Chapter 8 Flashcards
Elements in group 2
Alkaline earth metals that are reactive and do not occur in elemental form naturally - stable compounds like CaCO3
Why are the group 2 eleemnts reducing agents?
2 electrons in outer s sub shell that are lost to fomr 2+ ion - they act as reducing agents because the 2 electrons produced can be gained by smother species to reducd them. Eventually end up with electron config as a noble gas with each metal atom being oxidised
Grouo 2 reaction with oxygen
Forms metal oxide n; magneisum burns with a brilliant white light. Metal is oxidised - total changed balance out to net 0 ; totsl reduction and oxidation = 0.
Redox reactions with water
React with water to form sn slkaline hydroxide and hydrogen gas - reactivity increases down the group ghus reacfions get more and more vigorous
Redox reactions with dilute acids
Metal + acid - > salt + hydrogen ; reactivity increases down the group
Why does reactivity increase down group 2?
Requires first two ionisation energies ; both of which decrease going down the group because attrcation between nucleus and outer elecgrons decfeases as a result of increasing atomic radius and increasing shielding
Group 2 oxides + water
Releasing hydroxide ions and forming an alkaline solution of metal hydroxide
CaO + H2O -> Ca2+ (aq) + 2OH- (aq)
Griuo 2 hydroxides are only slightly soluble in watsr and when solution is saturated any further metal and hydroxide ions will form a solid precipitate
Ca2+ (aq) + 2OH- (aq) -> Ca(OH)2 (s)
Solubility of group 2 hydroxides
Increases down the group so the resulting solutions contain more OH- ions and are more alkaline; Mg(OH)2 us on,y slightly so,uble thus low alkaline Ph (10)
Ba(OH)2 is mcuh more soluble with greater OH- ion concentration and thus ph higher (13)
Group 2 compounds in agriculture
Calcium hydroxide is added to fields as lime by farmers to increase the pH of acidic soils - calcium hydroxide neytralised acid in the soil forming neutral water
Ca(OH)2 + 2H+ -> Ca2+ (aq) + 2H2O (l)
Grouo 2 compounds in medicine
Used as antacids for treating acid indigestion sith mang indigestion tablegs containg magnesium and calcium carbonates as main ingredients - milk of magnesia is a suspension of white magnesium hydroxide Mg(OH)2 in water ; acid in stomach is mainly HCl and thus neutralksation reaction takes place
Halogens
Do not appear in elementsl form - as negatively charged ions dissolved in sea water for example
Trends in boiling points
Increases down the group as there are more electrons thus stringer london forces so more energy required to break these intermolechlsr forces so boiling point increases
Fluorine
Pale yellow gas
Chlorine
Pale green gas
Bromine
Red brown liquid
Iodine
Shiny grey black solid
Astatine
Never been seen radioactive - decays rapidly
Redox reactions with halogens
5 electrons in outer p subshell - each halogen atom is reduced gaining an electron to form 1- ion
Cl2 + 2e- -> 2Cl-
Hallgens are the oxidising agent as another species js oxidised as a result of the hallgens losing electrons
Hallgen halide displacement reactions
Reactivity of hallgens decreases down the group - solution of each halogen added to halides ; if more reacfive than halogen displaces halide and solution changes colour to the halogen that is left
Solutions if halogens in water
Chlorine = pale green Bromine = orange Iodine = brown
Why is cyclohexane added?
Non-polar solvent with non polar hallgens disslving mlre readily - easjer to tell apart anf clntrast
Cl2 = pale green
Bromine = orange
Iodine = purp,e
Ionic equation of halogen halid reactions?
ALL AQUEOUS SOLUTIONS
Redox in halogen halide reactions
Halogen js reduced
Halide ion is oxidised
Fluorine and astatine?
Fluorine is too reactive with any substance
Astatine very rare and decays rapidly
Trend in reactivity
Halogens become less reactive down the group - atomic radius increases, more inner shells so shielidng increases thus less nuclear attraction to capture an electon from another species so reactivity decreases
Halogens as oxidisng agents?
Become weaker oxidising agents down the griup eith fluorine the best
Disporportionation
Same element is both oxidised and reduced - examples are chlorine with water and soidum hydroxide
Chlorine + water
Used in water purification - killing harmful bacteria
Cl2 + H2O -> HClO + HCl
Both reduced and oxidised
Chloric i acid actuallh kills bacteria + ClO- ions. Also acts as a weak bleach (the chloric acid) ; indicator turns res first snd then is bleached
Chlorine + aqueous sodium hydroxide
Cl2 (aq) + 2Na(OH (aq) -> NaClO (aq) + NaCl (aq) + H2O (l)
ClO- ions are used as a household bleach
Benefits and risks of chlorine use
Chlorine is an extremely toxic fas and is a respiratory irritant in small concebtrations - chlorine in drinking water dab react with organic hydrocarbons such as methance which coud cause cancer. However this is outweighed by risk of not adding chlorine which could reshkt kn typhoid/cholera
Precipitate reactions with aqueous silver ions
Ag+ (aq) + X- (aq) -> AgX (s)
Precipitage formed
Qualititave analysis
Observation rather than measurements
Carbonate tes
React wjth acuds to form CO2 gas - add dilute notric acid and id bubbles are seen then could be carbonage ; bubble gas through kimewater (calcium hydroxide) which then produces a cloudy lrecipitate of calcium carbonate
Sulfate test
Barkum sulfate is insoluble in water - add aqueous barium ions to the sulfate ions and if BaSO4 is produced a solid precipitate is creafed which is white. USE BARIUM NITRATE AS WITH BARIUM CHLORIDE YOU INTRODUCE HALIDE IONS + MAKE SURE IT IS NOT CARBONATE BY GETTING RID OF CARBONATE
Halide tests
Silver halides are insoluble # aqueous silver ions react with halide ions to form precipitates of silver balides. Add silver nitrate Silver chloride - white Silver bromjde - cream Silver iodiide - yellow Add ammonia Dilute ammonia dissolves chloride Conc smmonia dissolves bromide Iodide does not dissolve in either ALL AQUEOUS EXCEPT PRECIPITATE
Sequence of tests
Carbonate
Sulfate
Halides
Carbonate test sequence
Looking for effervescene from carbon dioxide gas - neither sukfate nor hakide jons bubbles with acid thus you can carry out carbonat test eithout possibility of an incorrect conclusion ; no carbonate present then move onto next
Sulfate test sequence
Barium carbonate is also a white insoluble precipitae like baroum sulfate thus make shre to do carbonate first and kn,y proceed if no carbonates are present
Halide test sequence
Silver ions are introduced -they can form a pfecipirate with carbonate and sukfate which is indoluble therefore carry it out last as the other two rule out any possibilities
A mixture of ions
1) Carbonate ions - if bubbles are seen add in nitric acid until it stops ; all carbknate ions removed and nond left ; USE HNO3 AND NOT H2SO4/HCL AS IT CONTAINS SULFATE IONS
2) Sulfate test - no carbknates left ; add exceek barium nitrate will form a white precipiratet ; filter out the barium sulfafe. Dont use BACL2 due to halide ions
3) Halide test - add AgNO3 ; anh carbonate or sulfate removed so only halide left and add NH3 to confirm (aqueous)
Test for am,onium ions
Add sodium hydroxide to form nh3 gas and water ; then heat and ammonium gas released ; use ph indicator paper should turn it fromr ed to blue (alkaline)
