Is it possible to hack a WhatsApp chat without a target phone?

Yes you can be hacked, there are some apps to use whatsapp web on mobile which provides feature to scan QR code with your phone on the app and then your whatsapp will be cloned by that app.

For android:
I prefer to use this (WhatsWeb – WebLite version for Whatsapp – Android Apps on Google Play ) on android phone because of some reasons below:
It has password or fingerprint lock feature to protect the conversations. It has best UI/UX comparing with others and without any major issue/bug.
Below are the clear steps to hack the WhatsApp account on any Smartphones:
Begin by setting up Hacking WhatsApp account on a new mobile phone using the phone number of your target.
During the setup process, WhatsApp will call the target’s phone number and will provide a PIN that needs to be entered for the authentication of the account.
If you already have access to the victim’s phone, you can just answer the phone call and grab the code with no efforts. Even if the victim has a lock screen enabled on the phone, you can receive the phone call to get the secret PIN.
Using this known and simple trick your colleagues can Hack WhatsApp Account easily.