Is it permissible to pray sitting on a chair due to the difficulty of standing resulting from a surgical operation, while being able to stand for some household chores at spaced intervals, and must previous prayers be made up if that is not permissible?

It is permissible for a person to pray while sitting if standing causes him apparent and severe hardship, and complete inability to stand is not a condition. This is because Allah does not burden a soul beyond its capacity, and because standing in this case would be a hardship (haraj). Furthermore, the Prophet, peace and blessings be upon him, prayed sitting when standing became difficult for him. Some scholars have defined the hardship that permits sitting as the hardship that removes khushu' (devotional humility). The reward of one who prays sitting due to an excuse is not less than the reward of one who prays standing.