Energy, rates and reactions - AQA SynergyCalculating energy changes - Higher
Energy changes can be represented using energy profiles. Catalysts (including enzymes) speed up chemical reactions. Energy changes can be calculated from 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 ×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
Worked example
Hydrogen and chlorine react to form hydrogen chloride gas:
H2 + Cl2 → 2HCl
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
H−H
436 kJ/mol-1
Cl−Cl
243 kJ/mol-1
H−Cl
432 kJ/mol-1
Bond
H−H
Bond energy
436 kJ/mol-1
Bond
Cl−Cl
Bond energy
243 kJ/mol-1
Bond
H−Cl
Bond energy
432 kJ/mol-1
Energy in = 436 + 243 = 679 kJ/mol-1
Energy out = (2 × 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 → H2 + Br2
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
H−Br
366 kJ/mol-1
H−H
436 kJ/mol-1
Br−Br
193 kJ/mol-1
Bond
H−Br
Bond energy
366 kJ/mol-1
Bond
H−H
Bond energy
436 kJ/mol-1
Bond
Br−Br
Bond energy
193 kJ/mol-1
Energy in = 2 × 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 → 2H2O
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
H−H
436 kJ/mol-1
O=O
498 kJ/mol-1
O–H
464 kJ/mol-1
Bond
H−H
Bond energy
436 kJ/mol-1
Bond
O=O
Bond energy
498 kJ/mol-1
Bond
O–H
Bond energy
464 kJ/mol-1
Energy in = (2 × 436) + 498
= 872 + 498
= 1370 kJ/mol-1
Energy out = 2 × 2 × 464
= 1856 kJ/mol-1 (there are two O–H bonds in each water molecule)
Energy change = in - out
= 1370 – 1856
= -486 kJ/mol-1
The energy change is negative. This shows that the reaction is exothermic.
