Bhagavad Gita

ಏವಮುಕ್ತ್ವಾsರ್ಜುನಃ ಸಂಖೇ ರಥೋಪಸ್ಥ ಉಪಾವಿಶತ್ ।
ವಿಸೃಜ್ಯ ಸಶರಂ ಚಾಪಂ ಶೋಕಸಂವಿಗ್ನಮಾನಸಃ ॥೪೭॥
saṃjaya uvāca |
evamuktvāsrjunaḥ saṃkhe rathopastha upāviśat |
visṛjya saśaraṃ cāpaṃ śokasaṃvignamānasaḥ ||47||

Sloka 1:47
Gist of the sloka:
Sanjaya said: Right in the battlefield, Arjuna kept aside his bow and arrows and sat down in the middle of his chariot.
Standing and speaking in the chariot for this long, Arjuna sat down in his chariot after discarding his bow and arrow. The mental confusion affected his physical body and Arjuna was unable to stand.
Arjuna by now had completely poured out his feelings and thoughts to Lord Krishna.
Before proceeding to the next chapter, let us first discuss few questions which we might have.
Q1. How did in the middle of the battlefield Lord Krishna and Arjuna spent so much time that they had a conversation of over 700 slokas? What were rest of the people doing at the time? How long did it take them to have the conversation?
We need to first understand some of the rules guiding the war. One was not supposed to start the fight until the opposite party was also ready. One could only fight with another having similar weapons and status. Which meant, armies would wait until Arjuna was ready.
Arjuna was regarded even before the war as one the great experts in relation to various dharmas, books and sacred knowledge. This aspect is mentioned in the earlier parts of Mahabharatha. Lord Krishna is the universal lord. Conversation between them would not necessarily happen using the 700 slokas which we now have. They would have had the conversation in crisp and in short form.
The Gita which we now have is the expanded form of that conversation given to us as a conversation between Sanjaya and Emperor Dhitrarastra, as written down for our benefit by sage Veda Vyasa. Even so, we need thousands of pages of commentary about Gita to give us that little bit of knowledge and wisdom.
Q2. Why was Lord Krishna instructing Arjuna instead of his elder brother Bheema?
The answer to this question is very easy. You give the medicine to those who need it, not to their elders or healthy. Similar is the case here. Arjuna needed help and guidance not Bheema.