WARNING! Long answer!
During respiration, glucose molecules are converted to other molecules in a series of steps.
They finally end up as carbon dioxide and water.
The overall reaction is
"C"_6"H"_12"O"_6 + "6O"_2 → "6CO"_2 + "6H"_2"O" + "2805 kJ"C6H12O6+6O2→6CO2+6H2O+2805 kJ
The reaction is exothermic because the "C=O"C=O and "O-H"O-H bonds in the products are so much more stable than the bonds in the reactants.
Bond energy is the average energy needed to break a bond.
Some bond energies are:
"C-C"C-C = 347 kJ/mol
"C-H"C-H = 413 kJ/mol;
"C-O"C-O = 358 kJ/mol;
"O-H"O-H = 467 kJ/mol;
"O=O"O=O = 495 kJ/mol;
"C=O"C=O = 799 kJ/mol
We can view the process as breaking all the bonds in the reactants to separate the atoms and then re-combining the atoms to form the bonds in the products.
A glucose molecule has the formula
It contains 5 "C-C"C-C, 7 "C-H"C-H; 7 "C-O"C-O, and 5 "O-H"O-H bonds.
The 6 "O"_2O2 molecules contain 6 "O=O"O=O bonds.
The products contain 12 "C=O"C=O and 12 "O-H"O-H bonds.
The overall process is
"5C-C"color(white)(l) + "7C-H" + "7C-O" +color(red)(cancel(color(black)("5O-H"))) + "6O=O" → "12C=O" + stackrelcolor(blue)(7)(color(red)(cancel(color(black)(12))))"O-H"
or
underbrace("5C-C"color(white)(l) + "7C-H" + "7C-O"+ "6O=O")_color(red)("break these bonds")→ underbrace("12C=O"color(white)(l) + "7O-H")_color(red)("form these bonds")
The energy differences in kilojoules per mole are
color(white)(l)"5C-C" +color(white)(l) "7C-H" + "7C-O"color(white)(l)+color(white)(l) "6O=O"color(white)(l) → "12C=O" +color(white)(l) "5O-H"
"5×347" + "7×413" + "7×358"+color(white)(l) "6×495"color(white)(l) → "12×799" + "7×467"
color(white)(l)1735color(white)(l) +color(white)(l)2891color(white)(l) +color(white)(l) 2506color(white)(l) +color(white)(ll) 2970 color(white)(m)→ color(white)(m)9588 color(white)(ll)+color(white)(l) 3269
color(white)(mmmmmmm)"10 102"color(white)(mmmmmmml) → color(white)(m) color(white)(mm)"12 857"
ΔH ≈ "(10 102 - 12 857) kJ/mol" = "-2755 kJ/mol"
Almost 75 % of the energy released comes from formation of the stable "C=O" bonds in "CO"_2.
