Question #81303

a. To balance the given reaction
#(-)"H"_2"O"+ (-)"CO"_2+"Sunlight"->10"C"_6"H"_12"O"_6+ (-)"O"_2#

Let us write the balanced equation as
#a"H"_2"O"+ b"CO"_2+"Sunlight"->10"C"_6"H"_12"O"_6+ c"O"_2#

By inspection, balancing #"C"# on both sides of the reaction we obtain #b=60#. The reaction becomes

#a"H"_2"O"+ 60"CO"_2+"Sunlight"->10"C"_6"H"_12"O"_6+ c"O"_2#

Next let us balance #"H"#. As there are #120 " atoms of H"# on right hand side of the reaction we obtain #a=60#, Now the reaction becomes

#60"H"_2"O"+ 60"CO"_2+"Sunlight"->10"C"_6"H"_12"O"_6+ c"O"_2#

Now left is only #"O"#. There are total #60+120=180# atoms of oxygen on the left had side of the reaction. On the right had side we have

#60+2c# atoms, Equating both we get
#60+2c=180#
or #c=(180-60)/2=60#. Thus we get the balanced reaction as

#60"H"_2"O"+ 60"CO"_2+"Sunlight"->10"C"_6"H"_12"O"_6+ 60"O"_2#

if we observe closely we see that number #10# can be factored out from both sides of the reaction. Therefore, final balanced reaction can be written as

#6"H"_2"O"+ 6"CO"_2+"Sunlight"->"C"_6"H"_12"O"_6+ 6"O"_2#

We know that the reaction represents the process of photosynthesis which takes place in plants, some algae and cyanobacteria.

Sometimes the reaction of photosynthesis is written as

#12"H"_2"O"+ 6"CO"_2+"Sunlight"->"C"_6"H"_12"O"_6+6"H"_2"O"+ 6"O"_2#

b. Like other endothermic reactions, photosynthesis does not absorb energy in the form of heat, therm is unit of heat. But there would no reaction in the absence of light energy.

In other words energy in the form of sunlight needs to be made available in order that photosynthesis reaction occurs. As such it should be called an endothermic reaction, though not in strict sense.

c. The sunlight energy is converted in to chemical energy which is stored in the form of glucose molecules.