · 140 เหรียญ + โบนัสเล็กน้อยในการทำครั้งแรกวันนี้
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: 100 g solution rises by 5.0 °C; specific heat capacity is 4.18 J per gram per degree. q = mcΔT. q = 100 × 4.18 × 5.0 = 2,090 J = 2.09 kJ.
- If 0.050 mol reacts, ΔH = -q/n = -2.09/0.050 = -41.8 kJ per mole.
Your turn: 50 g water rises 4 °C. Use c = 4.2 J per gram per degree to find q.
50 g water rises 4 °C. Use c = 4.2 J per gram per degree to find q.
ความยาก 2/5 · 140 เหรียญ + โบนัสเล็กน้อยในการทำครั้งแรกวันนี้
The result is 840 J. Known: 50 g of water; temperature rise 4 degrees C; c = 4.2 J per gram per degree. Use q = mc x temperature change. q = 50 x 4.2 x 4 = 840 J. This is the energy gained by the water; heat losses mean the reaction released at least this much.
Check the conclusion and its limits
- Heat loss usually lowers the observed temperature rise.
- The solution gaining heat and the reaction losing heat have opposite signs.
Not true: Bond breaking releases energy. Bond breaking is endothermic - it requires energy. Bond forming releases energy. Confusing the two reverses the sign of every energy calculation.
- Exothermic reactions transfer energy to surroundings.
- Use energy transferred = mass × specific heat capacity × temperature change.
- Watch for: heat loss usually lowers the observed temperature rise.
Write from 756 J and minus 37.8 kJ per mole
- 60 g of solution rises by 3 degrees. Use c = 4.2 J per gram per degree. 0.020 mol reacts.
- Find q and the enthalpy change per mole. Bond breaking requires energy.
Bond breaking is endothermic - it requires energy.
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True. Bond breaking is endothermic - it requires energy. Bond forming releases energy. Confusing the two reverses the sign of every energy calculation.
Transferring energy to the surroundings: write the technical term.
ความยาก 1/5 · 100 เหรียญ + โบนัสเล็กน้อยในการทำครั้งแรกวันนี้
exothermic means Transferring energy to the surroundings.
60 g of solution rises by 3 degrees and c is 4.2 J per gram per degree. 0.020 mol reacts. What is q, what is the enthalpy change per mole, and what does bond breaking do?
q is mass times specific heat capacity times temperature rise. The enthalpy change per mole is minus q divided by the amount for an exothermic temperature rise.
What is q for this temperature rise?
ความยาก 1/5 · 100 เหรียญ + โบนัสเล็กน้อยในการทำครั้งแรกวันนี้
60 times 4.2 times 3 is 756 J. The prompt uses 50 g, a 4 degree rise and the same specific heat capacity. Other options — 60 J (the mass read as energy); 840 J (the figure given in the question).