I've been trying to find a definitive answer to this question since long, and I still haven't found one! My teacher at school says that it can't be zero, according to the Arrhenius equation. She says if the activation energy were to be zero, every collision would result in a reaction, which does not take place, and hence activation energy is not zero. But doesn't a reaction take place when there is an effective collision, ie, with sufficient energy(>Ea) and with proper orientation (according to the collision theory)? Or is there something I have misunderstood about activation energy all along? It'd also be great if someone could tell me what the meaning of activation energy actually is. I've searched online, and I've come up with various answers, and now I'm confused.