Activation energy for diffusion represents the energy barrier for atomic jumps. How does increasing activation energy Q affect the diffusion coefficient?

• A. Increasing Q reduces diffusion coefficient D.
• B. Increasing Q increases diffusion coefficient D.
• C. Activation energy has no effect on diffusion.
• D. Diffusion coefficient becomes independent of Q.

Answer: (C)

Activation energy for diffusion sets the energy barrier atoms must overcome to jump between sites. The diffusion coefficient follows an Arrhenius-type relation: D = D0 exp(-Q / (R T)). This means that at a fixed temperature, increasing the activation energy Q makes the exponential factor smaller, so D decreases. In other words, a larger barrier reduces how often atoms can successfully jump, slowing diffusion. The pre-exponential term D0 encapsulates how often attempts occur and how far atoms move, but the barrier height Q is what controls the likelihood of those jumps. Even though higher temperature can mitigate the impact of a bigger barrier, the dependence on Q remains, so increasing activation energy reduces the diffusion coefficient.