Charles H. Spurgeon on Psalms 81:10

Open thy mouth wide. This implies,

1. Warmth and fervency in prayer. To open the mouth is in effect to open the heart, that it may be both engaged and enlarged... We may be said to open our mouths wide when our affections are quick and lively, and there is a correspondence between the feelings of the heart and the request of the lips; or when we really pray, and not merely seem to do so. This is strongly and beautifully expressed in Ps 119:131: I opened my mouth, and panted: for I longed for thy commandments.

2. It implies a holy fluency and copiousness of expression, so as to order our cause before him, and fill our mouths with arguments. When the good man gets near to God, he has much business to transact with him, many complaints to make, and many blessings to implore; and, as such seasons do not frequently occur, he's the more careful to improve them. He then pours out his whole soul, and is at no loss for words; for when the heart is full, the tongue overflows. Sorrow and distress will even make those eloquent who are naturally slow of speech.

3. Enlarged hope and expectation. We may be too irreverent in our approaches to God, and too peremptory in our application; but if the matter and manner of our prayer be right, we cannot be too confident in our expectations from him... Open thy mouth wide then, O Christian; stretch out thy desires to the uttermost, grasp heaven and earth in thy boundless wishes, and believe there is enough in God to afford the full satisfaction. Not only come, but come with boldness to the throne of grace: it is erected for sinners, even the chief of sinners. Come to it then, and wait at it, till you obtain mercy and find grace to help in time of need. Those who expect most from God are likely to receive the most. The desire of the righteous, let it be ever so extensive, shall be granted. Benjamin Beddome.