When you encrypt iPhone backup on Mac, a password entry dialog will pop up to ask you to enter your password and display a "Remember this password in my Keychain" check box. If you checked the checkbox, your iPhone encrypted backup password was saved in your Keychain files. So you can open the Keychain to show the encryption password once you forget it. 

If you did not check that "Remember this password in my Keychain" option, your iPhone backup password would not be saved in your Keychain. So it is impossible to find the encryption password in the Keychain or anywhere on your Mac. In this case, you have to use a third-party application to recover iPhone encrypted backup password on your MacBook.

This post would be helpful: http://www.isumsoft.com/it/find-iphone-encrypted-backup-password-in-keychain-on-macbook/