Energetics - OCR GatewayCalculating energy changes in chemical reactions
Exothermic reactions in solution give out energy and the temperature increases, while endothermic reactions take in energy and the temperature decreases. Bonds are broken and made in reactions.
The energy change in a reaction can be calculated using bond energyThe amount of energy needed to break one mole of a particular covalent bond.. A bond energy is the amount of energyThe capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. needed to break one moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 ×10²³ number of particles). of a particular covalent bondA bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons.. Different bonds have different bond energies. These are given when they are needed for calculations.
To calculate an energy change for a reaction:
add together the bond energies for all the bonds in the reactantA substance that reacts together with another substance to form products during a chemical reaction. - this is the ‘energy in’
add together the bond energies for all the bonds in the productA substance formed in a chemical reaction. - this is the ‘energy out’
energy change = energy in - energy out
Example
Hydrogen and chlorine react to form hydrogen chloride gas:
H−H + Cl−Cl → 2 × (H−Cl)
Use the bond energies in the table to calculate the energy change for this reaction.
Bond
Bond energy (kJ/mol)
H−H
436
Cl−Cl
243
H−Cl
432
Bond
H−H
Bond energy (kJ/mol)
436
Bond
Cl−Cl
Bond energy (kJ/mol)
243
Bond
H−Cl
Bond energy (kJ/mol)
432
Energy in = 436 + 243 = 679 kJ/mol
Energy out = (2 × 432) = 864 kJ/mol
Energy change = in - out
= 679 - 864
= -185 kJ/mol
The energy change is negative. This shows that the reaction is exothermic.
Example
Hydrogen bromide decomposes to form hydrogen and bromine:
2 × (H−Br) → H−H + Br−Br
Use the bond energies in the table to calculate the energy change for this reaction.
Bond
Bond energy (kJ/mol)
H−Br
366
H−H
436
Br−Br
193
Bond
H−Br
Bond energy (kJ/mol)
366
Bond
H−H
Bond energy (kJ/mol)
436
Bond
Br−Br
Bond energy (kJ/mol)
193
Energy in = 2 × 366 = 732 kJ/mol
Energy out = 436 + 193 = 629 kJ/mol
Energy change = in - out
= 732 - 629
= +103 kJ/mol
The energy change is positive. This shows that the reaction is endothermic.
Question
Hydrogen reacts with oxygen to form water:
2 × (H-H) + O=O → 2 × (H-O-H)
Use the bond energies in the table to calculate the energy change for this reaction.
Bond
Bond energy (kJ/mol)
H−H
436
O=O
498
O-H
464
Bond
H−H
Bond energy (kJ/mol)
436
Bond
O=O
Bond energy (kJ/mol)
498
Bond
O-H
Bond energy (kJ/mol)
464
Energy in = (2 × 436) + 498
= 872 + 498
= 1370 kJ/mo
Energy out = 2 × 2 × 464
= 1856 kJ/mol (there are two O-H bonds in each water molecule)
Energy change = in - out
= 1370 - 1856
= -486 kJ/mol
The energy change is negative. This shows that the reaction is exothermic.