In a shallow dish, combine the flour, garlic powder, salt and pepper; coat chicken. In a skillet, saute chicken in 2 tablespoons oil over medium-high heat for 8-10 minutes or until tender. Remove and set aside. , Saute the celery, green peppers and onion in remaining oil for 5 minutes or until crisp-tender. Return chicken pan. Combine the ketchup, lemon juice, pineapple and brown sugar; add to skillet. Bring to a boil; cook and stir for 1-2 minutes or until heated through. Serve over rice.