PRACTICAL QUESTIONS TITRATION AND ENTHALPY Questions
PRACTICAL QUESTIONSTITRATION AND ENTHALPYQuestionsDr Chris Clayhttp://drclays-alevelchemistry.com/
Practical QuestionsQ1.Read the following instructions that describe how to make up a standard solution of a solid in avolumetric flask.Answer the questions which follow.‘Take a clean 250 cm3 volumetric flask. Use the balance provided and a clean, dry container, toweigh out the amount of solid required. Tip the solid into a clean, dry 250 cm3 beaker and add about100 cm3 of distilled water. Use a stirring rod to help the solid dissolve, carefully breaking up anylumps of solid with the rod. When the solid has dissolved, pour the solution into the flask using afilter funnel. Add water to the flask until the level rises to the graduation mark.’(a)Suggest three further instructions that would improve the overall technique in this account.1 .2 .3 .(3)(b)In a series of titrations using the solution made up in part (a), a student obtained the followingtitres (all in cm3).Rough1225.725.2025.35State what this student must do in order to obtain an accurate average titre in this experiment.(2)(Total 5 marks)
Practical QuestionsQ2.In a titration experiment, a good technique is essential for an accurate result to be obtained.(a)Suggest a reason for removing the funnel after it has been used for filling the burette.(1)(b)Suggest one other source of error in using the burette to carry out a titration.(1)(c)During the titration, the inside of the conical flask is rinsed with distilled water.Suggest why rinsing improves the accuracy of the titre.(1)(d)Explain why adding this extra water does not change the volume of EDTA solution that isrequired in the titration.(1)(Total 4 marks)Q3.The maximum errors for the pipette and the burette are shown below. These errors take into accountmultiple measurements.Pipette 0.05 cm3Burette 0.15 cm3Estimate the maximum percentage error in using each of these pieces of apparatus.Use an average titre 24.25 cm3 to calculate the percentage error in using the burette.Show your working.Pipette .
Practical Questions.Burette .(Total 2 marks)Q4.A value for the enthalpy of combustion of an alcohol can be determined using the apparatus shown inthe diagram. The calorimeter is held in position by a clamp.This experiment can be repeated by using a different volume of water that would result in a moreaccurate value for the enthalpy of combustion because there would be a reduction in the heat lost.State a change in the volume of water that would cause a reduction in heat loss and explain youranswer.Change in volume: .Explanation: .(Total 2 marks)Q5.The alcohol 2-methylpropan-2-ol, (CH3)3COH, reacts to form esters that are used as flavourings by thefood industry. The alcohol can be oxidised to produce carbon dioxide and water.A student carried out an experiment on a pure sample of 2-methylpropan-2-ol to determine itsenthalpy of combustion. A sample of the alcohol was placed into a spirit burner and positioned undera beaker containing 50 cm3 of water. The spirit burner was ignited and allowed to burn for severalminutes before it was extinguished.The results for the experiment are shown in Table 1.
Practical QuestionsTable 1(a)Initial temperature of the water / C18.1Final temperature of the water / C45.4Initial mass of spirit burner and alcohol / g208.80Final mass of spirit burner and alcohol / g208.58Use the results from Table 1 to calculate a value for the heat energy released from thecombustion of this sample of 2-methylpropan-2-ol.The specific heat capacity of water is 4.18 J K–1 g–1.Show your working.(2)(b)Calculate the amount, in moles, of 2-methylpropan-2-ol burned in the experiment.Hence calculate a value, in kJ mol–1, for the enthalpy of combustion of2-methylpropan-2-ol.Show your working.(If you were unable to calculate an answer to part (a), you should assume that the heat energyreleased was 5580 J. This is not the correct value.).(3)(c)An equation for the combustion of 2-methylpropan-2-ol is(CH3)3COH(I) 6O2(g)4CO2(g) 5H2O(I)Table 2 contains some standard enthalpy of formation data.
Practical QuestionsTable 2 Hf/ kJ ��286Use the data from Table 2 to calculate a value for the standard enthalpy of combustion of 2methylpropan-2-ol. Show your working.(3)(d)An accurate value for the enthalpy of combustion of 2-methylpropan-2-ol in which water isformed as a gas is –2422 kJ mol–1.Use this value and your answer from part (b) to calculate the overall percentage error in thestudent’s experimental value for the enthalpy of combustion of 2-methylpropan-2-ol.(1)(e)Suggest one improvement that would reduce errors due to heat loss in the student’sexperiment.(1)(f)Suggest one other source of error in the student’s experiment. Do not include heat loss,
Practical Questionsapparatus error or student error.(1)(Total 11 marks)Q6.(a)Anhydrous calcium chloride is not used as a commercial de-icer because it reacts with water.The reaction with water is exothermic and causes handling problems.A student weighed out 1.00 g of anhydrous calcium chloride. Using a pipette, 25.0 cm3 ofwater were measured out and transferred to a plastic cup. The cup was placed in a beaker toprovide insulation. A thermometer was mounted in the cup using a clamp and stand. The bulbof the thermometer was fully immersed in the water.The student recorded the temperature of the water in the cup every minute, stirring the waterbefore reading the temperature. At the fourth minute the anhydrous calcium chloride wasadded, but the temperature was not recorded. The mixture was stirred, then the temperaturewas recorded at the fifth minute. The student continued stirring and recording the temperatureat minute intervals for seven more minutes.The student’s results are shown in the table below.Time / minutesTemperature / CTime / minutesTemperature / C0123419.6 19.5 19.5 19.545678910111224.6 25.0 25.2 24.7 24.6 23.9 23.4 23.0Plot a graph of temperature (y-axis) against time on the grid below.Draw a line of best fit for the points before the fourth minute.Draw a second line of best fit for the appropriate points after the fourth minute.Extrapolate both lines to the fourth minute.
Practical Questions(5)(b)Use your graph to determine an accurate value for the temperature of the water at the fourthminute (before mixing).
Practical QuestionsTemperature before mixing .(1)(c)Use your graph to determine an accurate value for the temperature of the reaction mixture atthe fourth minute (after mixing).Temperature after mixing .(1)(d)Use your answers from parts (b) and (c) to determine an accurate value for the temperaturerise at the fourth minute.Give your answer to the appropriate precision.Temperature rise .(1)(e)Use your answer from part (d) to calculate the heat given out during this experiment. Assumethat the water has a density of 1.00 g cm–3 and a specific heat capacity of 4.18 JK–1 g–1.Assume that all of the heat given out is used to heat the water.Show your working.(2)(f)Calculate the amount, in moles, of CaCl2 in 1.00 g of anhydrous calcium chloride (Mr 111.0).(1)(g)Use your answers from parts (e) and (f) to calculate a value for the enthalpy change, in kJ mol–1, for the reaction that occurs when anhydrous calcium chloride dissolves in water.CaCl2(s) aqCaCl2(aq).
Practical Questions.(2)(h)Explain why it is important that the reaction mixture is stirred before recording eachtemperature.(1)(i)Anhydrous calcium chloride can be prepared by passing chlorine over heated calcium.To prevent unreacted chlorine escaping into the atmosphere, a student suggested thediagram of the apparatus for this experiment shown below.(i)Suggest one reason why the student wished to prevent unreacted chlorine escaping intothe atmosphere.(1)(ii)Suggest one hazard of using the apparatus as suggested by the student for thisexperiment.(1)(Total 16 marks)Q7.A teacher noticed that a student had not cleared a large air bubble from below the burette tap inpreparing the burette for use before starting the titration. This air bubble was ejected during the firsttitration of the volumetric flask mixture.(a)State the effect that this mistake would have on the value of the first titre.
Practical Questions.(1)(b)State and explain the effect, if any, that this mistake would have on the average titre for thisexperiment.(2)(Total 3 marks)Q8.Alcohols such as methanol (CH3OH), ethanol (CH3CH2OH) and propan-1-ol (CH3CH2CH2OH) are goodfuels.(a)A student carried out an experiment to determine the enthalpy of combustion of methanol.Methanol was placed in a spirit burner and the mass of the spirit burner measured. Thestudent placed 100 g of water in a copper calorimeter and clamped it above the spirit burner.The burner was lit and allowed to burn for a few minutes. The flame was then extinguishedand the new mass of the spirit burner found.The measured temperature rise was 38.0 C. The specific heat capacity of water is4.18 J K 1 g 1.A diagram of the apparatus is shown alongside a table which shows the measurements thestudent recorded.Use the student’s data to calculate an experimental value for the enthalpy of combustion ofmethanol in kJ mol 1.
Practical Questions.(4)(b)Suggest one reason, other than incomplete combustion or heat transfer to theatmosphere, why the student’s value for the enthalpy of combustion of methanol isdifferent from that in a Data Book.(1)(c)The uncertainty in each of the temperature readings from the thermometer in thisexperiment was 0.25 C. This gave an overall uncertainty in the temperature rise of 0.5 C.Calculate the percentage uncertainty for the use of the thermometer in this experiment.(1)(d)The student said correctly that using a thermometer with an overall uncertainty for therise in temperature of 0.5 C was adequate for this experiment.Explain why this thermometer was adequate for this experiment.(1)
Practical Questions(e)The enthalpy of combustion of ethanol is 1371 kJ mol 1. The density of ethanol is0.789 g cm 3.Calculate the heat energy released in kJ when 0.500 dm3 of ethanol is burned.Give your answer to an appropriate number of significant figures.(3)(Total 10 marks)Q9.This question is about a white solid, MHCO3, that dissolves in water and reacts with hydrochloric acidto give a salt.MHCO3 HCl MCl H2O CO2A student was asked to design an experiment to determine a value for the Mr of MHCO3. Thestudent dissolved 1464 mg of MHCO3 in water and made the solution up to 250 cm3.25.0 cm3 samples of the solution were titrated with 0.102 mol dm 3 hydrochloric acid. The results areshown in the table.Initial burettereading / cm3Final burettereading / cm3Titre / .9010.009.509.759.65Calculate the mean titre and use this to determine the amount, in moles, of HCl that reactedwith 25.0 cm3 of the MHCO3 solution.(3)
(b)Practical QuestionsCalculate the amount, in moles, of MHCO3 in 250 cm3 of the solution.Then calculate the experimental value for the Mr of MHCO3.Give your answer to the appropriate number of significant figures.(3)(c)The student identified use of the burette as the largest source of uncertainty in the experiment.Using the same apparatus, suggest how the procedure could be improved to reduce thepercentage uncertainty in using the burette.Justify your suggested improvement.Suggestion .Justification .(2)(d)Another student is required to make up 250 cm3 of an aqueous solution that contains a knownmass of MHCO3. The student is provided with a sample bottle containing the MHCO3.Describe the method, including apparatus and practical details, that the student should use toprepare the solution.
Practical Questions.(6)(Total 14 marks)Q10.Magnesium carbonate, MgCO3, can occur as the anhydrous compound, or as hydrates with 2, 3 or 5molecules of water of crystallisation. All types of magnesium carbonate can be decomposed to formmagnesium oxide, an important starting material for many processes. This decomposition reactioncan be used to identify the type of magnesium carbonate present in a mineral.A chemist was asked to identify the type of magnesium carbonate present in a mineral importedfrom France. The chemist weighed a clean dry crucible, and transferred 0.25 g of the magnesiumcarbonate mineral to the crucible. The crucible was then heated for a few minutes. The crucible wasthen allowed to cool, and the crucible and its contents were reweighed. This process was repeateduntil the crucible and its contents had reached constant mass. The mass of the crucible and itscontents was then recorded.The experiment was repeated using different masses of the magnesium carbonate mineral.For each experiment the chemist recorded the original mass of the mineral and the mass ofmagnesium oxide left after heating to constant mass. The chemist’s results are shown in the tablebelow.Experiment123456Mass of mineral / g1.60 1.17 0.74 1.31 1.80 1.34Mass of magnesium oxide / g0.54 0.39 0.24 0.44 0.61 0.49(a)Plot a graph of the mass of the mineral (x-axis) against the mass of magnesium oxide on thegrid below.Draw a straight line of best fit on your graph.
Practical Questions(4)(b)Use the graph to determine the mass of the mineral which would have formed 0.50 g ofmagnesium oxide.Mass of the mineral .(1)
(c)Practical QuestionsCalculate the amount, in moles, of MgO present in 0.50 g of magnesium oxide.(1)(d)Use your answers from part (b) and from part (c) to calculate the Mr of the magnesiumcarbonate present in the mineral.(1)(e)Use your answer from part (d) to confirm that this mineral is MgCO3.2H2O(If you could not complete the calculation in part (d), you should assume that the experimentalMr value is 122.0 This is not the correct answer.).
Methanol was placed in a spirit burner and the mass of the spirit burner measured. The student placed 100 g of water in a copper calorimeter and clamped it above the spirit burner. The burner was lit and allowed to burn for a few minutes. The flame was then extinguished and the new mass of the spirit burner found.
What is redox titration? A TITRATION WHICH DEALS WITH A . REACTION INVOLVING OXIDATION AND . REDUCTION OF CERTAIN CHEMICAL . SPECIES. What is a titration? The act of adding standard solution in small quantities to the test solution till the reaction is complete is termed titration.
is that the visual method cannot be automated and is therefore difficult to validate and it lacks data integrity. This paper summarizes the steps involved in converting an existing manual titration procedure to semi-automated or automated titration procedures. It discusses topics such as selecting the right electrode and titration mode.
II. Weak acid- strong base titration In this part of the experiment you will do weak acid strong base titration. 0.05M, 20 mL CH 3 COOH is titrated with 0.05M NaOH. perform same procedure mentioned above. III. Mixture of weak acid, strong acid and strong base titration 0.02M, 10 mL HCl and 0.02M, 10 ml acetic acid mixture is titrated with 0.05 .
As in the weak acid-strong base titration, there are three major differences between this curve (in blue) and a strong base-strong acid one (in black): (Note that the strong base-strong acid titration curve is identical to the strong acid-strong base titration, but flipped vertically.) 1- The weak-acid solution has a lower initial pH.
Karl Fischer titration is the method of choice for water content determination. With the . with test certificate according to DIN 50049-2.3 First titrator (METTLER TOLEDO DL55) with online curves E f(t) and V f(t) for Karl Fischer titration. 1997 New DV705 KF Titration Stand with very low drift value ( 2μg/min) for the METTLER
Titration of amino acid: When an amino acid is dissolved in water it exists predominantly in the . isoelectric form. Amino acid is an . amphoteric. compound It act as either an acid or a base: Upon titration with acid it acts as a BASE (accept a proton). Upon titration with base it acts as an ACID (donate a proton)
The excess enthalpy, HE between the enthalpy of pure substances and mixtures can be calculated by using Eq. (4). HE H m (x1H1 x2H2)(4) where xi, H1 and Hm are the mole fraction, molar enthalpy of com-ponent i (i 1, 2) and mixture enthalpy, respectively. The
16. The standard enthalpy change is the change in enthalpy for a process in which the initial and final substances are in their standard states. The standard state is the pure substance at 1 bar. 17. Enthalpy changes are additive, as in sub H fus H vap H 18. The enthalpy change for a process and its reverse are related
