Exam questions Flashcards
Nitric acid
HNO3
Why is there a general increase in ionisation energies across period 2
There is an increases in nuclear charge so the electrons experience a bigger pull from the nucleus. There is little change in atomic radius and shading, so more energy is needed to remove the outermost electron so bigger nuclear attraction
What forms after the first ionisation energy
X+
Electronegativity
The ability of an atom to attract a pair of electrons into a covalent bond
Why do bonds have dipoles
Because there is a big difference in the electronegativity of the elements
How are p orbitals involved in the formation of a C=C double bond
The p orbitals overlap to form a pie bond there are now 2 bonds between the carbons, a pie bond and a sigma bond.
What type of bond fission is involved in the formation of bromine radicals
Homolytic fission
Why do both cis and trans isomers react to form the same product
They no longer have a double bond which restricts movement
What is the role of phosphoric acid in a reaction and why is it added slowly
It acts as a catalyst to speed up the reaction. It is added slowly to prevent high temperatures and side reactions
How is margarine formed
It is manufactured from the hydrogenation of alkenes. The oils are no longer saturated so are solid instead of being liquid. A nickel catalyst is used.
Why are polymers hard to dispose of
They are inert so are not biodegradable. They release toxic fumes when burnt which is bad for the environment.
Low volatility
Does not easily turn into a gas
Intermolecular forces
The forces between the molecules which must be broken in order for the substance to evaporate. They hold the shape of the molecule
Why is ethanal produced in distillation
Because it is distilled to prevent the ethanal from being further oxidised to ethanoic acid. It is heated below the ethanoic acid boiling point so that the ethanal will be collected but not the ethanoic acid.
Why is reflux used to form ethanoic acid
Reflux is used so that any ethanal originally produced will be further oxidised to ethanoic acid
What is also produced when ethanol is oxidised
Water
What would happen if chlorine gas is bubbled through potassium iodide solution followed by cyclohexane
The solution turns brown, then two layers are produced and the top layer is purple
Why can non-polar substances dissolve in non-polar solvents
Because there are only London force in the non-polar substance and solvent. London forces can occur between the non-polar substance and solvent, which weakens the intermolecular forces in the non-polar substance and breaks it up
What happens to the pH of solutions of group 2 hydroxides down the group
The pH increases down the group because they become more soluble down the group so there are more OH- ions in the water.
Bond enthalpy
The energy required to break one mole of a bond in a gaseous state
Suggest two reasons why the standard enthalpy of methanol is different from that calculate
Energy will be lost to the surroundings when calculated experimentally. Incomplete combustion may occur when it is calculated experimentally.
Why can some enthalpy change of reactions not be measured directly
Other products are more likely to be formed and the activation energy may be too high
Units for enthalpy change
KJmol-1
Why does an exothermic reaction occur spontaneously at low temperatures but does not occur at very high temperatures
DeltaS is negative because it goes from 2 moles of gas to one. As the temperature increases the magnitude of TdeltaS increases. At high T, deltaS is more negative then deltaH. DeltaG is positive and the reaction is no longer feasible.
Why would dissolving be exothermic
Because bonds are being broken which is exothermic. Enthalpy of hydration is bigger then lattice enthalpy. Enthalpy of hydration is exothermic and lattice enthalpy is endothermic
What makes a reaction more spontaneous
The lower the value of deltaG is.
How will the experimental lattice enthalpy differ from the calculated one
The experimental lattice enthalpy will be more negative then the theoretical lattice enthalpy, as the theoretical lattice enthalpy only takes into account the ionic bonding and not the covalent characteristics. They will therefore assume that the bond is weaker then it is, so the experimental value will be more negative.
How does electron affinity change as you go down group 7
Down the group the atoms get bigger and are less able to attract an electron due to the increased effect of shielding and atomic radius, this decreases the pull the atom exerts on the electron. Therefore the value for electron affinity becomes less negative down the group.
Define the first ionisation energy (3)
The energy required to remove one electron from each atom in a mole of gaseous atoms
Why is the second ionisation energy greater than the first ionisation energy (1)
There are more protons to electrons so the electrons feel a bigger pull from the nucleus and more energy is required to break this.
What bond will be broken in CF3Cl
The carbon-chlorine bond as it has the lowest bond enthalpy so would require the least energy to break.
Investigating the rate of hydrolyses of chloroethane, bromoethane and iodoethane (5)
You would ass each of the haloalkanes to a test tube, then ethanol and aqueous silver nitrate. You then put it in a water bath set at 50 degrees, and time how long it would take for a precipitate to form. Ag + X- –> AgX(s). X represents a halide. The yellow precipitate from the Iodine will form first, then the cream precipitate from bromine, and finally the white precipitate from the chlorine. Iodine has therefore reacted first because the carbon-iodine bond has the lowest bond enthalpy then the others so less energy is needed to break this bond
Describe and explain the trend in oxidising ability down group 7
The stronger oxidising agents are at the top of the group, as you go down the group atomic radius and shielding increases, this outweighs the increase in nuclear charge. This means that there is less nuclear attraction so it is harder to gain an electron.
How would you test for a carbonate
You add HCl then bubble the gas through lime water, it then turns milky white as a precipitate forms
How do you test for sulfates
Add HCl then BaCl2, a white precipitate will form
Describe the bonding in benzene
The overlap of p-orbitals of the delocalised electrons forms a region of electron density above and below the benzene ring
Evidence of benzene being the model it is
C-C bonds are the same length. Benzene does not undergo electrophilic addition. The enthalpy of hydrogenation is less exothermic then expected for alternating single / double bonds
Why is phenol more reactive then benzene (3)
The lone pair of electrons on the oxygen atom is donated into the pi system, this increases the electron density. So electrophiles are polarised more easilly
Why is a halogen carrier needed in the bromination of benzene but not cyclohexene
Benzene has delocalised pi electrons whereas cyclohexene has localised pi electrons. This means that benzene has decreased electron density, so a halogen carrier is needed to generate a positive charge on bromine.
What is more likely to react with benzene, bromine or iodine? (2)
Iodine because bromine is more electronegative, so Iodine is more likely to be positive and act as the electrophile
What type of mechanism is a ketone with HCN
Nucleophillic addition
What makes something a nucleophile
Contains a lone pair of electrons which can be donated
Why do isotopes have the same chemical properties
They have the same number of electrons
Explain in terms of kc why increasing pressure increases yield of product (3)
Kc does not change, the reaction is no longer in equilibrium. The reaction shifts to the right to restore equilibrium, more products are formed and less reactant
Why does a pressure increase cause more product to be produced
The equilibrium moves tot the right, to the side with the fewest particles
In order for a reaction to be feasible….
Delta g must be negative
What is a salt
When the H+ of an acid is replaced by a metal ion
Why does Mg have a higher melting point then Na
Magnesium ions have a greater charge, magnesium ions have more delocalised electrons, so there are stronger metallic bonds
Why does s8 have a higher melting point then cl2
S8 has London forces, s8 has stronger London forces because it has more electrons
Why does atomic radi decrease across the period
More protons in nucleus meaning the electrons feel a greater pull from the nucleus. Shielding remains the same
Example of drying agent
Anhydrous sodium sulfate
What are cyclohexenes electrons
Localised
What makes a substance a nucleophile
Contains a lone pair of electrons which can be donated
Explain the difference in ionisation energies of Li and Na (3)
As you go down the group atomic radius increases, there is more shielding. Nuclear attraction decreases as the shielding outweighs the increase in nuclear charge
How do you separate aqueous products
Fractional distilation
Why would magnesium have stronger ionic bonds then sodium (3)
The magnesium ions has a greater charge and is smaller meaning there is a greater attraction between the oppositely charged ions
What temperature would you want for an exothermic reaction where delta H is negative
You would want a high temperature so that the reaction would be at a higher rate. However this would shift the equilibrium to the left meaning less products are formed. So you would want a high enough temperature for a reasonable rate without compromising equilibrium yield.
How would a larger volume of water effect temperature change or enthalpy change of solution
Temperature change is less because the energy is spread over a larger volume. The enthalpy change is the same, as the energy is released per mole
Explain the dissociation of sulphuric acid
Complete dissociation would give [H+] = 0.2 (mol dm–3) pH from complete dissociation = –log 0.2 = 0.7. Stage 1 is complete dissociation and stage 2 is partial dissociation so it less acidic then expected
How would a metal interact with an amino acid
There will be a coordinate covalent bond between the protein and metal ion, the nitrogen atom will donate the electron pair
What makes a substance soluble in water
If it can form hydrogen bonds with the water molecules
What is the standard electrode potential of a half cell
The voltage of the half cell connected to a standard hydrogen half cell
Reducing agent
Species that adds electrons to the species being reduced
Fuel cell
A cell that uses the energy from a reaction of a fuel with oxygen to create a voltage.
How do you do titration to find out the concentration of Iron reacting with sodium thiosulphate (3)
Fill burette with sodium thiosulphate and use a pipette to measure out the solution of Iodine and then put the vessel containing Iodine below the burette. Add thiosulphate to the Iodine vessel, do it drop wise when it is near the end point. Use a starch indicator, the end point is shown by the colour change from blue-black to colourless
Why can a wire not act as a salt bridge
A wire does not allow the flow of ions
Solution used to make salt bridge
Potassium nitrate
Why might an electrode potential reaction not occur even if it is feasable
Concentration of ions may not be standard ie 1 mol dm. Standard electrode potentials only apply to aqueous solutions. Activation energy may be too high
How does sulphur differ from the trend in ionisation energies (3p2)
It has two electrons in the p orbital, they repel each other
What does a kc below one suggest about the position of equilibrium
Equilibrium is torwards the left more reactants are made
How does ligand substitution allow haemoglobin to transport oxygen
Oxygen combines with Iron, when required oxygen is released
What do you do when describing changes in delta S
The change of state, for example whether it went from liquid or gas or whether there are more moles of gas on the product side
Why do straight chain alkanes have a higher boiling point then branched alkanes
Straight chain alkanes have more surface contact so more London forces
What is a condensation polymer
When monomers join and another product is formed ie H2O
Atomic number
The number of protons in an atom
The enthalpy change of combustion
The enthalpy change required for complete combustion of one mole of a substance in excess oxygen
Why is it difficult to measure the enthalpy change of x
They don’t react together, activation energy would be too high, other hydrocarbons would form
Carbonate
Co3-2
Why is the mass of x not used in calculations for heat energy in this reaction
The heat capacity is negligible
Why was the temperature measured for 3 minutes before the reaction started
To ensure equilibration
Why is the temperature measured for a period of time after the reaction is complete
To determine maximum temperature change
Why is the reaction mixed
To ensure uniform temperature
Can sodium and sodium oxide conduct electricity when solid or liquid
Sodium is a metal which can conduct electricity as the ions can move and conduct the charge. Sodium oxide can not conduct electricity when solid as the ions are fixed relative to each other and are unable to move and carry the charge. Sodium oxide can conduct electricity when liquid as the bonds between the ions have broken down and the ions are free to move and carry the charge
Why is ethanal rather then ethanoic acid collected in distillation (3)
Ethanal has a lower boiling point. So ethanal distils out of the mixture meaning it is separated before it can be further oxidised
What can be used to test for both an alcohol and carboxylic acid
SOCl2, misty fumes are produced
Why does bromomethane have a larger boiling point then chloromethane (3)
Bromine has more electrons meaning it has stronger London forces. Though the Cl-C bond is more polar then the C-Br bond, the induced dipole has a bigger effect then the permanent dipole.
Explain why the enthalpy change of solution of KF is more negative then RbF (4)
The Potassium ion is smaller, so the lattice enthalpy of KF is more negative then RbF. The enthalpy change of hydration of potassium is more negative then Rb as potassium has a greater attraction for the water molecules. The enthalpy change of solution is more affected by lattice enthalpy then enthalpy change of hydration, so the enthalpy change of solution is more negative for KF
What needs to be considered when choosing a suitable indicator (1)
Does the vertical section match the pH range of the indicator
