Chem 4 organic chemistry question helpWatch
If you're doing AQA, this is actually a mechanism from AS! CH3CH2CH2Cl is the haloalkane 1-chloropropane and the ammonia is the nucleophile. Therefore, the ammonia substitutes the chlorine. The excess ammonia takes one of the hydrogens from the original ammonia (now attached to the carbon chain) to become an ammonium ion. Hope this helps!