Common Questions A level chemistry unit 1 Questions and Answer

Common Questions A level chemistry unit 1 Questions and Answer Define the term relative atomic mass of an element. (2) mean mass of an atom of an element divided by one twelfth the mass of a carbon-12 atom State how the molecular mass of a covalent compound is obtained from its mass spectrum (1) the highest m/z value Explain why isotopes have identical chemical properties (1) they have the same electron configuration State what is meant by the term isotopes (1) atoms of the same element containing the same number of protons but a different number of neutrons Explain how, in a mass spectrometer, ions are detected and their abundance measured (2) they are detected by colliding with a negatively charged plate and causing a current to flow abundance is measured by the size of the current State the general trend in first ionisation energies from carbon to neon. Deduce the element that deviates from this trend and explain why it does this. (4) increases oxygen first element to have a pair of electrons in the 2p orbital (doubly occupied) electrons repel each other Explain why the second ionisation energy of carbon is higher than its first ionisation energy (1) it is harder to remove an electron from a positive ion than from an atom State and explain the general trend in first ionisation energies of period 3 elements (3) increases increasing nuclear charge and similar shielding Explain why the second ionisation energy of sodium is greater than the second ionisation energy of magnesium (3) for sodium you have to remove an electron from the 2p orbital whereas for magnesium it has to be removed from the 3s orbital 2p is closer to the nucleus with less shielding How is electron impact ionisation done? (4) high energy electrons are fired out of an electron gun at the atoms. They knock one electron off an atom to form a 1+ ion. How is electrospray ionisation done? (4) sample is dissolved in a volatile solvent then injected through a fine hypodermic needle to create a fine mist. The tip of the needle has a high voltage so when the sample atoms leave it the gain a proton. Which method of ionisation is most likely to lead to the fragmentation of ions? (1) electron impact Why is a negatively charged plate used to accelerate the ions? (2) the positively charged ions will be attracted to it Explain why different ions take different times to travel through the flight tube (2) heavier ions travel slower than lighter ions time of flights depends on mass Explain why on a mass spectrum there are peaks with much lower m/z ratios and peaks with higher m/z rations. (2) much lower ones due to the fragmentation caused by electron impact ionisation higher ones because of heavier isotopes Explain how ions are accelerated, detected and have their abundance determined in a TOF mass spectrometer (3) accelerated because of attraction to an electric field detected by gaining electrons at a negatively charged metal plate abundance determined by the size of the current Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer. (2) they will interact with and be accelerated by the electric field only ions will create a current when hitting the detector Explain whether you would expect MgCl2 or NaCl to have a higher boiling point. (3) MgCl2 magnesium forms 2+ ions whereas sodium forms 1+ ions magnesium has a greater attraction to negative chloride ions which requires more energy to overcome Explain whether you wuld expect MgCl2 or CaCl2 to have a higher boiling point (4) magnesium and calcium both have a 2+ charge. However Mg is a smaller ion meaning it has a higher charge density and a stronger electrostatic attraction to the oppositely charged ion, taking more energy to overcome. Explain whether sodium or aluminium would have the greatest electrical conductivity. (3) Al it has 3 delocalised electrons per ion whereas Na has 1 they are free to carry the current throughout the structure Explain whether CO2 or SiO2 would have the highest boiling point (5) CO2 has a simple covalent structure whereas silicon dioxide has a giant covalent structure. This means there are many strong covalent bonds between the atoms requiring a lot of energy to overcome. On the other hand CO2 has weak intermolecular forces which take little energy to overcome. This means SiO2 has the higher boiling point. Explain why metals are malleable (3) layers of ions able to slide over each other ions still attracted to delocalised electrons Explain how the ions are held together in solid sodium metal. Strong electrostatic forces of attraction between the delocalised electrons and positive metal ions - metallic bonding. Explain how the ions are held together in solid sodium chloride. ionic bonding - strong electrostatic forces of attraction between the oppositely charged ions. The melting point of sodium chloride is much higher than that of sodium chloride. What can be deduced from the information? (1) there are stronger electrostatic forces of attraction in sodium chloride which need more energy to overcome. Compare the electrical conductivity of solid sodium metal with that of solid sodium chloride. Explain your answer. (3) solid sodium metal conducts electricity but solid sodium chloride does not. Sodium metal has free electrons that move about the structure and carry the charge. Sodium chloride does not have free electrons because the ions can't move because it is solid. State the block in the Periodic Table in which sulphur is placed and explain your answer. (2) p. Its outer electrons are in the p subshell. By reference to all of the atoms involved, explain in terms of electrons how Na2S is formed from its atoms. (2) electron transfer from Na to S - 1 electron from each of the Na atoms. Name the type of bond formed when PH3 reacts with a H+ ion and explain how this bond is formed. (3) coordinate/dative lone pair from P on PH3 donated to H+ Explain how the strongest type of IMF in liquid HF arises. (1) large difference in electronegativity between H and F Explain why the O-H bond in a methanol molecule is polar. (2) oxygen is more electronegative causing H to have a partial positive charge and O to have a partial negative charge. State the property of atoms which causes a bond to be polar (1) electronegativity Why is each bond angle exactly 120 in BCl3? (2) 3 bonding pairs of electrons repel equally Give an example of a tetrahedral molecule (1) CH4 Why do diamond and graphite both have high melting points? (3) giant covalent. Many strong covalent bond require a lot of energy to break them. Why is graphite a good conductor of electricity? (1) delocalised electrons in the structure Why is graphite soft? (2) planes weak VDW forces between planes State how 2 carbon atoms form a C-C bond in graphane. (1) shared pair of electrons Suggest why the melting point of sodium iodide is lower than the melting point of sodium bromide. (1) iodide ion is bigger so less attraction to the Na+ ion Describe the structure of and bonding in graphite and explain why its melting point is very high (4) layers of carbon atoms in hexagonal rings each carbon atom forms 3 covalent bonds within layers - one delocalised electron per carbon atom intermolecular forces between layers to melt graphite you have to break many strong covalent bonds which takes a lot of energy State why iodine is a very poor conductor of electricity (1) no delocalised electrons Give 1 reason why ice is less dense than water (1) the molecules are further apart Explain why iodine vaporises when heated gently (2) you only have to overcome weak van der waals forces between molecules Why is a chloride ion polarised more by an aluminium ion than a magnesium ion? (2) aluminium forms 3+ ions whereas magnesium forms 2+ ions. They are a similar size. In terms of IMF's explain why PH3 is almost insoluble in water. (1) it doesn't have hydrogen bonding Predict the type of bonding in aluminium chloride (1) polar covalent Identify one similarity in the bonding and one difference in the structure of iodine and diamond. Explain why these 2 solids have very different melting points. (6) both have covalent bonds iodine has a simple molecular structure diamond has a giant covalent structure to melt iodine you only have to overcome weak VDW which doesn't take much energy to melt diamond you have to break many strong covalent bonds which takes a lot of energy Explain why nonane has a higher boiling point than 2,4-dimethylheptane. (2) greater VDW forces as nonane molecules can pack closer together. Explain why PVC melts at a higher temperature than chloroethene (2) bigger molecule so stronger intermolecular forces Explain why the boiling point of water is much higher than the boiling point of hydrogen sulphide (1) hydrogen bonding is stronger Explain why boiling points increase from H2S to H2Te. (2) molecules get larger/more electrons increased VDW forces In terms of electronic structure, explain why the van der waals forces in liquid argon are very weak. (2) single atoms with electrons closer to the nucleus electron cloud is not easily distorted Deduce why the bonding in nitrogen oxide is covalent rather than ionic (1) there isn't much difference in electronegativity Suggest the type of crystal shown by OF2 (1) molecular Explain why nickel is ductile (1) the layers can slide over each other Name the type of crystal structure shown by silicon dioxide and suggest why it does not conduct electricity when molten (2) giant covalent there are no charged atoms Suggest why magnesium is a liquid over a much greater temperature range compared to bromine. the forces of attraction in the liquid are stronger In terms of bonds broken and bonds formed, explain why the calculated values of enthalpies of combustion of consecutive alcohols, when plotted against Mr, follow a straight line. (2) each alcohol in the series increases by CH2 combustion breaks 1 more C-C bond and 2 more C-H bonds and forms one more mole of CO2 and 1 more mole of H2O Give 2 reasons why the experimental values for enthalpy of combustion are lower than calculated values using enthalpy of formation data. (2) heat loss to the surroundings incomplete combustion reactants and products may not have been in their standard states State why the value for the standard enthalpy of formation of Mn (s) is 0. (1) it is an element Suggest 1 reason why the calculated value is less accurate than the data book value for enthalpy change. (1) heat loss incomplete reaction State the meaning of the term carbon neutral. (1) an activity which has no net carbon emissions to the atmosphere State why the value for the standard enthalpy of formation of Na(l) is not 0. (1) it is not in its standard state - should be solid Give 1 reason why the calculated bond enthalpy is different from the mean bond enthalpy quoted in a data book (1) in a data book it is averaged across a range of compounds whereas the calculated value is specific to this reaction In a calorimetry experiment, explain why it is important the reaction mixture is stirred before recording each temperature. (1) so all the solution is at the same temperature The data book value for the enthalpy of combustion of butanone (-2430) refers to the formation of carbon dioxide and water in the gaseous state. How would this value differ if it referred to the formation of water in the liquid state? Explain your answer. (3) it would be more negative water would have to be converted to the gaseous state first and heat would be required to vaporise it With reference to the Maxwell-Boltzmann distribution, explain why a decrease in temperature decreases the rate of reaction. (2) fewer reactant particles have energy greater than or equal to the activation energy leading to fewer successful collisions. State the effect, if any, a catalyst will have on the shape of a Maxwell-Boltzmann distribution. (1) no effect Explain what must happen for a reaction to occur between molecules of 2 different gases. (2) they must collide with the required activation energy Explain why a small increase in temperature has a large effect on the initial rate of reaction (1) many more particles will have the required activation energy Explain the process that causes some molecules in this sample to have very low energies. (3) collision causes some molecules to slow down and lose energy Explain why, even in a fast reaction, a very small percentage of collisions lead to a reaction. very few collisions have the required activation energy State what is meant by activation energy. (1) the minimum energy required to start a reaction State in general terms how a catalyst increases the rate of a chemical reaction. (2) it lowers the activation energy by providing an alternative reaction pathway. State how the value of most probable energy changes as the total number of molecules is increased at constant temperature. (1) stays the same State how the number of molecules with the most probable energy changes as temperature is decreased without changing the total number of molecules. (1) increases State how the number of molecules with energy greater than the activation energy changes as temperature increases without changing total number of molecules. (1) increases State how the area under the distribution curve changes with a catalyst. (1) stays the same Give the meaning of the term catalyst. (2) a substance that increases the rate of reaction without being used up At any time most molecules have less energy than the activation energy. Suggest why most of the sample eventually decomposes. (2) molecules gain energy when they collide with each other State and explain 2 ways of speeding up a gas phase reaction other than changing the temperature. (4) increase the pressure - increases the collision frequency add a catalyst - lowers the activation energy Explain why catalysts are often powdered. (1) increases the surface area Explain why a reaction curve's gradient decreases as time increases. lower concentration of reactants so lower frequency of successful collisions State Le Chatelier's principle. (1) if there is a change in concentration, pressure or temperature in a reversible reaction, the equilibrium position will shift to oppose that change State the effect of a catalyst on the time taken to reach equilibrium and on the position of equilibrium (2) reduces it, no effect State how the rate of the forwards reaction and the rate of the backwards reaction are related to each other at equilibrium. (1) they go at equal rates Define oxidisation in terms of electrons. (1) loss of electrons What is the oxidisation state of an atom in an uncombined element (1) 0 By referring to electrons, explain the meaning of the term oxidising agent (1) accepts electrons Explain the meaning of the term oxidisation state (1) the charge on the ion In terms of electron transfer what does the reducing agent do in a redox reaction? loses electrons then gives them to the species being reduced What can be deduced from the fact that the value of Kc is larger in an equilibrium at a lower temperature? the forward reaction is exothermic