6.2.2Chemical Change: Rate and Extent
Energy Changes and Reversible Reactions — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 11 of 16
Section 1 of 7Reversible reactions
Two-way change
- Two-way change In a reversible reaction the products can react to form the original reactants again.
- It is shown with a double arrow: A + B ⇌ C + D.
Forward and reverse
- Forward and reverse The reaction going left to right is the forward reaction;
- right to left is the reverse reaction.
- Changing the conditions can make either direction dominate.
Not every reaction
- Not every reaction Many reactions, such as burning, are effectively one-way and do not reverse under normal conditions.
Section 2 of 7Energy in both directions
Same energy, opposite sign
Exothermic one way
- Exothermic one way If the forward reaction releases heat to the surroundings, the reverse reaction must take in heat.
Endothermic the other
- Endothermic the other The reverse of an exothermic change is endothermic, so heat is absorbed from the surroundings.
Equal amounts
- Equal amounts The amount of energy released in one direction is exactly the amount absorbed in the other direction.
Section 3 of 7Example: hydrated copper(II) sulfate
1Start with blue crystals
- Start with blue crystals Hydrated copper(II) sulfate, CuSO4·5H2O, is blue.
2Heat it
- Heat it Water of crystallisation is driven off as steam, an endothermic change.
3White powder remains
- White powder remains Anhydrous copper(II) sulfate, CuSO4, is white.
4Add water
- Add water The white solid turns blue again and gets warm, as the exothermic reverse reaction takes place.
Section 4 of 7Equation and uses
| Direction | Change | Energy | Observation |
|---|---|---|---|
| Forward (heating) | CuSO4·5H2O → CuSO4 + 5H2O | Endothermic | Blue crystals turn white; steam given off |
| Reverse (adding water) | CuSO4 + 5H2O → CuSO4·5H2O | Exothermic | White powder turns blue and the temperature rises |
Section 5 of 7Other examples to know
Ammonium chloride
- Ammonium chloride On heating, solid ammonium chloride splits into ammonia and hydrogen chloride gases;
- they recombine on cooler glass to make the solid again.
Test for water
- Test for water Anhydrous copper sulfate is used to test for water because it turns from white to blue.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Reversible reaction | A reaction in which products can turn back into reactants. |
| Exothermic | A change that releases heat energy to the surroundings. |
| Endothermic | A change that absorbs heat energy from the surroundings. |
| Hydrated | Containing water of crystallisation within the crystal structure. |
| Anhydrous | Containing no water of crystallisation. |
Section 7 of 7Exam technique
1
Writing a single arrow for a reversible reaction instead of ⇌.
2
Thinking both directions are exothermic; one is exothermic and the other endothermic.
3
Describing the heating of copper sulfate as burning, when it is thermal decomposition by water loss.
4
Mixing up the colourshydrated is blue and anhydrous is white.
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