Which of the following statements is true?
a. the entropy of a system always decreases for a spontaneous process.
b. a spontaneous reaction is always a fast reaction.
c. perpetual motion machines are a possibility in the near future.
d. there is a "heat tax" for every energy transaction.
e. none of the above is true.

The correct answer is D.

Explanation:

In every process that includes an energy transaction or transition, inevitably a little amount of energy in heat form will be lost. Regardless if the process is spontaneous or not, the loss of energy (heat tax) will be present always. Also, it is impossible to make a perpetual machine, not always a spontaneous reaction will be fast, and only in certain cases the entropy could decrease during a spontaneous process.

Therefore the correct answer is D.

  the   balanced   equation   for   reaction of   ca metal   with   o2 to   produce   cao   is as below

2 ca+ o2→ 2 cao2

  explanation

the reaction is   balanced     since the   number of atoms   in reactant side is   equal to   number of atoms   in   product side.   for   example   they   are 2   atom of   ca in   reactant   side   and   2   atoms of ca   in   the product side.

  the   process   is a   meet the   requirement   of   redox   reaction because :     calcium   is   oxidized   from   oxidation state 0   to 2+   while   oxygen   is reduced from   oxidation   state   0   to 2-