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  • In this lesson, you will learn about how a system at equilibrium responds to changes

  • in temperature.

  • LeChatelier's Principle states that a chemical system at equilibrium always works to restore

  • equilibrium when it is stressed. To consider what happens to a system at equilibrium

  • when temperature is changed, you must first consider the energetics of the reaction in

  • question. If the forward reaction is exothermic, then

  • the reverse reaction must be endothermic. Let's examine this hypothetical reaction

  • A + B C + D ΔH = -75kJ This means that 75kJ of energy is released

  • when the forward reaction occurs, and 75kJ is absorbed when the reverse reaction occurs.

  • So an increase in temperature would mean that the endothermic reaction would be favoured,

  • to remove the excess heat, therefore counteracting the imposed stress.

  • Decreasing the temperature would cause the system to produce more energy; therefore the

  • exothermic reaction would be favoured. An increase in temperature favours the endothermic

  • reaction. A decrease in temperature favours the exothermic

  • reaction. Challenge:

  • The dimerization of nitrogen dioxide to dinitrogen tetroxide is exothermic.

  • NO2(g) N2O4(g) ΔH = -57.2kJ brown colourless

  • Nitrogen dioxide is a brown gas, whereas dinitrogen tetroxide is colourless.

  • What observations do you think can be made when temperature is decreased?

  • How about when temperature is increased? Please pause the lesson to think about this,

  • and resume once you are done. A decrease in temperature favours the exothermic

  • reaction, so more dinitrogen tetroxide is produced.

  • Since it is a colourless gas, the mixture should appear paler.

  • An increase in temperature favours the endothermic reaction, so more nitrogen dioxide is produced.

  • The mixture should therefore appear darker brown.

  • Addition of a catalyst does not affect the position of equilibrium as it increases the

  • rate of both the forward and reverse reactions. It only quickens the attainment of equilibrium.

  • Let's return to the example of you digging a hole and your friend refilling it while

  • you dig. Imagine that you are both given much larger

  • shovels. The size of the hole still remains constant,

  • but with each dig or fill, more soil is removed or filled.

  • In conclusion, when the temperature of a system at equilibrium is increased, the endothermic

  • reaction is favoured. When the temperature of a system at equilibrium

  • is decreased, the exothermic reaction is favoured. Adding a catalyst has no effect on the position

  • of equilibrium.

In this lesson, you will learn about how a system at equilibrium responds to changes

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B2 中上級

ル・シャトリエの原理。第二部|化学の旅|布施の学校 (Le Chatelier's Principle: Part 2 | The Chemistry Journey | The Fuse School)

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    Wayne Lin に公開 2021 年 01 月 14 日
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