Question

A student performs a reaction twice. In the second trial, she raises the temperature by 20¨¬C and notices that the reaction takes place more quickly. She concludes that the reaction must be endothermic. Explain why the student is incorrect.

Answer 1

@thomaster

Answer 2

@paki

Answer 3

@ParthKohli

Answer 4

You can't conclude that the reaction is endothermic purely because it sped up at a higher temperature, because ALL reactions (endothermic and exothermic) proceed faster as you increase the temperature.

This comes down to collision theory - the higher the temperature, the higher the average kinetic energy of reactant particles. This means they're moving around faster, and more likely to collide with one another with enough energy to overcome the activation energy of the reaction.

What determines whether a reaction is endothermic or exothermic is your net change in enthalpy after the reaction. If the change is positive, the reaction was endothermic because the reactants absorbed heat energy to form the products (heat was added to the system). If the change is negative, the reaction was exothermic because the reactants released heat energy to form the products (heat was subtracted from the system).