CGP EXAM QUESTIONS: OZ Flashcards
Many covalent molecules have a permanent dipole, due to differences in electronegativities
Define the term electronegativity
The ability of an atom to attract the bonding electrons in a covalent bond
Many covalent molecules have a permanent dipole, due to differences in electronegativities
What are the trends in electronegativity as you go across a period and down a group in the periodic table?
Electronegativity increases across a period and decreases down a group
Which of the following molecules is polar?
A) H2O
B) Br2
C) CCl4
D) SF6
A) H2O
Draw a diagram to show the shape of Boron trichloride (BCl3) a covalently bonded molecule. Mark any partial charges on your diagram
Draw a diagram to show the shape of Dichloromethane (CH2Cl2) a covalently bonded molecule. Mark any partial charges on your diagram
Explain whether or not Boron trichloride (BCl2) or Dichloromethane (CH2Cl2), both covalently bonded molecules, have an overall dipole.
The polar bonds in BCl3 are arranged so that they cancel each other out, so the molecule has no overall dipole
CH2Cl2 does have an overall dipole because the polar bonds are not orientated so they are pointing in opposite directions so they don’t cancel each other out
The molecules in the table all have the molecular formula C5H12
Explain the differences in the boiling points of these molecules
The boiling point of a substance depends on the energy needed to overcome the intermolecular forces between the molecules. Pentane is the most linear molecule so it has the greatest surface contact, and so has the strongest instantaneous dipole-induced dipole bonds. This gives it the highest boiling point
The surface contact of 2-methylbutane is less than that of pentane and that of 2,2-dimethylpropane is smaller still, meaning that these substances have weaker instantaneous dipole-induced dipole bonds and consequently lower boiling points
What intermolecular bonds are present in methane?
A) Instantaneous dipole-induced dipole bonds only
B) Permanent dipole-permanent dipole bonds only
C) Both instantaneous dipole-induced dipole bonds and permanent dipole-permanent dipole bonds
A) Instantaneous dipole-induced dipole bonds only
State and explain which of Ammonia (NH3) and methane (CH4) will have the higher boiling point.
Ammonia will have the higher boiling point
Because it can form hydrogen bonds
There are stronger/take more energy to overcome than the intermolecular bonds between the molecules of CH4
State and explain which of water (H2O) and hydrogen sulfide (H2S) will have the higher boiling point.
Water will have the higher boiling point
Because it can form hydrogen bonds
There are stronger/take more energy to overcome than the intermolecular bonds between the molecules of H2S
State and explain which of butane and propan-1-ol will have the higher boiling point.
Propan-1-ol will have the higher boiling point
Because it can form hydrogen bonds
There are stronger/take more energy to overcome than the intermolecular bonds between the molecules of butane
An organic compound used as antifreeze is ethane-1,2-diol
Its structure is shown below
The boiling point of ethane-1,2-diol is 197oC, whereas the boiling point of ethanol is 78oC. Suggest a reason for this difference
Ethane-1,2-diol has stronger intermolecular bonds because there are two alcohol groups, twice as many as in ethanol. Therefore ethane-1,2-diol can form twice as many hydrogen bonds as ethanol
Nitrogen monoxide (NO) and ozone (O3) sometimes react to produce nitrogen dioxide (NO2) and oxygen (O2)
A collision between the two molecules does not always lead to a reaction. Explain why.
The molecules don’t always have enough energy
Collisions don’t always happen in the right orientation
Explain why an increase in temperature results in an increase in the rate of a reaction. Draw a suitable graph to support your answer
At higher temperatures, more particles will have the activation energy and will be able to react
So collisions that result in a reaction/have the activation energy will happen more frequently
The graph below shows the rate that oxygen is evolved when hydrogen peroxide decomposes at 25oC
Which of the curves, X, Y or Z shows the rate that oxygen is evolved when the same amount of hydrogen peroxide decomposes at 15oC? Explain your answer
Y
The same amount of hydrogen peroxide is decomposed, so the same amount of oxygen will be evolved (so it can’t be curve Z)
Curve Y shows that oxygen is evolved more slowly and reaction rates are slower at lower temperatures
Explain how homogenous catalysts speed up chemical reactions
The homogenous catalyst forms intermediate compounds
The activation enthalpy needed to form the intermediates (and to form the products from the intermediates) is lower than that needed to make the products directly from the reactants
Enzymes are proteins that catalyse specific biological reactions
Draw a fully labelled enthalpy profile for an enzyme-catalysed reaction and an uncatalysed reaction
A student follows the rate at which magnesium reacts with hydrogen chloride by measuring the volume of gas evolved over 5 minutes. The reaction is:
Mg(s) + 2HCl(aq) –> MgCl2(aq) + H2(g)
Her results are shown in the table below
Plot a graph to follow the course of the reaction
A student follows the rate at which magnesium reacts with hydrogen chloride by measuring the volume of gas evolved over 5 minutes. The reaction is:
Mg(s) + 2HCl(aq) –> MgCl2(aq) + H2(g)
Her results are shown in the table below
What is the rate of formation of hydrogen after 50 seconds?
0.15 cm3 s-1 (± 0.005 cm3 s-1)
A student follows the rate at which magnesium reacts with hydrogen chloride by measuring the volume of gas evolved over 5 minutes. The reaction is:
Mg(s) + 2HCl(aq) –> MgCl2(aq) + H2(g)
Her results are shown in the table below
What other suitable method could the student have used to follow this reaction?
measuring the mass loss
measuring the change in pH
taking samples and titrating
measuring a temperature change
A student follows the rate at which magnesium reacts with hydrogen chloride by measuring the volume of gas evolved over 5 minutes. The reaction is:
Mg(s) + 2HCl(aq) –> MgCl2(aq) + H2(g)
Her results are shown in the table below
Explain how the rate of the reaction changes over time
The rate of reaction decreases over time because as the concentration of reactants decreases, they are likely to collide and react less frequently
2-bromopropane reacts with water in the following reaction:
CH3CHBrCH3 + H2O –> CH3CH(OH)CH3 + HBr
Name this type of reaction
nucleophilic substitution
2-bromopropane reacts with water in the following reaction:
CH3CHBrCH3 + H2O –> CH3CH(OH)CH3 + HBr
Give the systematic name of the organic product
propan-2-ol
2-bromopropane reacts with water in the following reaction:
CH3CHBrCH3 + H2O –> CH3CH(OH)CH3 + HBr
Under the same conditions, 2-iodopropane was used in place of 2-bromopropane in the reaction above. What difference would you expect in the rate of the reaction? Explain you answer
The reaction would be faster with 2-iodopropane
This is because the C-I bond is weaker than the C-Br bond
2-bromopropane reacts with water in the following reaction:
CH3CHBrCH3 + H2O –> CH3CH(OH)CH3 + HBr
Draw the mechanism for the reaction between 2-bromopropane and aqueous potassium hydroxide.
diagram
When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.
Write an overall equation to show the formation of bromomethane from methane and bromine.
CH4 + Br2 –(uv)–> CH3Br + HBr
When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.
Write down the two equations in the propagation step for the formation of CH3Br
Br• + CH4 –> HBr + •CH3
•CH3 + Br2 –> CH3Br + Br•
When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.
Explain why a tiny amount of ethane is found in the product mixture
Two methyl radicals bond together to form an ethane molecule
When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.
Write the equation for the formation of ethane in this reaction
•CH3 + •CH3 –> CH3CH3
The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface
Explain how ozone forms in this part of the atmosphere
Ozone is formed by the effect of UV radiation from the Sun on oxygen molecules
The oxygen molecules split to form oxygen free radicals
which react with more oxygen molecules to form ozone
The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface
How does the ozone layer absorb harmful radiation without getting permanently destroyed?
The ozone molecules absorb harmful UV radiation, breaking down to form an oxygen molecule and a free oxygen radical (O3 + hv –> O2 + O)
The radical produced forms more ozone with an O2 molecule (O2 + O –> O3) so ozone is continually destroyed and remade
The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface
Write equations to show how the CFC CCl4 could catalyse the destruction of ozone in the stratosphere
CCl4 + hv –> •CCl3 + Cl•
Cl• + O3 –> ClO• + O2
ClO• + O3 –> Cl• + O2
Overall: 2O3 –> 3O2
Explain why large amounts of ground-level ozone can be a problem. Why are these problems more frequently encountered in heavily industrialised areas?
E.g. it causes photochemical smog which can cause respiratory problems
cause harm to animals/plants/materials
Ground-level ozone forms as a result of sunlight acting on mixtures of nitrogen dioxide and hydrocarbons, which are emitted by power stations/vehicles
Ozone is a molecule that absorbs ultraviolet light with a wavelength of 255nm
What frequency of ultraviolet radiation does ozone absorb?
Wavelength in m = 255 x 10-9 = 2.55 x 10-7m
v = c/λ
v = (3.00 x 108)/(2.55 x 10-7) = 1.18 x 1015 Hz
Ozone is a molecule that absorbs ultraviolet light with a wavelength of 255nm
What is the energy of the ultraviolet light that ozone absorbs?
E = (hc)/λ
= ((6.63 x 10-34) x (3.00 x 108))/(2.55 x 10-7)
= 7.80 x 10-19 J
What is the concentration of ozone in ppm if its percentage concentration by volume is 0.000021%?
0.21 ppm
Calculate the energy absorbed when one molecule of H2O changes from its ground electronic level to the next level, given that the frequency of radiation absorbed is 1.80 x 1015 Hz
E = hv
= (6.63 x 10-34) x (1.80 x 1015)
= 1.19 x 10-18 J
