I have some questions about silent prayer.
Is praying silently, in my mind, as powerful as praying aloud?
Can God hear silent prayers?
When I pray silently, should I call God "God" or "Father"?
Prayers, in of themselves, have no power. What has the power is God. That said, it matters not whether we pray aloud or in our heads---God is omniscient, and He hears (and knows) everything. The Bible teaches that He knows what our needs are even before we ask (Matthew 6:8), but yet God commands us to pray (I Thessalonians 5:17).
Consider also the account in Genesis 18:9-15, in which God tells Abraham that his wife Sarah will bear a child. Sarah laughs within herself (verse 12), and God calls her out on it (verse 15). Nothing escapes the all-knowing mind of God.
Also beware that the Bible warns to not make prayer a pubic spectacle. This could be talking either about spoken or silent prayers. The object is to avoid displaying yourself to others, as though you are seeking the attention of people. Instead, a person should retire to a quiet place when praying (Matthew 6:5-6). The "quiet place" could be something as simple as the person's mind.
In Matthew 6:9, Christ states to His disciples that we are to pray Our Father... We are, of course, to show the proper respect for God when praying. However, in Mark 15:34, Christ, while suffering on the cross, calls out, My God, My God, why hast Thou forsaken Me?
Christ has instructed us to use "Father" when praying, but it seems that proper respect is of utmost importance. Many of the psalms, for example, could easily be considered prayers, and "Father" is not always used. "God," "Lord," "LORD God," etc., are found.
I hope this helps.
May God bless you,
Ezekiel 20: 45-47:
Moreover the Word of the LORD came unto me, saying,
Son of man, set thy face toward the south, and drop (thy word) toward the south, and prophesy against the forest of the south field;
And say to the forest of the south, Hear the Word of the LORD; Thus saith the Lord GOD; Behold, I will kindle a fire in thee, and it shall devour every green tree in thee, and every dry tree: the flaming flame shall not be quenched, and all faces from the south to the north shall be burned therein.