What must happen in a chemical reaction? a. energy is created b. energy is transferred. c. atoms are destroyed. d. energy is destroyed

What’s the scientific notation for the number 6,840,000,000

Can someone me with these questions plz.

100 grams of molten lead (600°c) is used to make musket balls. if the lead shot is allowed to cool to room temperature (21°c), what is the change in entropy (in j/k) of the lead? (for the specific heat of molten and solid lead use 1.29 j/g⋅°c; the latent heat of fusion and the melting point of lead are 2.45 × 104 j/kg and 327°c, respectively.)