Chem!stry. Identification of Gases

Transcription

1 Chem!stry Name: ( ) Class: Date: / / Identification of Gases Gas Ammonia, NH3 Chlorine, Cl2 Water vapour, H2O Sulphur dioxide, SO2 Carbon dioxide, CO2 Oxygen, O2 Observations Colourless. Pungent. Damp red litmus paper turns blue. White fumes of NH4Cl are observed when a glass rod dipped in concentrated hydrochloric acid is brought near the gas. Pale green-yellow. Pungent. Damp blue litmus paper turns red and is then bleached white. Colourless. Odourless. Anhydrous cobalt(ii) chloride paper changes colour from blue to pink. Colourless. Pungent. Acidified potassium manganate(vii) changes colour from purple to colourless. Colourless. Odourless. White precipitate of CaCO3 forms when the gas is bubbled into limewater. Colourless. Odourless. Relights a glowing splint. Hydrogen, H2 Hydrogen chloride, HCl Colourless. Odourless. Lighted splint is extinguished with a pop sound. Colourless. Pungent. Damp blue litmus paper turns red. White fumes of NH4Cl are observed when a glass rod dipped in aqueous ammonia is brought near the gas. Identification of Cations Part #1 Test-tube Reactions Cation Using aqueous sodium hydroxide NaOH Using aqueous ammonia NH4OH Aluminium cation, Al 3+ Calcium cation, Ca 2+ Zinc cation, Zn 2+ Lead(II) cation, Pb 2+ Iron(II) cation, Fe 2+ Iron(III) cation, Fe 3+ Copper(II) cation, Cu 2+ Ammonium cation, NH4 + White precipitate of Al(OH)3 soluble in excess reagent giving a colourless solution. White precipitate of Ca(OH)2 insoluble in excess reagent. White precipitate of Zn(OH)2 soluble in excess reagent giving a colourless solution. White precipitate of Pb(OH)2 soluble in excess reagent giving a colourless solution. Green precipitate of Fe(OH)2 insoluble in excess reagent. Turns red-brown on standing. Red-brown precipitate of Fe(OH)3 insoluble in excess reagent. Blue precipitate of Cu(OH)2 insoluble in excess reagent. No precipitate ammonia gas produced on warming (turns damp red litmus paper blue). White precipitate of Al(OH)3 insoluble in excess reagent. No observed reaction. No precipitate formed. White precipitate of Zn(OH)2 soluble in excess reagent giving a colourless solution. White precipitate of Pb(OH)2 insoluble in excess reagent. Green precipitate of Fe(OH)2 insoluble in excess reagent. Turns red-brown on standing. Red-brown precipitate of Fe(OH)3 insoluble in excess reagent. Blue precipitate of Cu(OH)2 soluble in excess reagent to give a dark blue solution. Test not applicable. 1

2 Identification of Cations Part #2 Flame Tests Cation Sodium, Na + Potassium, K + Calcium, Ca 2+ Barium, Ba 2+ Copper(II), Cu 2+ Observation Yellow / orange flame colour. Lilac flame colour. Brick red flame colour. Apple green flame colour. Green flame colour. Identification of Anions Anion Carbonate, CO3 2 Chloride, Cl Iodide, I Nitrate, NO3 Sulfate, SO4 2 Observation Add dilute acid. Effervescence is observed. Carbon dioxide gas is produced (carbon dioxide gas produces a white precipitate of CaCO3 when bubbled through limewater). Add dilute nitric acid followed by dilute aqueous silver nitrate. A white precipitate of AgCl, which is soluble in aqueous ammonia, but insoluble in dilute nitric acid, confirms chloride ions. Add dilute nitric acid followed by dilute aqueous silver nitrate. A yellow precipitate of AgI, which is insoluble in aqueous ammonia and insoluble in dilute nitric acid confirms iodide ions. Add aqueous sodium hydroxide followed by aluminium foil and warm the mixture. Ammonia gas is produced (turns damp red litmus paper blue). Should exclude NH4 + before testing for NO3. Add dilute nitric acid followed by dilute aqueous barium nitrate. A white precipitate of BaSO4 indicates the presence of sulfate ions. Effect of Heat on a Solid Observation Carbonate, CO3 2 Group 1 nitrate, NO3 Other nitrates, NO3 Ammonium salt, NH4 + Hydrated salt, X H2O Generally decompose on strong heating to produce carbon dioxide gas. Carbon dioxide gas produces a white precipitate when bubbled through limewater. Decompose on strong heating to produce oxygen gas and the Group 1 metal nitrite. Oxygen gas will relight a glowing splint. Decompose on strong heating to produce oxygen gas and nitrogen dioxide gas. Oxygen gas will relight a glowing splint. Nitrogen dioxide is orange / brown in colour. Sublime on heating. White solid will be observed on the cooler regions of the test-tube. Produces steam on strong heating. Steam will condense on the cooler regions of the test-tube. Water causes anhydrous cobalt(ii) chloride paper to change colour from blue to pink. Test for Oxidising Agents and Reducing Agents Observation Oxidising agent Reducing agent Add an aqueous solution of FeSO4. Colour changes from pale green to brown. Add an aqueous solution of KI. Colour changes from colourless to brown blue / black with starch. Add an acidified solution of KMnO4. Colour changes from purple to colourless. Add an acidified solution of K2Cr2O7. Colour changes from orange to green. 2

3 Chem!stry Name: ( ) Class: Date: / / Questions on Qualitative Analysis for Class Discussion #1 Question One: Study the reaction sequence shown below: a) Give the name and formula of the element, compound or ion: A B C D E F G H I J b) Write the balanced chemical equation for the formation of the dirty-green precipitate G: c) Write the ionic half-equation that shows the dichromate(vi) ion functioning as an oxidising agent: 1

4 Question Two: Study the reaction sequence shown below: a) Give the name and formula of the element, compound or ion: S T U V W X Y Z b) Write the balanced chemical equation for the formation of the white precipitate U: c) Write the balanced chemical equation for the formation of the white precipitate V: d) Write the ionic equation for the formation of the white precipitate X: e) Write the ionic equation for the formation of the colourless gas Y: f) Write the balanced chemical equation for the formation of the white precipitate Z: 2

5 Answers Question One: a) A Nitric acid, HNO3 C Hydrogen, H2 E Either chloride (Cl ), bromide (Br ) or iodide (I ) ion G Iron(II) hydroxide, Fe(OH)2 I Either iron(iii), Fe 3+ or chromium(iii), Cr 3+ ion B Iron, Fe D Iron(II) nitrate, Fe(NO3)2 F Ammonia, NH3 H Iron(III) hydroxide, Fe(OH)3 J Either iron(iii), Fe 3+ or chromium(iii), Cr 3+ ion b) Fe(NO3)2(aq) + 2NaOH(aq) Fe(OH)2(s) + 2NaNO3(aq) c) Cr2O7 2 (aq) + 14H + (aq) + 6e 2Cr 3+ (aq) + 7H2O(l) Question Two: a) S Barium chloride, BaCl2 U Silver chloride, AgCl W Barium nitrate, Ba(NO3)2 Y Carbon dioxide, CO2 T Sodium chloride, NaCl V Barium carbonate, BaCO3 X Barium sulphate, BaSO4 Z Calcium carbonate, CaCO3 b) AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) c) BaCl2(aq) + Na2CO3(aq) BaCO3(s) + 2NaCl(aq) d) Ba 2+ (aq) + SO4 2 (aq) BaSO4(s) e) 2H + (aq) + BaCO3(s) Ba 2+ (aq) + CO2(g) + H2O(l) f) Ca(OH)2(aq) + CO2(g) CaCO3(s) + H2O(l) 3

6 Chem!stry Name: ( ) Class: Date: / / Questions on Qualitative Analysis for Class Discussion #2 Question One: The following tests were carried out to identify an unknown salt A. The white salt A gave an apple green flame test. An aqueous solution of A produced a white precipitate of compound B when treated with dilute sulphuric acid. A second aqueous solution of A, when treated with aqueous sodium carbonate, gave a solution of compound C and a white precipitate of compound D. The precipitate of D was removed by filtration. Solid D, when treated with dilute hydrochloric acid, produced a gas, which gave a white precipitate when bubbled through limewater. A portion of the filtrate, containing C, was treated with dilute nitric acid followed by aqueous silver nitrate. An off-white precipitate of compound E was formed. This precipitate of E dissolved easily when concentrated aqueous ammonia was added but only partially with dilute aqueous ammonia. When chlorine water was added to the remainder of the filtrate, containing C, a yellow / orange solution of substance F was formed. a) Identify the cation present in A. (1 mark) b) Identify, by name or formula, the substances B, C, D, E, F and A. Identity of B... Identity of C... Identity of D... Identity of E... Identity of F... Identity of A... (6 marks) c) Write an equation for the reaction between A and aqueous sodium carbonate. (2 marks) d) Identify, by name or formula, the gas evolved when solid D is treated with dilute hydrochloric acid. (1 mark) 1

7 e) When chlorine water is added to an aqueous solution of C, a yellow / orange solution is formed. Write an ionic-equation for this reaction. Give the name of the type of reaction involved. Equation: Name of reaction type: (2 marks) Question Two: When ionic compounds are dissolved in water, they dissociate and form positive ions (cations) and negative ions (anions). When two solutions containing ions are mixed, sometimes the oppositely charged ions that are present will combine to form a precipitate or insoluble compound. Sodium hydroxide produces sodium ions and hydroxide ions when dissolved in water: NaOH(s) Na + (aq) + OH (aq) Iron(III) chloride produces iron(iii) ions and chloride ions when dissolved in water: FeCl3(s) Fe 3+ (aq) + 3Cl (aq) When an aqueous solution of sodium hydroxide is added to an aqueous solution of iron(iii) chloride, a reddish brown precipitate of iron(iii) hydroxide is formed. If this mixture is left to stand for several minutes, tiny particles of the precipitate settle to the bottom of the test tube. The clear liquid on the top is called the supernatant. a) Write a balanced chemical equation for the reaction between the aqueous sodium hydroxide and aqueous iron(iii) chloride: (1 mark) A student performed a series of precipitation reactions to determine the concentration of a sample of iron(iii) chloride. She placed 5.0 cm 3 of the iron(iii) chloride solution into a test tube and then added 1.0 cm 3 of 0.1mol dm 3 sodium hydroxide solution. She placed a stopper in the mouth of the test tube and then shook it. She repeated the procedure using different volumes of the sodium hydroxide solution. She allowed the contents of each test tube to stand for 10 minutes and then measured the height of the precipitate. Her results are shown below: Volume of NaOH(aq) / cm Height of precipitate / mm b) Using the student s results, calculate the concentration of the iron(iii) chloride solution: 2 (3 marks)

8 Question Three: Substance X is a mixture of a salt and a metal. a) Give the name and formula of the element, compound or ion: P Q R S T X (6 marks) b) Write the balanced chemical equation for the formation of the yellow precipitate P: (2 marks) c) Write the ionic equation for the formation of the white precipitate Q: (2 marks) d) Write a balanced chemical equation for the formation of the white precipitate T: (2 marks) 3

9 Question Four: Study the reaction sequence shown below: a) Give the name and formula of the element, compound or ion: A B C D E F G H I J K L M (13 marks) b) Write the balanced chemical equation for the formation of the black solid D: (2 marks) c) Write the balanced chemical equation for the formation of the white precipitate G: (2 marks) 4

10 Answers Question One: a) Barium ion, Ba 2+ b) B Barium sulfate, BaSO4 D Barium carbonate, BaCO3 F Bromine, Br2 C Sodium bromide, NaBr E Silver bromide, AgBr A Barium bromide, BaBr2 c) BaBr2(aq) + Na2CO3(aq) BaCO3(s) + 2NaBr(aq) d) Carbon dioxide, CO2 e) Cl2(aq) + 2Br (aq) 2Cl (aq) + Br2(aq) This is a redox reaction. The chlorine is reduced to chloride while the bromide is oxidised to bromine. Question Two: a) FeCl3(aq) + 3NaOH(aq) Fe(OH)3(s) + 3NaCl(aq) b) Between 1.0 and 5.0 cm 3 of sodium hydroxide, the iron(iii) chloride is in excess. This can be seen because the level of the precipitate increases. At 6.0 cm 3 of sodium hydroxide, the ratio of sodium hydroxide to iron(iii) chloride is correct for both chemicals to react completely. Between 7.0 and 10.0 cm 3 of sodium hydroxide, the sodium hydroxide is in excess. This can be seen because the level of the precipitate remains almost constant. Therefore, 5.0 cm 3 of FeCl3 reacts with 6.0 cm 3 of NaOH. moles of NaOH = c v 10 3 = = From the balanced chemical equation: moles of FeCl3 = moles of NaOH 3 = = Therefore, there are mol of FeCl3 in 5.0 cm 3 of solution. moles of FeCl3 in 1000 cm 3 = ( ) 1000 = 0.04 = 0.04 mol dm 3 Question Three: a) P Lead(II) iodide, PbI2 R Zinc chloride, ZnCl2 T Silver chloride, AgCl Q Calcium hydroxide, Ca(OH)2 S Hydrogen, H2 X Calcium Iodide (CaI2) and zinc (Zn) b) Pb(NO3)2(aq) + CaI2(aq) PbI2(s) + Ca(NO3)2(aq) c) Ca 2+ (aq) + 2OH (aq) Ca(OH)2(s) d) ZnCl2(aq) + 2AgNO3(aq) Zn(NO3)2(aq) + 2AgCl(s) 5

11 Question Four: a) A Copper(II) carbonate, CuCO3 C Calcium carbonate, CaCO3 E Sulphuric acid, H2SO4 G Barium sulfate, BaSO4 I Zinc, Zn K Zinc sulfate, ZnSO4 B Carbon dioxide, CO2 D Copper(II) oxide, CuO F Copper(II) sulfate, CuSO4 H Copper(II) hydroxide, Cu(OH)2 J Copper, Cu L Zinc oxide, ZnO M Sulphur trioxide or sulphur(vi) oxide, SO3 b) CuCO3(s) CuO(s) + CO2(g) c) CuSO4(aq) + Ba(NO3)2(aq) Cu(NO3)2(aq) + BaSO4(s) 6

12 Chem!stry Name: ( ) Class: Date: / / Questions on Qualitative Analysis for Class Discussion #3 Question One: Study the reaction sequence given below: a) Give the name and formula of the element, compound or ion: A B C D E F G H b) Write the balanced chemical equation for the formation of C: c) Write the ionic equation for the formation of G: 1

13 Question Two: A student has been provided with five reagent bottles, each containing one of the following solutions: Nitric acid HNO3(aq) Barium chloride BaCl2(aq) Sodium chloride NaCl(aq) Sodium carbonate Na2CO3(aq) Potassium sulfate K2SO4(aq) Only the bottle of nitric acid is labelled. With only test tubes, and a test tube rack at your disposal, explain how you will correctly identify the four unknown laboratory reagents in only three steps. Write a balanced chemical equation to describe the essential reaction that is taking place in each of the three steps. Procedure: Observation: Inference: Step One: Step Two: Step Three: Question Three: Study the reaction sequence given below. Note: potassium oxalate has the formula K2C2O4. a) Give the name and formula of the element, compound or ion: A B C D E F b) Write the ionic equation for the formation of either B, E or F: 2

14 Question One: a) Answers A Lead(II) nitrate, Pb(NO3)2 B Oxygen, O2 C Lead(II) oxide, PbO D Nitrogen dioxide, or nitrogen(iv) oxide, NO2 E Lead(II) hydroxide Pb(OH)2 F Tetrahydroxolead(II) ion, [Pb(OH)4] 2 G Lead(II) iodide, PbI2 H Ammonia, NH3 b) 2Pb(NO3)2(s) 2PbO(s) + 4NO2(g) + O2(g) c) Pb 2+ (aq) + 2I (aq) PbI2(s) Question Two: Procedure: Observation: Inference: Step One: Add a few drops of nitric acid to a sample of each unknown. Effervescence is observed in one of the test tubes. The test tube in which the effervescence is observed contains the sodium carbonate solution. Step Two: Add a few drops of the sodium carbonate solution to a sample of each of the remaining unknowns. A white precipitate will be observed in one of the test tubes. The test tube in which the white precipitate is observed contains the barium chloride solution. Step Three: Add a few drops of the barium chloride solution to a sample of each of the remaining unknowns. A white precipitate will be observed in one of the test tubes. The test tube in which the white precipitate is observed contains the potassium sulfate solution. The remaining test tube contains the sodium chloride solution. Reaction for Step One: 2HNO3(aq) + Na2CO3(aq) 2NaNO3(aq) + CO2(g) + H2O(l) Reaction for Step Two: Na2CO3(aq) + BaCl2(aq) BaCO3(s) + 2NaCl(aq) Reaction for Step Three: BaCl2(aq) + K2SO4(aq) BaSO4(s) + 2KCl(aq) Question Three: a) A Potassium manganate(vii), KMnO4 B Iron(III) ion, Fe 3+ C Iron(III) hydroxide, Fe(OH)3 E Manganese(II) ion, Mn 2+ D Carbon dioxide, CO2 F Iodine, I2 b) The ionic half-equation with respect to the manganate(vii) ion is: MnO4 (aq) + 8H + (aq) + 5e Mn 2+ (aq) + 4H2O(l) This should be combined with any one of the following ionic half-equations after the total number of electrons in the two ionic half-equations have been balanced and cancelled out: Fe 2+ (aq) Fe 3+ (aq) + e C2O4 2 (aq) CO2(g) + 2e 2I (aq) I2(aq) + 2e 3

15 Chem!stry Name: ( ) Class: Date: / / Questions on Qualitative Analysis for Class Discussion #4 Question One: Complete the crossword puzzle given below: Across Down 2. Cations of this metal produce a white precipitate with aqueous sodium hydroxide. The precipitate is insoluble in excess reagent (7). 1. Cations of this metal produce a white precipitate with aqueous ammonia. The precipitate is insoluble in excess reagent (9). 4. This anion produces a white precipitate with aqueous barium nitrate. The precipitate is insoluble in nitric acid (8). 3. When an acid is added to this anion, a colourless gas is produced which gives a white precipitate when bubbled through lime water (9). 6. This anion is reduced to form an alkaline gas when warmed with sodium hydroxide and Devarda s alloy (7). 5. Compounds of this Group I metal produce a characteristic lilac flame colour (9). 9. Cations of this metal produce a white precipitate with aqueous ammonia. The precipitate is soluble in excess reagent (4). 7. In the following ionic half-equation, the chromium is...? (7) Cr2O7 2 (aq) + 14H + (aq) + 6e 2Cr 3+ (aq) + 7H2O(l) 10. Compounds of this metallic element are all soluble in water (6). 8. This colourless gas, which turns moist red litmus paper blue, is produced when an ammonium salt is warmed with a hydroxide (7) This anion produces a white precipitate with aqueous silver nitrate. The precipitate is insoluble in nitric acid (8). In general, cations of these metals form coloured aqueous solutions and coloured precipitates (10). 1

16 Question Two: When aqueous barium nitrate is added to an aqueous solution of compound T, a white precipitate is formed. a) What information about compound T can be deduced from this information? Support your answer by writing relevant ionic equation(s): b) What further test(s) would need to be performed in order to clarify the answer given in part a)? What would be observed? Support your answer by writing relevant ionic equation(s): Question Three: Study the reaction sequence shown below: a) Give the name and formula of the element, compound or ion: S T U V W X b) Write the balanced chemical equation for the formation of V: c) Write the balanced chemical equation for the formation of W: d) Write the balanced chemical equation for the formation of X: e) Write the ionic equation for the formation of X: 2

17 Question One: Answers A C A L C I U M L A S U L P H A T E N I T R A T E M O B I T R O N A E A Z I N C I S O D I U M A U S U M T M I C H L O R I D E U E N M D I T R A N S I T I O N Question Two: a) The white precipitate could be either barium carbonate or barium sulfate, because both are insoluble in water. Therefore, compound T could be either a carbonate or a sulfate. Ba 2+ (aq) + CO3 2 (aq) BaCO3(s) Ba 2+ (aq) + SO4 2 (aq) BaSO4(s) b) The addition of nitric acid will confirm the existence of either the carbonate or the sulfate. If T is a carbonate, effervescence will be observed. If T is a sulfate, there will be no observed reaction. BaCO3(s) + 2H + (aq) Ba 2+ (aq) + H2O(l) + CO2(g) Note: a similar rule applies to the use of silver nitrate. Nitric acid should always be added to any precipitates that are formed to eliminate the risk of misinterpreting a precipitate of Ag2CO3 for AgCl, AgBr or AgI. Question Three: a) S Calcium carbonate, CaCO3 U Calcium oxide, CaO W Calcium hydroxide, Ca(OH)2 T Carbon dioxide, CO2 V Calcium nitrate, Ca(NO3)2 X Ammonia, NH3 b) CaO(s) + 2HNO3(aq) Ca(NO3)2(aq) + H2O(l) c) CaO(s) + 2H2O(l) Ca(OH)2(aq) d) Ca(OH)2(aq) + 2NH4Cl(s) CaCl2(aq) + 2NH3(g) + 2H2O(l) e) OH (aq) + NH4 + (aq) NH3(g) + H2O(l) 3