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Lesson · Grade 11 · Chemistry

Exothermic and endothermic reactions: the sign of ΔH

  • Grade 11
  • Chemistry
  • 1 min 59 s
  • British Columbia
  • BC11-SC-26

One enthalpy diagram builds up step by step. Burning methane has ΔH = −890 kJ/mol: its products are lower, so energy is released and the reaction is exothermic. Arrows through a level of separate atoms show that breaking bonds absorbs energy, that forming bonds releases it, and that ΔH is the difference between the two. The same diagram then shows calcium carbonate breaking down, with ΔH = +178 kJ/mol: its products are higher, so energy is absorbed and the reaction is endothermic. The video ends on the rule that a negative ΔH means exothermic and a positive ΔH means endothermic.

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In one sentence

The question this lesson answers

Does a reaction give out energy or take it in? An energy diagram shows which.

Up the side is enthalpy, H: the heat energy stored in the chemicals.

Transcript

What the lesson says

Does a reaction give out energy or take it in? An energy diagram shows which.

Up the side is enthalpy, H: the heat energy stored in the chemicals.

Delta H, the enthalpy change, is the enthalpy of the products minus that of the reactants.

First, methane burns in oxygen. Methane and oxygen are the reactants.

The products, carbon dioxide and water, have less enthalpy, so they sit lower.

Products minus reactants is negative: delta H is minus 890 kilojoules per mole.

That energy goes to the surroundings as heat. A reaction that releases energy is exothermic.

Picture every bond in the reactants breaking into separate atoms. Breaking bonds absorbs energy.

Careful: breaking bonds never releases energy. It always takes energy in.

The atoms then form carbon dioxide and water. Forming bonds releases energy.

Delta H is energy absorbed minus energy released. More is released here, so delta H is negative.

Now heat calcium carbonate: it breaks down into calcium oxide and carbon dioxide.

The products have more enthalpy, so delta H is positive: plus 178 kilojoules per mole.

This reaction absorbs heat from the surroundings. That makes it endothermic.

Here, breaking bonds absorbs more energy than forming bonds releases.

So negative delta H means exothermic, energy released. Positive delta H means endothermic, energy absorbed.

Curriculum

Where this fits

British Columbia, Grade 11

Chemistry 11, Reactions: reactionsBC11-SC-26

Previous: Reaction types and predicting their productsNext: Smelting: Getting Iron Out of Ore

British Columbia, Grade 12

Chemistry 12, Reaction rates: energy change during a chemical reactionBC12-SC-49

This lesson is also listed in Grade 12.

The outcomes and strands follow the British Columbia curriculum.

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