bond energy calculations

Question 1

what are exothermic energy changes shown as in calculations and why

Answer 1

negative

energy has been transferred from the chemicals

Question 2

what are endothermic energy changes shown as in calculations and why

Answer 2

positive

energy has been gained

Question 3

describe the energy loss/gain in exothermic reactions

Answer 3

energy is transferred from the chemicals to the surroundings

Question 4

describe the energy loss/gain in endothermic reactions

Answer 4

energy is gained from the surroundings

Question 5

what is required when breaking a bond

what type of reaction is this

Answer 5

required energy

endothermic

Question 6

what happens when making a bond

what type of reaction is this

Answer 6

releases energy

exothermic

Question 7

what does energy value tell us

Answer 7

every chemical has an energy value that tells us the energy required to break that bond

Question 8

calculate the energy change for this reaction
hydrogen + chloride—>hydrogen chloride
H2 + Cl2 —> 2HCl
H-H + Cl-Cl —> H-Cl
energy values:
H-H : 436kJ/mol 
Cl-Cl: 242kJ/mol
H-Cl: 431kJ/mol

Answer 8

H2 + Cl2 —> 2HCl
H-H + Cl-Cl —> H-Cl

+436 +242 —> {431 x 2}
|______| |______|
+678 -862

+678
-862
———
-184kJ

this reaction is exothermic

Question 9

what happens on the left and right and side on a chemical reaction
H-H + Cl-Cl —> H-Cl

Answer 9

left hand side, the bonds are being broken

right-hand side, the bond is being made

Question 10

calculate the energy change for
Hydrogen + Oxygen —> water
2H2 + O2 —> 2H2O
H-H + O=O —> H-O-H

H-H : 436kJ/mol
O=O : 495kJ/mol
O-H : 463kJ/mol

Answer 10

2H2 + O2 —> 2H2O
H-H + O=O —> H-O-H

[+436 x 2] +495 —> [-463 x 4]
|_________| |_______|
+1367kJ -1852

+1367
-1852
———
-485

this reaction is an exothermic reaction

each water molecule has 2 bonds and we are making 2 of these molecules so it’s 4 bonds

Question 11

what does the positive sign of an energy value tell us

Answer 11

we are putting energy in

Question 12

calculate the energy change for
methane + oxygen --> carbon dioxide + water
CH4 + 2O2 —> CO2 + 2H2O
      H
 H -C- H + O=O —> O=C=O + H-O-H
      H

C-H : 413 kJ/mol
O=O : 495 kJ/mol
C=O : 799 kJ/mol
O-H : 463 kJ/mol

Answer 12

CH4 + 2O2 —> CO2 + 2H2O
H
H -C- H + O=O —> O=C=O + H-O-H
H

-808kJ

[+413 x 4]+[+495 x 2]-> [-799 x 2]+[-463 x 4]
|______| |______| |______| |_____|
+1652 +990 -1598 + -1852
|______________| |____________|
+2642 -3450

+2642
-3450
———
-808kJ