Exothermic and Endothermic Reactions: Question 2
Syllabus 5.1
Rail maintenance engineers use the thermite reaction to weld two sections of steel track together on site. Powdered aluminium is mixed with iron(III) oxide and ignited; the reaction produces molten iron, which fills the gap between the rails, and solid aluminium oxide.
A reaction pathway diagram for this reaction has energy on the vertical axis and progress of reaction on the horizontal axis. The curve starts at a high point labelled "reactants: aluminium + iron(III) oxide", rises briefly to a peak, and then falls to a much lower point labelled "products: iron + aluminium oxide".
(a) Using the reaction pathway diagram described above, state whether the thermite reaction is exothermic or endothermic. Give a reason for your answer based on the relative energy levels of the reactants and products. [2]
(b) Describe what happens to the temperature of the surroundings during the thermite reaction, and explain why this happens. [2]
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Worked solution
Part (a): Classifying the reaction from the diagram
On the described reaction pathway diagram, the curve starts high (labelled “reactants”) and, after an initial peak, ends much lower (labelled “products”).
Since the products end up at a lower energy level than the reactants, energy has been released overall during the reaction. The reacting particles have lost energy to their surroundings.
This means the thermite reaction is exothermic.
Part (b): Effect on the temperature of the surroundings
Because the reaction is exothermic, it transfers thermal energy to the surroundings (the surrounding rail, air and the steel being welded).
As a result, the temperature of the surroundings increases, dramatically so in this case, since the reaction releases enough thermal energy to produce molten iron.
Final answers
- (a) Exothermic. The products are at a lower energy level than the reactants on the diagram, so energy is released overall.
- (b) The temperature of the surroundings increases, because the exothermic reaction transfers thermal energy to them.