Slow Cooker Fiesta Chicken and Rice Casserole

Ingredients:
1 1/2 lbs. boneless skinless chicken breasts
1/4 tsp. pepper
1/4 garlic powder
1/2 tsp. chili powder
10.5 oz. can cream of chicken soup don't add water
1/2 cup diced white onion
11 oz. can Mexicorn, drained (this is corn with peppers in it)
1 seeded jalapeno minced
These ingredients are added at the end:
6 oz. box Farmhouse Spanish rice (follow stove-top instructions)
14 oz. can diced tomatoes needed to make the rice
2 Tbsp. salted butter needed to make the rice
1 1/2 cups water needed to make the rice
3 cups shredded sharp cheese divided
1/2 cup sour cream