Inorganic chem Oxidation Unit 2 Pt3
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What is a redox reaction?
- A reaction in which electrons are transferred.
- Reduction and oxidation happen simultaneously.
What is an oxidising agent and a reducing agent?
- Oxidising agent - accepts electrons and get reduced.
- Reducing agent - donates electrons and get oxidised.
- An increase in "positiveness" due to loss of electrons.
- Addition of oxygen or removal of hydrogen.
- Decrease in "positiveness" due to gain of electrons.
- Removal of oxygen or addition of hydrogen.
One element can be both oxidised and reduced in the same reaction. What is this reaction called?
A disproportionation reaction.
What is the usual oxidation number of oxygen? What are exceptions?
- Exception - in peroxides (H2O2) = -1
- and when it combines with fluorine = positive
What is the usual oxidation number of hydrogen? Exceptions?
- Exception - in metal hydrides = -1
What is the usual oxidation number of chlorine? Exceptions?
- Exception - with oxygen & fluorine = positive
What is usual oxidation number of bromine (Br)? Exceptions?
- Exception - with O, F, Cl = positive
What is the usual oxidation number of iodine? Exceptions?
- Exception - when combined with O, F, Cl, Br = positive
Oxidation number of fluorine? Exceptions?
(First) ionisation energy becomes what as you go down Group 2? Why?
- Ionisation energy decreases down the group (so easier to ionise)
- More electron shielding from inner shells/ energy levels.
- More distance from the positive nucleus. (Positive charge of nucleus does increase, but nuclear charge effect overridden)
Describe the reaction of Group 2 elements with water.
- Gives metal hydroxide and hydrogen (gas) [for calcium, strontium & barium]
- Increase in reactivity down the group. (Be doesn't react, Mg very slowly, Ba rapidly - with COLD water)
- [Mg reacts rapidly with steam to form magnisium oxide (MgO) and hydrogen.]
Describe the reaction of Group 2 elements with oxygen.
- Form solid white oxides.
- Magnesium element burn with brilliant white flame, and other burn with their characteristic flame colours.
What is formed when a Group 2 element reacts with chlorine?
Solid white chloride formed.
What does the reaction of Group 2 oxides with water form?
- Metal hydroxides, which dissolve to produce alkali solution.
- Oxides form more strongly alkaline solutions down the group, because the hydroxides get more soluble.
- (BeO doesn't react though)
What do Group 2 metal oxides form when reacting with dilute acid?
They neutralise it, and therefore form a salt (eg. MgCl2, or Ca(NO3)2 ) and water.
What is formed when Group 2 hydroxides react with dilute acid?
Salt and water produced.
Describe the solubility of Group 2 hydroxides and Group 2 sulphates. What is the general pattern?
- Solubility increases down group for hydroxide.
- Solubility decreases down group for sulfate.
- Generally, Group 2 compounds that contain singly charged negative ions (eg OH-) increase in solubility down the group, and those containing doubly charged negative ions (eg SO42-, CO32-) decrease in solubility down group.
- (Barium sulfate is insoluble)
Roughly, what is thermal decomposition and thermal stability?
- Thermal decomposition is when a substance breaks down (decomposes) when heated.
- The more thermally stable a substance is, the more heat it will take to break it down.
Thermal stability of Group 1&2 carbonates and nitrates ______ down the group. Why?
- Carbonate and nitrate anions are large, and can be made unstable by presence of cation. Cation polarises anion, distorting it.
- The greater the distortion, the less stable the anion.
- Smaller cations (have higher charge density) and distorts anions more.
- THUS, as ionic radius increases down the group, the more distorted anions are going to be at the top, and therefore more thermally stable down the group.
Group2 carbonates and nitrates are ____ stable than those of Group 1. Why?
- Group2 c&n are less stable than those of G1.
- Because G2 cations have 2+ charge, whereas G1 have 1+, therefore G2 cations distort the anion more, and thus less thermally stable.
Group 2 carbonates decompose to form what?
- the Oxide and Carbon dioxide
- (MgCO3 ------ MgO + CO2)
Group 2 nitrates decompose to form what?
- The oxide, nitrogen dioxide and oxygen.
- eg. Ca(NO3)2 (s) ---- 2 CaO (s) + 4NO2(g) + O2(g)
Group 1 nitrates decompose to form what?
- The nitrite and oxygen.eg. 2KNO3(s) ----- 2KNO2 (s) + O2 (g)
- (Except Lithium nitrate which decomposes to form Li2O, NO2 and O2)
Give 2 ways of testing thermal stability of nitrates.
- How long it takes until oxygen is produced (ie. to relight a glowing splint) OR
- How long it takes until a brown gas (NO2) is produced. [This needs to be done in fume cupboard as nitrogen dioxide is toxic]
Give a way to test the thermal stability of carbonates.
Measure how long it takes for carbon dioxide to be produced. (Bubble through limewater which turns cloudy with carbon dioxide).
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