DF4 - Bond enthalpies Flashcards
What is energy density?
How much energy you get per kilogram of fuel
What is a chemical bond?
Electrical attraction between atoms/ions. (Breaking them involves overcoming these attractive forces)
What needs to happen to break a chemical bond completely?
The atoms/ions need (theoretically) to be an infinite distance apart
What is bond enthalpy
The quantity of energy needed to break one mole of a bond in a molecule to give separate atoms all in the gaseous state. (also known as bond dissociation enthalpy)
Is bond-breaking an endothermic or exothermic process?
Bond-breaking is endothermic - so bond enthalpies are always positive
Is bond-making endothermic or exothermic?
Bond-making is exothermic
The stronger the bond, the higher its bond enthalpy. True or false? Explain why.
True. More energy is needed to break it, so it has a higher bond enthalpy.
Describe and explain the attraction between e.g two H atoms (H2) and bond length
- When a covalent bond between the two H atoms form, the atoms move towards one another because of the attractive forces between nuclei + electrons
- There are also repulsive forces between the nuclei of the two atoms
- These get bigger as atoms approach until the atoms stop moving together
- Distance between them is now equilibrium bond length. The shorter the bond length, the stronger the attraction between the atoms
Why are average bond enthalpies usually used in calculations?
The exact value of bond enthalpy depends on particular compound in which the bond is found
What is the difference between the bond enthalpies of single, double and triple bonds?
- Double bonds have much higher bond enthalpies than single bonds
- Triple bond enthalpies are even higher
What is relationship between bond enthalpy + bond length?
- In general, the higher the bond enthalpy, the shorter the bond
- This is because there are more electrons between the atoms being attracted to the positive nuclei - more attraction makes shorter bonds
How and why are bond enthalpies measured indirectly?
Why?
* It isn’t easy to measure bond enthalpies as there is often more than one type of bond in a compound
* It’s also very difficult when everything is in the gaseous state
How?
* They are measured indirectly using enthalpy cycles
Why do small atoms form strong covalent bonds
This is because the electrons are closer to the nucleus
Bond enthalpies are stated at standard conditions. True or false?
True
When methane burns, does the reaction involve the breaking or making of bonds?
It involves both making + breaking bonds
You have to break all the old bonds before you can make new ones. True or false?
False. New bonds start forming as soon as the first of the old bonds have broken
All reactions initially need energy to stretch + break bonds. True or false?
True
Describe what bond breaking + making look like on an enthalpy cycle
- Reactants —> Products
- Bond-breaking arrow points from reactants (in top line) to elements that make up reactants/products
- Bond-making arrow point from elements that make up reactants/products to the products (in top line)
- Remember that all of the compounds/atoms are in their gaseous states (include g state symbol)
Why is the value calculated for the bond enthalpy different from the value given in the data sheets? (Two main reasons)
- Value of bond enthalpy value is not actually the standard value - e.g in the equation for combustion of methane the water product is gaseous, not liquid, as it would be liquid under standard conditions - gaseous form of water is used as when using bond enthalpies you have to work it out in gaseous state
- Bond enthalpies are given often as averages - this means the values are not always precise
What are common errors that you should be aware of when calculating bond enthalpies?
- Forgetting that bonds in O2 are broken
- Not noticing that there are two C=O bonds per CO2
- Not noticing that there are two O-H bonds per H2O
In exothermic reactions, describe how energy is involved in the bonds made and broken
More energy is released when bonds are formed in the products than is used to break the bonds in the reactants
In endothermic reactions, describe how energy is involved in the bonds made and broken
More energy is absorbed when bonds in reactants are broken than is released when new bonds are formed in the products
