Why does God call Israel his firstborn son?

Pharaoh sees Israel as slaves. But God sees them as his son. The slavery is unacceptable to him. He sees his child in distress. In Israel the firstborn son had a special position. He was the future for the family and the coming head. He got an important part of the inheritance. Israel gets the Promised Land as inheritance (Exodus 15:17). God liberates them from slavery and will give Canaan, not only to live there, but to serve (worship) him (see Exodus 3:12). The Promised Land does not only refer to the physical place, but also to live with their Father. Only a few times in the Old Testament is Israel called God’s son(s), but it does happen, and God is their Father (e.g., Isaiah 63:16). God is a family man who rescues his children in need as the kinsman-redeemer.