No, if it is not a regular thing. If you make it regular, their attention will stay on getting the reward rather than studying. This might inculcate a bad habit. Hence consider aligning their focus to their study more than on the reward.
However, you can do so in a different manner. For example, if they completed a hard task and you rewarded them, that will be a symbol of praising them for doing the work well. Let them only expect a reward after some tasks.