come back chem Flashcards
A hydrocarbon is found to contain 92.3% by mass of carbon. What is its empirical formula?
b)The hydrocarbon mentioned in the previous question is found to have an MR of 78. What is its molecular formula?
Carbon (92.3%) Hydrogen (7.7%) 92.3/12 = 7.69 7.7/1 = 7.7 1 : 1 ratio Therefore, empirical formula is CH.
b)Empirical formula is CH. Relative mass of CH = 13. 78 / 13 = 6 Therefore, molecular formula is C6H6.
Define functional group.
An atom, or group of atoms, in a molecule which determines its chemical properties.
What are the three characteristics of a homologous series?
b)What does a dot represent in a free radical mechanism?
Same general formula
· Same functional group · Same chemical reactivity
b)The dot represents an unpaired electron
Define structural isomerism.
b)explain why alkenes show E-Z isomerism but alkanes do not.
Two or more compounds with the same molecular formula but different structural formula
b)Alkenes contain a double bond that is planar and unable to rotate. Therefore, the substituent groups are locked in place and E-Z isomerism can arise. Alkanes contain only single covalent bonds that are able to rotate and, therefore, E-Z isomerism cannot occur.
State and explain the reactivity of iodoalkanes compared to fluoroalkanes.
Iodoalkanes are more reactive than fluoroalkanes because the C-I bond is weaker (has lower bond enthalpy) than the C-F bond. This outweighs the increased partially positive carbon present in the fluoroalkane
Why are two ammonia molecules necessary for nucleophilic substitution reaction to form butane nitrile occur?
The first ammonia molecule acts as a nucleophile, the second acts as a base.
(Ch2ch2ch2ch2nh2) and (nh4)
Compare and contrast the role of the hydroxide ion in nucleophilic substitution and elimination reactions.
In nucleophilic substitution reactions the hydroxide ion acts as a nucleophile (a lone pair donor) whereas in elimination reactions the hydroxide ion acts as a base (a proton acceptor).
Why is the ozone layer important for life on earth?
The ozone layer absorbs ultraviolet radiation from the Sun and prevents it from reaching the surface of the Earth. Without the ozone layer, plant and animal life as we know it would struggle to exist
Why is the ozone layer important for life on earth?
The ozone layer absorbs ultraviolet radiation from the Sun and prevents it from reaching the surface of the Earth. Without the ozone layer, plant and animal life as we know it would struggle to exist.
Write equations showing how CFCs break down the ozone layer.
What is it acting as
Cl· + O3 —> O2 + ClO·
ClO· + O3—> 2O2 + Cl·
Catalyst
Write an overall equation for the decomposition of ozone into oxygen.
2O3 —-> 3O2
Why are HCFCs a better alternative to CFCs?
Although HCFCs contain a C-Cl bond, they are far less stable than CFCs and, therefore, most of them are destroyed in the low atmosphere before reaching the ozone layer.
Name the type of reaction and the product formed when ethyl hydrogen sulfate reacts with water.
A hydrolysis reaction that produces ethano
Describe and explain the reactivity of addition polymers.
.
Addition polymers are unreactive because they are made of long saturated hydrocarbon chains with strong covalent C-C bonds that are difficult to break
How can scientists change the properties of polymers?
By changing the monomer or by changing the conditions under which the polymer is made.
Give some typical uses of poly(chloroethene), otherwise known as PVC.
Poly(chloroethene), otherwise known as PVC, is used in piping, packaging, window frames and many other areas.
How does a plasticiser affect the properties of a polymer?
Plasticisers are used to make polymers softer and more flexible.
State and explain the difference in properties between thermosetting and thermosoftening polymers.
Thermosetting polymers do not melt when heated and cannot be reshaped. This is due to the strong covalent bonds linking polymer chains together.
Thermosoftening polymers melt when heated and can be reshaped. This can happen because the only intermolecular forces between polymer chains are weak van der Waals forces.
What is a single covalent bond?What is a double covalent bond?
A shared pair of electrons between two atoms.
b)Two shared pairs of electrons between two atoms.
What is a coordinate (dative covalent) bond? Draw the coordinate bond between ammonia and boron trifluoride.
A covalent bond where both shared electrons originated from one atom.
b)e.g. H3N:–> BF3
Describe the bonding in a metal.
A regular lattice of positively charged metal ions surrounded by a sea of delocalised electrons.
What structure are the following crystals? 1. Sodium chloride 2. Magnesium 3. Diamond 4. Graphite 5. Iodine 6. Ice
Ionic 2. Metallic 3. Macromolecular 4. Macromolecular 5. Molecular 6. Molecular
Why do iodine crystals have such a low melting point?
b) Does iodine conduct electricity?
Iodine has a simple molecular structure. The intramolecular bonds in iodine are strong covalent, but the much weaker intermolecular Van der Waals between molecules are easily overcome by a small amount of heat energy.
b) No, there are no charged particles (delocalised electrons or free ions) that are able to move and carry a charge.
Why can graphite conduct electricity but diamond can’t? ·
The carbon atoms in graphite form three covalent bonds to other carbon atoms. · Each carbon’s fourth electron is delocalised and is free to move and carry a charge. · In diamond all electrons are covalently bonded and unable to move.
Explain why carbon tetrachloride (CCl4) has polar bonds but not a permanent dipole.
Although there are partial charges, the molecule is symmetrical and, therefore, the partial charges cancel each other out, leaving the molecule to be non-polar.
