Final answer:
The main impact of westward expansion on African slaves in the 1800s was the significant increase in demand for slave labor and the perpetuation of the institution of slavery. Enslaved Africans endured the horrors of the domestic slave trade, with families being forcibly separated and sold to meet the demand for labor in the South.
Step-by-step explanation:
The main impact of westward expansion on African slaves in the 1800s was the significant increase in demand for slave labor. As cotton agriculture expanded, white planters in the South sought fertile lands in newer states to the southwest, resulting in the rapid growth of the cotton kingdom. This expansion led to the entrance of nine new slave states into the Union, exacerbating and perpetuating the institution of slavery. Enslaved Africans endured the horrors of the domestic slave trade, as families were forcibly separated and sold to meet the South's demand for labor.