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CHEM UNIT 2 PAST PAP

QuestionAnswer
What colour would phenolphthalein turn at the end-point in this titration? Colourless.
The best piece of apparatus to accurately measure out 10.0 cm3 is a A pipette.
What volume of 0.200 mol dm–3 hydrochloric acid solution was added by the end-point? 12.50 cm3.
Which of these metal hydroxides is the most soluble in water? Barium hydroxide.
Which of these metals will give a lilac flame colour? K
Which of these is a tertiary alcohol? D 2-methylpentan-2-ol
Which of these statements about fluorine is not correct? It is less electronegative than chlorine
What is the oxidation number of oxygen in dioxygen difluoride, O2F2? +1
Which of these four molecules, PCl3, CO, CO2 and CCl4, are polar? PCl3 and CO
Which intermolecular forces exist between molecules of ethoxyethane? Instantaneous dipole – induced dipole and permanent dipole – permanent Dipole.
The following liquids all have the same number of electrons in each molecule.
Which one is likely to have the lowest boiling point? CH3C(CH3)2CH3
Which of these is likely to be the best solvent for cyclohexanol? CH3COCH3(l)
The ability of a liquid to flow is linked to the strength of its intermolecular forces. Suggest which of these liquids flows the slowest when poured. Propane-1,2,3-triol.
What type of species forms when a bond breaks homolytically? Free radical
In the reaction between Ag+(aq) ions and Fe2+(aq) ions, what would be the effect of increasing the concentration of Ag+(aq) ions? Ag+(aq) + Fe2+(aq) Rate of reaction increases, yield of Fe3+(aq) increases.
Which one of these reactions is not a disproportionation reaction? Cl2(aq) + 2Br–(aq) → 2Cl–(aq) + Br2(aq)
Molecules absorb IR radiation because they change their polarity when they vibrate.
How many of the following molecules will absorb IR radiation? H2O N2 CH4 O2 CO2 1 O H stretching in alcohols at 3750 − 3200 cm–1 2 C=O stretching in aldehydes at 1740 − 1720 cm–1 3 C=O stretching in ketones at 1700 − 1680 cm–1. 4 C=O stretching in carboxylic acids at 1725 − 1700 cm–1 -
To identify the formation of the product when propan-1-ol has been partially oxidized, you can look for absorptions in the IR spectrum at absorption range To monitor whether all of the sample of propan-2-ol has been oxidized, you can look for.
State the reagents and conditions needed to convert the following halogenoalkanes into the named product. 1-bromobutane into butan-1-ol (Dilute) /NaOH / KOH (1) (Heat/warm in)aqueous solution /Mixture of water and ethanol (1)
(ii) 1-iodobutane into butylamine Ammonia / NH3 (in ethanol) (1) Heat and pressure / heat in a sealed tube
(iii) 2-chloropropane into propene (concentrated) Sodium hydroxide / potassium hydroxide / NaOH / KOH (1) (heat and)dissolved in ethanol.
Chloroethane can be prepared by reacting ethanol with PCl in the presence of concentrated S04 . Explain why a similar reaction using potassium iodide and concentrated sulfuric acid should not be used to prepare iodoethane. Sulfuric acid is a (strong enough) oxidising agent (1) To oxidise iodide ions/hydrogen iodide (to iodine) Two gaseous halogenoalkanes that could be used as fire retardants have the structural formulae CF2ClBr and CF3CHF2.
Give the systematic name of CF2ClBr. Bromochlorodifluoromethane
Suggest TWO reasons to explain how these compounds can help put out fires. Halogenoalkanes; Absorb heat from fire ,Prevent oxygen from reaching the fire/form a layer that excludes oxygen, Absorb free radicals in combustion propagation, Strength of C-F bond makes molecules, inert / strength of C-F bond makes it hard to break.
Suggest a reason why the scientific community still supports the use of fire retardants containing CF3CHF2.
Halogenoalkanes such as CF2ClBr can release Cl free radicals • Cl free radicals react with O3 • Ozone layer depletes • Leading to greater levels of UV exposure
Greater risk of skin cancer CF3CHF2 has strong C-F bonds so does not release F radicals, 10.0 cm3 of a solution containing Cu2+(aq) ions was added to excess potassium iodide solution and the following reaction occurred.
2Cu2+(aq) + 4I−(aq) → 2CuI(s) + I2(aq)
What happens to the Cu2+(aq) during this reaction? Justify your answer. Reduction (1) Has gained 1 electron / oxidation number has decreased (from (+)2 to (+)1) (1) Oxidation = 0
Indicator Starch (1) Blue-black / Blue / black to colourless Colour change at end-point
The whole of the solution containing iodine was used in one titration. Explain how this affects the reliability of your answer to They are not reliable as the experiment was only carried out once so there is no evidence that the result is repeatable.
This question is about boron and nitrogen compounds. Draw and name the shape of a boron trifluoride, BF3, molecule. Suggest the FBF bond angle. Trigonal planar ,120°
Ammonia has the formula NH3. Its HNH bond angle is less than the FBF bond angle in boron trifluoride. Estimate the HNH bond angle in NH3. 108° – 106°
Explain why the HNH bond angle is less than that for FBF. 3 electron pairs around central B atom but 4 electron pairs around central N
atom (hence less space) / ammonia has an extra pair of e- around N
Name the strongest intermolecular force between BF3 molecules Instantaneous dipole – induced dipole / temporary dipole – induced dipole / Induced dipole – Induced dipole / / London forces / van der Waals' forces
Name the strongest intermolecular force between NH3 molecules. Hydrogen bonds / H-bonds
Ammonia will react with oxygen in the presence of a platinum catalyst at 750°C forming water and nitrogen(II) oxide, NO.
What is the oxidation number of nitrogen in ammonia, NH3? -3
The diagram below shows the distribution of molecular energies in the reaction at 750°C. On the same diagram, draw a curve to show the distribution at 500°C and explain what effect this change in temp would have on the rate of the reaction.
Curve with higher peak to left of 750°C peak (1) Smaller area under curve above Ea (1) Reaction rate slower as fewer particles have E ≥ Ea (so fewer successful collisions per second)
Explain how a catalyst speeds up the rate of a reaction. Provides alternative mechanism / route / pathway (1) Of lower activation energy (1) Hence a greater proportion of molecules can react (at a given T)
The half-equation for the oxidation of ethanol is CH3CH2OH + H2O → CH3COOH + 4H+ + 4e- . to write a full equation for the overall reaction between acidified dichromate(VI) ions and ethanol. 3 CH3CH2OH + 2Cr2O72- + 16H+ → 3CH3COOH + 4Cr3+ + 11H2O ,3:2 ratio.
Why are the concentrated sulfuric acid and sodium dichromate(VI) added a drop at a time in the laboratory process? To prevent the mixture heating too rapidly /prevent ethanol evaporating
Draw a labelled diagram of the apparatus that could be used to heat the mixture under reflux. Suitable flask & heat indicated Liebig Condenser ,Correct water flow
What colour would the mixture be after it was heated under reflux? Green / blue.
Explain why the other products and any excess reactants are left behind in the distillation flask. They have a (very) high boiling temperature / are ionic compounds
Suggest a method to separate pure ethanoic acid, boiling temperature 118°C, from the water. Fractional distillation / distil of water then distil of ethanoic acid / absorb water with anhydrous drying agent
would increasing the pressure have on the yield of ethanoic acid? Justify your answer. Yield of lab process may be low Cost of oxidising agent.
Toxicity of oxidising agent ,Disposal of Cr3+,Control of temperature/rate in scaled up reaction could be difficult,The lab process has a lower atom economy. Energy costs to separate ethanoic acid from reaction mixture Lab procedure is a batch process
Suggest TWO reasons why it might be difficult, or undesirable, to produce ethanoic acid in industry by scaling up the laboratory process. - Energy costs to separate ethanoic acid from reaction mixture,Lab procedure is a batch process.
An alternative industrial process for the production of ethanoic acid is the oxidation,of butane using a transition metal catalyst at 150°C and 55–60 atm. 2C4H10(l) + 5O2(g) → 4CH3COOH(aq) + 2H2O(l) Evaluate the ‘ greenness’ and sustainability of the two industrial processes.
Suggest TWO additional pieces of information that would help you make a more informed decision. Discussion of four aspects of processes e.g four from Cativa runs at lower P Hence less energy required (for compression) Cativa has 100% atom economy, Cativa runs at higher temp so greater energy requirements for heating ,
2011:
Which of the following bond angles occur in a molecule of ethanol, C2H5OH? 104.5° and 109.5°
Which of the following molecules is linear? Carbon dioxide, CO2
Which of the following molecules contains polar bonds but is not a polar molecule? Tetrachloromethane, CCl4
Which of the following has dipole-dipole interactions between its molecules, but no hydrogen bonding? Hydrogen iodide, HI
Which list below shows the compounds in order of increasing boiling temperature? CH4, HCl, HF
Which of the following has the highest boiling temperature? Hexane, CH3CH2CH2CH2CH2CH3
Which of the following could not be an element in Group 2? An element with a chloride which is liquid at room temperature.
Chlorides of Group 1 elements produce coloured flames when excited electrons move from a higher to a lower energy level.This question is about the following compounds. Which compound gives a green colour in a flame test?
Which compound gives a lilac colour in a flame test and does not decompose on heating? Potassium bromide.
20 cm3 of H2SO4 , concentration 0.25 mol dm–3, was neutralized in a titration with BaOH , concentration 0.50 mol dm–3. The equation for the reaction is Ba(OH)2(aq) + H2SO4(aq) → BaSO4(s) + 2H2O(l)
The volume of barium hydroxide required was 10 cm3
During the titration, the barium hydroxide was added until it was present in excess. The electrical conductivity of the titration mixture decreased and then increased.
Which of the following trends occurs going down the elements in Group 2? The solubility of the hydroxides increases.
Which of the following is not a true statement about hydrogen iodide? It forms a cream precipitate with silver nitrate solution.
Which of the terms above best describes what happens to the chlorine in the following reactions? Cl2(g) + H2O(l) → HCl(aq) + HOCl(aq) disproportionation
Cl2(g) + 2Na(s) → 2NaCl(s) reduction
NaCl(s) + H2SO4(l) → HCl(g) + NaHSO4(s) no change in oxidation number
When chloroethane is heated with a concentrated solution of potassium hydroxide in ethanol, the reaction which occurs is
Elimination.
Chloroethane reacts with aqueous potassium hydroxide solution, producing ethanol as the organic product.
The hydroxide ion is acting as a nucleophile.
Magnesium nitrate, Mg(NO3)2, decomposes when it is heated, nitrogen dioxide.
Write an equation for this reaction.. 2Mg(NO3)2 → 2MgO + 4NO2 + O2 Correct formulae (1)
Calcium nitrate decomposes like magnesium nitrate, higher temperature. Explain why the two nitrates have different stability to heat. Mg2+ / Magnesium ion smaller or fewer electron shells / greater charge density (1)OR Magnesium ion has same charge (as calcium ion) but is smaller (1) Causes more polarisation /distortion of nitrate/ anion (electron clouds) / N―O (bond)(1)
OR MgO produced has stronger lattice (1) OR production of MgO is more exothermic (1) OR reverse argument based on Ca2+
Sodium nitrate decomposes to give different products to magnesium nitrate. Write an equation for the decomposition of sodium nitrate. 2NaNO3 → 2NaNO2 + O2.
Scientists have found that the bonds between nitrogen and oxygen in the nitrate ion are all the same length. Is the student’s suggestion supported by this evidence? Explain your answer. No as….double bond would be shorter (than single bond) / shorter than dative (covalent) bond. Double and single bonds have different lengths
What would you see when an equilibrium mixture of these gases is warmed gently? Explain your answer. Goes darker (brown) (1) Goes browner. Equilibrium moves in the endothermic direction (1) OR Equilibrium moves left as forward reaction is exothermic (1) Equilibrium moves left to counteract addition of heat.
Explain why an equilibrium mixture of these gases eventually becomes paler in colour when the pressure on it is increased. Equilibrium moves right (ALLOW forwards) (so NO2 concentration decreases) (1) OR Reaction reduces pressure (1) As fewer moles / molecules(ALLOW particles) (of gas)
Use the distributions to explain why gases react faster when the temperature is increased. At T2 more molecules/collisions have energy greater than (or equal to) EA (1) This can be shown on the diagram by indicating areas to right of vertical line,
When Cl is added to a solution containing Br or I2 ions, a colour . change occurs. What solvent add to mixture to confirm identity of halogen A hydrocarbon (solvent) / volasil / named hydrocarbon solvent / tetrachloromethane
Give the result for the test with this solvent in a reaction in which bromine is produced. Red / brown /orange / amber / yellow Or any combination No TE on incorrect / no reagent 2HBr(g) + H2SO4(l) → SO2(g) + Br2(l) + 2H2O(l)
Show, by use of oxidation numbers for sulfur, that the sulfuric acid has been reduced. Oxidation number of S in H2SO4 =(+)6, Oxidation number of S in SO2 =(+)4 (1) Oxidation number had decreased (1)
State TWO observations, which would differ from those with potassium bromide, when potassium iodide reacts with concentrated sulfuric acid. Black / (shiny) grey solid (1) Purple / violet / pink vapour / fumes (1) Smell of (bad) eggs (1) Yellow solid (1)
One product of the reaction with potassium iodide is hydrogen sulfide, H2S. How does this show that iodide ions are more powerful reducing agents than bromide ions? Energy must be at least EA for successful collision / for reaction (1) OR So more collisions have sufficient energy to react(1)
Oxidation number of S has reduced more / to –2 (in H2S) (1) OR increase in temperature(1) OR Reaction removes added heat by moving left (1)
Oxidation number of S is lower in H2S (than in SO2)
In areas where the natural concentration of fluoride ions in rocks is low, some water authorities add fluoride..
to the water supply to improve the dental health of children. An alternative would be to supply free fluoride tablets.
Give ONE reason why it could be considered more ethical to supply free fluoride tablets than add fluoride to the water.
People can choose whether to take extra fluoride /Fluoride is not released into the environment
Butan-1-ol and three other alcohols, X, Y and Z, are isomers. Give TWO make when any one of the alcohols reacts with sodium. Effervescence / fizzing / bubbles (of colourless gas) (1) Mixture gets hot (1)
White solid (ALLOW ppt) produced / sodium dissolves or disappears (1)
Give the molecular formula of the organic product of the reaction. C4H9ONa / C4H9O–Na+ /structural or displayed formulae of any of the isomers: CH3CH2CH2CH2ONa , (CH3)2CHCH2ONa, (CH3)3CONa,CH3CH(ONa)CH2CH3
What would be the principal difference between the infrared spectrum of Q and the infrared spectrum of 2-methylpropanoic acid? You are not expected to quote absorption values O━H absorption / peak in 2-methylpropanoic acid / No O━H absorption / peak in Q ALLOW C━O absorption / peak in 2-methylpropanoic acid / No C━O absorption / peak in Q One of the isomers, X, Y or Z can be converted to 2-chlorobutane.
What reagent would you use to carry out this conversion? PCl5 / PCl3 / conc HCl / SOCl2 / mixture of NaCl + H2SO4 / mixture of KCl + H2SO4.
2-chlorobutane reacts with silver nitrate in a mixture of ethanol and H20 as a solvent. What would you see?
White precipitate/ white solid Both ethanol and water contain hydrogen bonds. (ALLOW intermolecular forces) stronger in water (1) I2(aq) + 2S2O3 2–(aq) → 2I–(aq) + S4O6 2–(aq)
By considering the hydrogen bonding on these two solvents, suggest why 2-chlorobutane is more soluble in ethanol than in water. Water has 2 hydrogen bonds per molecule (on average) whereas ethanol only has 1 (1) ALLOW Water has more hydrogen bonds (per molecule) than ethanol Needs more energy to break H bonds in water (so less soluble) / H bonding
Name a suitable indicator which could be used for this titration Starch (solution)
The amount of sulfur dioxide in the atM can be measured by passing a known volume of air through iodine solution
.. . SO2(g) + I2(aq) + 2H2O(l) → SO4 2–(aq) + 4H+(aq) + 2I–(aq).
100 m3 air were passed through 100 cm3 I2, conc 0.0100 mol dm–3. The remaining iodine titrated with Nathiosulfate and reacted with ... 12.60 cm3 of sodium thiosulfate, concentration 0.100 mol dm–3.
How many moles of iodine were present in the solution of the iodine at the start of the experiment? I2 at start = 1 x 10-3 / 0.001 (mol)
How many moles of iodine remained in the solution at the end of the experiment? 1.26 x 10-3 (mol) thiosulfate (1) 6.3(0) x 10-4 / 0.00063 (mol) I2 (1)
Calculate the number of moles of iodine which reacted with the sulfur dioxide, and hence the number of moles of sulfur dioxide in 100 m3 of air. I2 used = (1 x 10-3 - 6.30 x 10-4) = 3.70 x 10-4 (mol) (1) Mol SO2 = mol I2 = 3.70 x 10-4 / 0.00037 (mol) (1)
Calculate whether the sulfur dioxide in this sample of air was within this limit. One mole of sulfur dioxide has mass 64.1 g. Mass SO2 in 100 m3 =(64.1 x 3.70 x 10-4 ) (1) Mass SO2 in 1 m3 = 64.1 x 3.70 x 10-4 /100 = 237(.2) x 10-6 g = 2.37 x 10-4g (1) (= 237.2 / 237 / 240 μg)
Explain whether the changes below would or would not improve the experimental procedure for Improved because titration may be repeated /averages could be taken ALLOW Smaller titration reading so greater (%) error
The concentration of sodium thiosulfate used to titrate the iodine was changed from 0.100 mol dm–3 to 0.050 mol dm–3. Larger titration reading (1) So smaller (%)error in titration reading (1)OR Smaller mass of sodium thiosulfate used to make solution (1) So greater %) error in the mass measurement (1) 150 m3 of air was passed through the iodine.
Created by: ufuoma