Chapter 8 Reactivity Trends Flashcards
What elements have been oxidised or reduced in
2Mg + O2 —> 2MgO
Mg is oxidised from 0 in Mg to +2 in MgO
O is reduced from 0 in O2 to -2 in MgO
What has been oxidised and reduced in
Sr + 2H2O —> Sr(OH)2 + H2
Sr is oxidised from 0 in Sr to +2 in Sr(OH)2
H is reduced from +1 in H2O to 0 in H2
What has been oxidised and reduced in the reaction
Mg + 2HCl —> MgCl2 + H2
Mg is oxidised from 0 in Mg to +2 in MgCl2
H is reduced from +1 in HCl to 0 in H2
What is the trend in melting and boiling points of halogens?
Melting and boiling point increases going down the group, this is because there are more electrons so the London forces are stronger. This means more energy is required to overcome the intermolecular forces.
What is the appearance and state of Fluorine at RTP?
Pale yellow gas
What is the appearance of Chlorine at RTP?
Pale green gas
What is the appearance of Bromine at RTP?
Orange liquid
What is the appearance of Iodine at RTP?
Silver- black solid
What is the appearance of chlorine in an aqueous solution?
Pale green
What is the appearance of bromine in an aqueous solution?
Orange in an aqueous solution
What is the appearance of iodine in an aqueous solution?
Brown in an aqueous solution
What is the appearance of Chlorine in cyclohexane?
Pale green
What is the appearance of bromine in cyclohexane?
Orange
What is the appearance of iodine in cyclohexane?
Purple
During redox, are halogens oxidised or reduced?
Each halogen atom is reduced, gaining one electron to form a -1 halide ion.
What are halogen-halide displacement reactions?
A more reactive halogen will displace a less reactive halide ion from its solution.
How are halogen-halide displacement reactions carried out?
A solution of a halogen is added to an aqueous solution of a halide ion.
If the halogen is more reactive it displaces the less reactive halide.
A colour change at the end of a reaction indicates the halogen which is present.
How can halogen identification be made easier?
Cyclohexane is added
This is helpful when distinguishing between iodine and bromine which are similar colours in aqueous.
Explain the trend in reactivity of halogens down the group
Reactivity decreases down the group because the tendency to gain an electron from another species decreases. This is because atomic radius increases so there is greater distance between the nucleus and outer electron. This also increases the amount of inner electron shielding. Therefore, going down the group there is less nuclear attraction to capture an electron from another species.
What is disproportionation?
A redox reaction in which the same element is both oxidised and reduced.
Show the general ionic equation of the test for halide ions
Ag+ (aq) + X- (aq) —> AgX (s)
What are the uses of Chlorine?
It can be added to drinking water to kill harmful bacteria such as cholera.
It can be reacted with cold, dilute sodium hydroxide to make bleach.
What is the equation for the reaction between Chlorine and water?
Cl2 + H2O —> HCl + HClO
What type of agents are Group 2 elements?
Reducing agents
Explain why Group 2 elements are reducing agents
Because Group 2 elements can lose their 2 outer electrons from the outer s sub shell to form a 2+ ion. Therefore, another species will gain these two electrons and become reduced.
What is the trend in reactivity of group 2 elements?
Reactivity increases going down Group 2
Why does reactivity increase going down group 2?
Because atomic radius increases- so there is a greater distance between the nucleus and the outer electrons.
Shielding also increases as there is a greater number of inner electron shells.
Therefore, the attraction between the nucleus and outer electrons decreases so electrons are lost more easily so less energy is required to lose an electron
The first and second ionisation energies decrease going down the group
What are the reactions of group 2 elements?
Group 2 elements can carry out redox reactions with oxygen, water and dilute acids.
What is the product of the reaction between a group 2 element and oxygen?
Metal (group 2) oxide
Eg, 2Mg + O2 —> 2MgO
What is the product of the reaction between a group 2 element and water?
Alkaline hydroxide and hydrogen gas
Eg, Sr + 2H2O —> Sr(OH)2 + H2
What is the product of the reaction between a group 2 element and a dilute acid?
Metal + dilute acid —> salt + hydrogen
Eg, Mg + 2HCl —> MgCl2 + H2
What are the reactions of group 2 compounds?
Group 2 oxides can react with water releasing hydroxide ions, OH-, and forming alkaline solutions of the metal hydroxide.
What is the trend in the solubility of hydroxides?
The solubility of hydroxides in water increases going down group 2, so solutions contain more OH- (aq) ions so become more alkaline.
Which group 2 hydroxide will have a greater pH:
Mg(OH)2 or Ba(OH)2
Ba(OH)2 is more soluble in water. The solution will have a greater concentration of OH- (aq) ions so will have larger pH.
While Mg(OH)2 is only slightly soluble in water. The solution will have a lower OH- (aq) ion concentration with a lower pH.
How can you test for the solubility of group 2 hydroxides?
1- Add a spatula of each Group 2 oxide to water in a test tube
2- Shake the mixture. Due to insufficient water, a saturated solution of each metal hydroxide will form as well as some undissolved white solid precipitate
3- Measure the pH of each solution.
What are the uses of group 2 compounds?
Can be used to neutralise acids in agriculture and medicine
What is Calcium hydroxide used for?
Neutralising acidic soils
The calcium hydroxide neutralises acid in the soil forming neutral water.
Eg, Ca(OH)2 + 2HCl —> CaCl2 + 2H2O
What is Magnesium hydroxide used for?
Antacid to neutralise excess stomach acid - used in indigestion remedies
Mg(OH)2 + 2HCl —> MgCl2 + 2H2O
What is Calcium carbonate used for?
Antacid to neutralise excess stomach acid - used in indigestion remedies
CaCO3 + 2HCl —> CaCl2 + H2O +CO2
What type of structure do halogens form in their solid state?
Form lattices with simple molecular structures
What type of agents are halogens?
Oxidising agents
Explain why halogens are oxidising agents
Halogens have 7 electrons within their outermost shell so they accept one electron to form a -1 halide ion. Therefore, the halogen has oxidised the species from which it has accepted the electron.
What is the substance that kills bacteria in drinking water?
Chlorate (I) ion ~ClO-
What is the reaction between chlorine and sodium hydroxide?
Cl2 + 2NaOH —> NaCl + NaClO + H2O
What are the advantages of using chlorine in drinking water?
-kill harmful bacteria so reduce waterborne diseases likel cholera
-prevents algae growth in water
-eliminates bad taste and smells
-persists in water so lasts longer compared toO3 or UV alternatives
What are the disadvantages of using chlorine in drinking water?
-Chlorine is toxic and respiratory irritant
-Chlorine reacts with organic matter in water forming chlorinated hydrocarbons which are carcinogenic.
What is the test for halide ions?
Add a small volume of dilute nitric acid to the aqueous halide ion to remove any carbonate ions
Then, add a small volume of aqueous silver nitrate solution and observe the colour of the silver halide precipitate
Finally, a small volume of dilute ammonia can be added to the precipitate to distinguish between the halides.
What is the colour of the silver chloride precipitate?
White precipitate
What is the colour of the silver bromide precipitate?
Cream precipitate
What is the colour of the silver iodide precipitate?
Yellow precipitate
What is the solubility of aqueous (dilute) ammonia in silver chloride?
Soluble in dilute NH3
What is the solubility of aqueous (dilute) ammonia in silver bromide?
Partially soluble in dilute NH3, fully soluble in concentrated NH3
What is the solubility of aqueous (dilute) ammonia in silver iodide?
Insoluble in both dilute and concentrated NH3
What is the test for carbonates?
Add dilute nitric acid
If a carbonate is present, bubbles of CO2 should appear
To confirm the presence of CO2, bubble it through limewater - Ca(OH)2- it will turn cloudy if carbon dioxide is present.
What is the test for sulfates?
Firstly, add a small volume of dilute nitric acid to the solution to remove any carbonate ions that could give false positives of the sulfate ion by reacting with the Ba^2+ ion.
Add a small volume of Ba^2+ ions either Barium chloride or barium nitrate
A white precipitate of Barium sulfate should form indicates the presence of the SO4^2-
This is because Barium Sulfate is very insoluble.
Write the ionic equation for the sulfate test
Ba^2+ (aq) + SO4^2- (aq) —> BaSO4 (s)
Write the ionic equation for the carbonate test
CO3^2- + 2H+ —> CO2 + H2O
What is qualitative analysis?
Relies on simple observations rather than measurements.
Give the ionic equation for the reaction between a Bromide ion and aqueous silver nitrate
Ag+ (aq) + Br- (aq) —-> AgBr (s)
What is the test for ammonium?
Add a small volume of dilute Sodium hydroxide NaOH (aq) and warm gently
Test the gas produced with damp red litmus paper
In the presence of ammonia gas it will turn blue
What is the ionic equation for the ammonium test?
NH4^+ (aq) + OH- (aq) —-> NH3 (g) + H2O (l)
What is the correct order of the qualitative tests?
Carbonate
Sulfate
Halide
(Remember Ca-S-H as a acronym)
Explain why, if you are testing a mixture, it is important to use dilute nitric acid rather than sulfuric or hydrochloric acid, for the carbonate test?
If sulfuric acid is used, sulfate ions can show up in the sulfate test with Ba^2+
If hydrochloric acid is used, chloride ions are added and show up in the halide test with Ag^+
What are the conditions needed when Chlorine reacts with Sodium hydroxide?
Cold and dilute NaOH
Cyclohexane is a non-polar solvent. Halogens are soluble in non-polar solvents, since the halogens are non-polar molecules.
Explain why the halogens are non-polar?
Halogens are diatomic molecules with the same electronegativity so no permanent dipole is present.
Give the name of one reagent that could be added to distinguish between separate samples of the following pairs of chemicals and write the expected observations.
AgBr and AgI
Concentrated ammonia
Concentrated ammonia will dissolve in AgBr, but is insoluble in AgI
Give the name of one aqueous reagent that could be added to distinguish between separate samples of the following pairs of chemicals and write the expected observations.
NaNO3 and Na2SO4
Reagent BaCl2 or Ba(NO3)2
NaNO3 - No visible reaction
Na2SO4 - white precipitate of BaSO4
Explain why Bromine has a lower boiling point than iodine mono chloride, ICl, even though they have the same molecular mass
Because there is a permanent dipole-dipole interaction as well as London forces between the molecules of ICl as Chlorine is more electronegative than Iodine.
While Br2 only contains London forces, therefore more energy is required to overcome the London forces and permanent dipole interactions between ICl than just the London forces of Br2.
