Calculating energy changes - Higher
Using bond energies
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 脳10虏鲁 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
Worked example
Hydrogen and chlorine react to form hydrogen chloride gas:
H2 + Cl2 鈫 2HCl
贬鈭扝 + 颁濒鈭扖濒 鈫 2 脳 (贬鈭扖濒)
Use the bond energies in the table to calculate the energy change for this reaction.
Bond | Bond energy |
贬鈭扝 | 436 kJ/mol-1 |
颁濒鈭扖濒 | 243 kJ/mol-1 |
贬鈭扖濒 | 432 kJ/mol-1 |
Bond | 贬鈭扝 |
---|---|
Bond energy | 436 kJ/mol-1 |
Bond | 颁濒鈭扖濒 |
---|---|
Bond energy | 243 kJ/mol-1 |
Bond | 贬鈭扖濒 |
---|---|
Bond energy | 432 kJ/mol-1 |
Energy in = 436 + 243 = 679 kJ/mol-1
Energy out = (2 脳 432) = 864 kJ/mol-1
Energy change = in - out
= 679 - 864
= -185 kJ/mol-1
The energy change is negative. This shows that the reaction is exothermicReaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases..
Worked example
Hydrogen bromide decomposes to form hydrogen and bromine:
2HBr 鈫 H2 + Br2
2 脳 (贬鈭払谤) 鈫 贬鈭扝 + Br-Br
Use the bond energies in the table to calculate the energy change for this reaction.
Bond | Bond energy |
贬鈭払谤 | 366 kJ/mol-1 |
贬鈭扝 | 436 kJ/mol-1 |
叠谤鈭払谤 | 193 kJ/mol-1 |
Bond | 贬鈭払谤 |
---|---|
Bond energy | 366 kJ/mol-1 |
Bond | 贬鈭扝 |
---|---|
Bond energy | 436 kJ/mol-1 |
Bond | 叠谤鈭払谤 |
---|---|
Bond energy | 193 kJ/mol-1 |
Energy in = 2 脳 366 = 732 kJ/mol-1
Energy out = 436 + 193 = 629 kJ/mol-1
Energy change = in - out
= 732 - 629
= +103 kJ/mol-1
The energy change is positive. This shows that the reaction is endothermicReaction in which energy is taken in..
Question
Hydrogen reacts with oxygen to form water:
H2 + O2 鈫 2H2O
2 脳 (贬鈭扝) + O=O 鈫 2 脳 (H鈭挵库揌)
Use the bond energies in the table to calculate the energy change for this reaction.
Bond | Bond energy |
贬鈭扝 | 436 kJ/mol-1 |
O=O | 498 kJ/mol-1 |
翱鈥揌 | 464 kJ/mol-1 |
Bond | 贬鈭扝 |
---|---|
Bond energy | 436 kJ/mol-1 |
Bond | O=O |
---|---|
Bond energy | 498 kJ/mol-1 |
Bond | 翱鈥揌 |
---|---|
Bond energy | 464 kJ/mol-1 |
Energy in = (2 脳 436) + 498
= 872 + 498
= 1370 kJ/mol-1
Energy out = 2 脳 2 脳 464
= 1856 kJ/mol-1 (there are two 翱鈥揌 bonds in each water molecule)
Energy change = in - out
= 1370 鈥 1856
= -486 kJ/mol-1
The energy change is negative. This shows that the reaction is exothermic.
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