Why breaking bonds takes energy and making bonds releases it, and calculating the overall energy change of a reaction. Higher tier content.
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1.Is breaking bonds exothermic or endothermic?
Endothermic. Energy must be supplied to break a bond.
2.Is making bonds exothermic or endothermic?
Exothermic. Energy is released when a bond forms.
3.What is bond energy?
The energy needed to break one mole of a particular bond, which is the same as the energy released when it forms.
4.State the equation for the overall energy change.
energy change = energy taken in to break bonds minus energy released when bonds form
5.What does a negative energy change mean?
The reaction is exothermic: more energy was released forming bonds than was needed to break them.
6.What does a positive energy change mean?
The reaction is endothermic: more energy was needed to break bonds than was released forming them.
7.Breaking bonds takes 3000 and forming takes 3500. What is the energy change?
3000 minus 3500 = minus 500, so the reaction is exothermic and releases 500.
8.Breaking bonds takes 2000 and forming releases 1750. What is the energy change?
2000 minus 1750 = plus 250, so the reaction is endothermic.
9.What unit are bond energies given in?
Kilojoules per mole, kJ/mol.
10.What is the first step in a bond energy calculation?
List every bond in the reactants and every bond in the products, using the balanced equation.
11.Why must the equation be balanced first?
The number of each type of bond depends on the balancing numbers, so an unbalanced equation gives the wrong totals.
12.How many bonds are broken in one molecule of methane, CH4?
Four carbon to hydrogen bonds.
13.How many bonds are in one molecule of oxygen, O2?
One double bond between the two oxygen atoms.
14.How many bonds are in one molecule of water?
Two oxygen to hydrogen bonds.
15.Why is combustion always exothermic?
The bonds formed in carbon dioxide and water are stronger overall than those broken in the fuel and oxygen, so more energy is released than taken in.
16.Why are calculated bond energy values only approximate?
Bond energies are averages taken across many different compounds, and the exact value varies with the molecule.
17.If bonds in the products are stronger than in the reactants, what type of reaction is it?
Exothermic, because forming those stronger bonds releases more energy than breaking the weaker ones required.
18.What happens to energy during the bond breaking stage?
It is absorbed from the surroundings.
19.What happens to energy during the bond forming stage?
It is released to the surroundings.
20.Why does a reaction profile peak between the reactants and products?
The peak is where bonds have been broken but new ones have not yet formed, which is the highest energy point.
21.A reaction has an energy change of minus 890 kJ/mol. Describe it.
It is exothermic and releases 890 kilojoules for every mole reacting.
22.Which is stronger, a bond with a bond energy of 400 or one of 800?
The one with 800, because more energy is needed to break it.
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