How does the mood of Romeo and Juliet change during act 3?

The mood of the play in Acts 1 and 2 very greatly from Act
3, at which point the plot is shocked back into the world of the feud. In Act 1, Romeo
is heartbroken at the thougt of not having his Rosaline, while Juliet tells her mother
that marriage is "an honor" she has not thought of (Act 1).  This full distance between
their lives serves to enhance the passionate exchanges they hold in the final scene of
Act I into the balcony scene in Act 2. . In Act 2, we see the two arranging their
wedding with the help of Friar Laurence and the Nurse. If nothing else, the mood can
best be described as hopeful here, as stated by Friar Laurence when he states that he
hopes the union will serve to end the feud.

Act 3 presents
the dramatic twist of events. The harsh reality of the feud is brought back to the
forefront with the deaths of Mercutio and Tybalt, and the tragedy is further enhanced by
Romeo's banishment by the Prince for his role in the deaths. The hopeful mood has been
washed away and replaced by the violent and desperate mood of Act 3's
events.