Module 3: Chapter 8 (Reactivity Trends) Flashcards
What is another name for Group 2?
Alkaline Earth Metals
Describe the physical properties of Group 2
- reactive metals
- never found in their native state
- compounds are common throughout nature, e.g. calcium carbonate
Discuss Group 2 in terms of redox reactions
- Called a reducing agent as it reduces another species, gains the two outer electrons from the group 2 element, forms a 2+ group 2 ion
Group 2: Describe redox reactions with oxygen
- forms a metal oxide, MO
- For example, 2Mg + O2 -> 2MgO (solid)
- Magnesium is oxidised and Oxygen is reduced
Group 2: Describe redox reactions with water
- forms an alkaline hydroxide, M(OH)2, and hydrogen gas
- reaction becomes more vigorous down group (more reactive)
- for example: Sr + 2H2O -> Sr(OH)2
Group 2: Describe reactions with dilute acids
metal + acid -> salt + hydrogen (MASH)
- reactivity increases down the group
- e.g. Mg + 2HCl -> MgCl2 + H2
Why does reactivity increase down group 2?
- Ionisation energies decrease down the group because of increasing atomic radius and increasing shielding meaning less nuclear attraction for outer electron and lower ionisation energy
- Elements in group 2 react by losing 2 electrons (forming +2 ions), that is two ionisation energies in terms of energy input
Group 2 oxides reacting with water
- react with water to release OH- ions forming an alkaline solution of a metal hydroxide
- CaO (solid) + H2O -> Ca2+ + 2OH-
- the group 2 hydroxides are sparingly soluble in water, if the solution is saturated, a solid ppt will form:
Ca2+ + 2OH- -> Ca(OH)2
What is the trend of solubility of group 2 hydroxides?
- increases down the group
- more soluble = more OH- ions in solution = more alkaline
- for example Magnesium hydroxide forms a solution of a low concentration of pH 10 and barium forms a more concentrated solution, pH 13
What are the uses of Group 2 compounds?
Agriculture:
Ca(OH)2 added to fields to increase the pH of acidic soils: Ca(OH)2 + 2H+ -> Ca2+ + 2H2O
Medicine:
- Acid in stomach is mainly HCl, can be neutralised
- often used as antacids (treating acid indigestion)
- Mainly Magnesium and Calcium carbonates used
What is another name for group 7 elements?
Halogens
Describe general properties about halogens
- most reactive non-metal
- only occur as halide ions in nature (never in elemental form)
- in elemental form, exist as diatomic molecules, X2
- structure is simple molecular
Describe the elemental states of the halogens at RTP
Fluorine = pale yellow gas Chlorine = pale green gas Bromine = an orange-red volatile liquid Iodine = grey-black crystalline solid
What is the trend in boiling points in the halogens?
- increases down the group
- more electrons
- stronger London forces
- more energy required to break intermolecular forces
Describe the redox reactions of halogens
- outer shell configuration: s2p5
- gaining one electron achieves noble gas electronic configurations, so redox reactions are the most common reactions for halogens
- e.g Cl2 +2e- -> 2Cl- (half equation), oxidising agents as the chlorine is reduced
Describe halide-halide displacement reactions
- reactivity decreases down the group
- if a more reactive halogen is added to less reactive halide a displacement reaction will take place (halogen atom displaces ion in solution)
Describe the colour of aqueous halogens
Solution in water:
Cl = pale green
Br = orange
I = brown
top layer colour with cyclohexane:
Cl = pale green
Br = orange
I = violet
Explain why as you go down Group 7 the element become weaker oxidising agents?
- increasing atomic radius
- increased shielding from more inner shells means less nuclear attraction to capture an electron from another species
- reactivity decreases down the group/weaker oxidising agent
What is a disproportionation reaction?
A redox reaction in which an element is oxidised and reduced
What is the redox reaction for chlorine and water?
Cl2 (g) -> H2O (l) -> HClO (aq) + HCl
HClO = chloric (1) acid, kills bacteria and acts a weak bleach
- Chlorine 0 to +1 and -1 (disproportionation)
- Chlorine is used to treat drinking water by killing harmful bacteria, the equation above is the reaction of Cl being added to water
What is the reaction between chlorine and dilute sodium hydroxide?
- dilute NaOH will allow more chlorine to dissolve (solubility is lower in water)
Cl2 (g) + 2NaOH (aq) -> NaClO (aq) + NaCl (aq) + H20 (l)
NaClO (sodium chlorate) is a much stronger bleach, large concentration of ClO- ions, chlorate ions
- disproportionation of Cl2, 0 to +1 (in NaClO) and -1
What are the benefits and risks of chlorine?
- a toxic gas
- will react with methane(/organic hydrocarbons) in water, methane is formed by decaying vegetation, -> chlorinated hydrocarbons are suspected carcinogens
- without chlorine in our water we would be at risk of disease such as typhoid and cholera
What are the properties of Group 1 metal halides?
- white crystalline solids
- very soluble in water forming colourless solutions
- insoluble in cyclohexane
Describe the carbonate test
- acids react with carbonates to form a salt, water and carbon dioxide gas (effervescence)
- CO2 produced can be detected by bubbling the gas through lime water, a white precipitate produced (turned limewater cloudy white), calcium carbonate
Describe the sulphate test
- barium sulphate is insoluble in water, a white precipitate will form (add Ba2+ ions to a solution so BaSO4 will form, forming a white ppt)
- Application in medicine as a barium meal, drink containing barium sulphate will coat the lining of part of the body (e.g. gut) to detect problems or anomalies
Describe the test for ammonium ions
- Add aqueous sodium hydroxide to a solution of ammonium ions and warm.
- Ammonia gas is produced.
- Ammonia is alkaline so hold a damp
piece of pH indicator paper near the
mouth of the test tube and observe what happens.
Describe the halide test
- Silver ions (in the form of silver nitrate AgNO3) are added to an aqueous halide ions to detect the halide ion present.
- Silver chloride = white ppt, bromide = cream ppt and iodide = yellow
- ppts formed have similar colours (use aqueous ammonia to distinguish halides), addition of dilute ammonia dissolve white chloride ppt, addition of concentrated ammonia dissolves the bromide cream ppt
What order should you carry out the tests in?
- carbonate test only using nitric acid
- sulfate test, using barium nitrate
- halide test using silver nitrate and aqueous ammonia
Describe the results of the halogen displacement reactions
Chlorine displacing bromine:
orange colour from Br2 formation
Chlorine displacing iodine:
violet colour from I2 formation
Bromine displacing iodine:
violet colour from I2 formation
