Topic 2: Bonding and Structure Flashcards
What type of structure does sodium chloride have? [1]
- Giant ionic lattice
Would you expect sodium chloride to have a high or low melting point? Explain your answer. [2]
- High melting point;
- A lot of energy is required to overcome the strong electrostatic attraction between the positive and negative ions
How would you expect the melting point of NaBr to compare with NaCl? Explain your answer. [3]
- NaBr would have a lower melting point than NaCl;
- Bromide ions have one more electron shell than chloride ions, so have a larger ionic radius. This means the ions in NaBr can’t pack as closely together as the ions in NaCl;
- Ionic bonding gets weaker as the distance between the ions increases, so the ionic bonding in NaBr is weaker than in NaCl
Solid CaO does not conduct electricity, but molten CaO does. Explain this with reference to ionic bonding. [3]
- In solid, ions are held in place by string ionic bonds;
- When molten, the ions are mobile;
- so carry charge through the substance
In terms of electron transfer, what happens when sodium reacts with fluorine to form sodium fluoride? [3]
- Sodium loses one electron to form Na+;
- Fluorine gains one electron to form F-;
- Electrostatic forces of attraction between oppositely charge ions forms an ionic lattice.
Would you expect an N-N single bond to be shorter or longer than an N=N bond? Explain your answer. [3]
- Longer;
- There are 4 shared electrons in an N=N bond and only 2 shared in an N-N bond, so the electron density between the two nitrogen atoms is the nitrogen double bond is greater;
- Increases the strength of the electrostatic attraction between the positive nuclei and the negative electrons in the N=N bond, making the bond shorter
Would you expect an N-N single bond to be shorter or longer than an N=N bond? Explain your answer. [3]
- Longer;
- There are 4 shared electrons in an N=N bond and only 2 shared in an N-N bond, so the electron density between the two nitrogen atoms is the nitrogen double bond is greater;
- Increases the strength of the electrostatic attraction between the positive nuclei and the negative electrons in the N=N bond, making the bond shorter
Explain why the shapes of NCl3 and BCl3 are different. [3]
- BCl3 has 3 electron pairs around B;
- NCl3 has 4 electron pairs around B;
- including 1 lone pair
Explain what is meant by metallic bonding. [1]
- The attraction between positive metal ions and a sea of delocalised electrons between them
Explain why calcium has a higher melting point than potassium. [1]
- Calcium has two delocalised electrons per atom but potassium only has one delocalised electron per atom. So calcium has more delocalised electrons and therefore stronger metallic bonding
Silicon dioxide is a covalent compound that melts at 1610°C. Explain the high melting point of silicon in terms of its bonding. [2]
- Silicon dioxide has a giant covalent lattice structure;
- so, to melt it, lots of strong covalent bonds must be broken, which requires high temperatures
Explain why graphite is able to conduct electricity. [2]
- Graphite consists of sheets of carbon atoms, where each carbon atom is bonded to three others;
- So each atom has one free electron not involved in bonding and these free electrons allow graphite to conduct electricity
Electrical grade copper must be 99.99% pure. If sulphur and oxygen impurities react with the copper ions, its electrical conductivity is reduced. Use your knowledge of metallic and ionic bonding to explain this. [3]
- Copper is metallically bonded and so delocalised electrons are free to move (carry electric charge);
- Oxygen and sulphur form copper oxide/sulphur, fixing some electrons (as anions);
- This prevents them from moving and carrying charge
Carborundum (silicon carbide) has the formula SiC and is almost as hard as diamond.
What sort of structure would you expect carborundum to have as a solid? [1]
- Giant covalent
Carborundum (silicon carbide) has the formula SiC and is almost as hard as diamond.
Apart from hardness, give two other physical properties you would expect carborundum to have. [2]
Any two from:
1. High melting point;
2. Electrical insulator;
3. Insoluble;
4. Good thermal conductor
Define the term electronegativity. [1]
- The ability of an atom to attract the bonding electrons in a covalent bond
What are the trends in electronegativity as you go across a period and down a group in the periodic table? [1]
- Electronegativity increases across a period and decreases down a group
Explain why the boiling point of pentane is higher than that of 2-methylbutane, which is higher than that of 2,2-dimethylpropane, even though they all have the molecular formula C5H12. [3]
- Boiling point depends on energy needed to overcome intermolecular forces between molecules;
- Pentane is the most linear molecule, so has greatest surface constant, so the strongest London forces, and so the highest boiling point;
- Surface contact if 2,2-dimethylpropane is smaller than that if 2-methylbutane, which is lower than that if pentane, so these substances have weaker London forces and so lower boiling points
What intermolecular forces are present in chloroethane (CH3CH2Cl)? [1]
- London forces and permanent dipole-permanent dipole bonds
N2 and NO are both gases are room temperature. Predict, with reasoning, which has has a higher boiling point. [2]
- NO has higher boiling point. Both molecules have similar number of electrons so strength of London forces will be similar;
- NO is a polar molecule so also forms permanent dipole-permanent dipole bonds, so has stronger intermolecular forces between molecules than N2, which can only form London forces
Explain why water’s boiling point is higher than expected in comparison to other similar molecules. [2]
- Contains hydrogen covalently bonded to oxygen so can form hydrogen bonds;
- Hydrogen bonds are stronger than other types of intermolecular forces, so more energy is needed to break them
Explain why alcohols often dissolve in water but halogenoalkanes don’t. [4]
- Hydrogen bonds;
- form between the alcohol and water molecules;
- The (hydrogen) bonds between water molecules are stronger;
- than bonds that would form between water and the halogenoalkane molecules
Explain the process by which potassium iodide dissolves in water to form hydrated ions. [4]
- K+ ions attracted to the δ- ends of the water molecules;
- I- ions are attracted to the δ+ ends;
- The ions are pulled away from the lattice;
- and surrounded by water molecules forming hydrated ions
An unknown substance, X, is suspected to be a non-polar simple covalent molceule. Describe how you could confirm this by testing with two different solvents. Name the solvents chosen and give the expected results. [3]
Explain these results in terms of the intermolecular bonding within X and the solvents. [4]
- Try to dissolve the substance in water;
- and hexane;
- If X is non-polar, it is likely to dissolve in hexane, but not in water.
- X and hexane have London forces between their molecules;
- and form similar bonds with each other;
- Water has hydrogen bonds;
- which are much stronger than the bonds it could form with a non-polar compound
