Crown replaced and back tooth and large gap traping food in between the tooth and the one behind it.

Yes, trapping food between a crown and the tooth behind it is a problem. I suggest bringing this to the attention of your dentist. They should be willing to correct the problem for you.

Yes, food impaction can become a problem. It can lead of further decay underneath the new crown or in the adjacent tooth and gum issues. I would recommend going back to your dentist to have the crown replaced. 

You should return to your dentist and express your concerns with the excess space.  This gap is a food trap which can cause decay/cavity and periodontal (bone and gum) problems.  Your dentist should be able to provide some ideas for correcting the issue.  

A gap may be able to be sealed to prolong the life of the crown and prevent food from getting caught, a ledge should be able to be recontoured so the crown and tooth are flush with eachother. If the gap is too big, you may have to have the crowns redone. These types of issues with crowns (and any kind of tooth restoration for that matter) should be dealt with since they are areas that are more susceptible to the development of decay (cavities).