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Elements of the sea (OCR B Chemistry)
Terms in this set (68)
What is dynamic equilibrium?
When the forward and backward reactions are occurring at the same rate at equilibrium in a closed system
What is a closed system?
A system where no new reactants can enter
What do you need to know to calculate the equilibrium constant (Kc)?
The molar concentrations of each substance at equilibrium
How does the Kc value relate to the position of the equilibrium?
The larger the KC = further right
Smaller Kc = further left
What happens when you change the concentration of products or reactants at equilibrium?
The position of the the equilibria shifts to keep the Kc constant
How does increasing and decreasing pressure affect equilibria?
Increasing: shift to side with least moles reducing pressure
Decreasing: Shifts to side with more moles increasing pressure
How does increasing and decreasing temperature affect equilibria?
Increasing: shifts to endothermic side to absorb heat
Decreasing: shifts to exothermic side to replace lost heat
What is Le Chatelier's principle?
When the conditions of dynamic equilibrium are changed, the position of equilibria is changed to minimise the affects
How can halide ions be tested for?
-Add dilute nitric acid to remove impurities
-Add silver nitrate
What colour does each halide precipitate form when silver nitrate is added?
Fluoride- none (AgF if soluble)
What can be added after the test for halides in case the result is still unclear?
- Add ammonia solutions
Chloride- precipitate dissolves
Bromide- dissolves only in concentrated solution
Iodide- doesn't dissolve at all
Why is storing and transporting chlorine dangerous?
-Its toxic and corrosive (damages skin)
-Irritates the respiratory system when breathed in
-Has to kept away from flammable materials because its an oxidising agent
What is done to make transporting chlorine more safe?
It is transported as liquid under pressure in small cylinders
Why is chlorine useful?
Its anti-bacterial so it can be used to sterilise water, so its handy for drinking water and also making bleach
What is atom economy?
A measure of how efficient a reaction is (how much reactant is not used in the reaction)
How do you calculate atom economy %?
Molecular mass of desired product /
Sum of all reactants Mr
Why is a reaction with higher atom economy better?
It is cheaper and more environmentally friendly
Why do percentage yields and atom economies both need to be considered in manufacturing?
Because a reaction with high atom economy and low yield is not as efficient as a reaction where they are both moderate
Why is a reactions conditions important in industry?
Because high pressures and temperatures are expensive to maintain
How can hydrogen halides be made?
By adding a concentrated acid like phosphoric acid to a solid ionic halide
Why cant sulfuric acid be sued to make all hydrogen halides?
Because it is a strong oxidizing agent and so it becomes involved in redox reactions
What happens to hydrogen fluoride and chloride when heated?
They become stable and so they dont split up into hydrogen and halide ions
What happens to the strength of hydrogen halide bonds down group 7 and why?
They become weaker:
-Halogen atoms increase in size
-Bonding electrons are further from the nucleus
-bonding electrons shielded by inner electron shells
What happens when hydrogen halides dissolve in water?
They all form strong acids except fluoride as they dissociate into the hydrogen and halide ion
How do hydrogen halides react with ammonia?
They form ammonium halides as the ammonia can accept the hydrogen
How does hydrogen fluoride and chloride react with sulfuric acid?
It forms the halogen gas and water
What is electrolysis?
The breaking down of a substance using electricity
What state does an ionic substance have to be in to preform electrolysis?
Liquid so either molten or aqueous (this is known as the electrolyte)
What moves to the each electrode during electrolysis and what are they usually made of?
Anode: Negative non-metal anions
Cathode: Positive metal cations
Made of inert substance like carbon or platinum
How do you know what is formed at the cathode during electrolysis of an aqueous solution?
If the metal is less reactive than hydrogen like silver and copper then the metal is formed
How do you know what is formed at the anode during electrolysis of an aqueous solution?
-If the solution doesn't contain a halide, oxygen will form
-If the solution is concentrated and contains a halide, the halogen will be formed
How can electroysis be used for purification?
If the anode is made of impure copper and the cathode is made of pure copper
How can most halogens be produced?
By extracting them from concentrated halide solutions causing the halogen to be produced at the anode
How is chlorine extracted?
It is extracted from brine which is a solution with a high concentration of salt (mainly sodium chloride
What happens at the cathode during the electrolysis of brine?
2 hydrogen ions accept 2 electrons to become one hydrogen molecule
What happens at the anode during the electrolysis of brine?
2 chloride ions lose their electrons and become a chlorine molecule
What is left in the solution after the electrolysis of brine?
-Sodium ions left behind (more reactive than hydrogen)
-hydroxide ions left from water
So sodium hydroxide is left
Why does the sodium chloride in brine need to be concentrated for electrolysis to work properly?
-Because in dilute solutions the chloride ions aren't discharged
-So they hang on to their electrons
-This means OH- ions lose electrons and oxygen and water form at the anode not chlorine
How can bromine and iodine be extracted from brine?
By bubbling chlorine through the brine which displaces the the halide, so the less reactive halogen can be collected
What are oxidation states?
A number which tells you how many electrons an atom has lost or gained
Why are oxidation states useful?
-Make it easier to see what is being oxidized and reduced in redox reactions
-Helps to name compounds or find out a compounds formula
How does an oxidation state change for oxidation and reduction?
-In oxidation it is increased
-In reduction it is decreased
What are the 4 rules you have to remember about oxidation states?
1) If its not combined with another element its oxidation state is 0
2) The sum of oxidation states in a neutral compounds will equal 0
3) The sum of oxidation states in an ions is equal to its charge
4) The more electronegative element in a substance is given a negative oxidation state
What is oxygen's common oxidation state and also its exceptions?
Oxygen = -2 (Except in:)
Fluorides= OF2 (+2) O2F2 (+1)
What is hydrogen's common oxidation state and also its exceptions?
Hydrogen= +1 (Except in:)
Why do ions ending in (ate) need an oxidation state?
Because they contain oxygen
What is the formula of a nitrate or nitrate(V) ion?
What is the formula of a sulfate or sulfate(VI) ion?
What is the formula of a carbonate ion?
What is the formula of a manganate (VII) ion?
What is the formula of an ammonium ion?
What is the formula of a hydrogen carbonate ion?
What is the formula of a sulfide ion?
What ions have a charge of -1?
What ions have a charge of -2?
What is oxidation and reduction?
-Oxidation is is loss of electrons (oxidation state increases by 1)
-Reduction is gain of electrons (oxidation state decreases by 1)
What is a redox reaction?
This is when an oxidation and reduction happens simultaneously
What is usually oxidized and reduced in a chemical reaction?
Metals = oxidized + lose electrons (reducing agents)
Non-metals = Reduced + gain electrons (oxidizing agents)
What do you need to do to balance a redox reaction?
-Balance the atoms
-Balance the charge
How do you balance the charges in a redox reaction?
Find the change in oxidation state of the metal and non-metal and balance out the change
What can iodine-sodium thiosulfate titrations be used for?
They can be used to find the concentration of a non-metal oxidizing agent?
How do you carry out an iodine-sodium thiosulfate solution with potassium iodate(v)
1) Measure out 250cm3 of potassium iodate
2) Add this to excess acidic potassium iodide and iodate ions will oxidise some of the iodide ions
3) Titrate this solution with sodium thiosulfate in a flask until it turns a yellow, then add 2cm3 of starch solution (the solution should turn dark blue/black)
4) Then continue the titre until it turns colourless
5) Then use the volume and concentration to find the moles of the halogen in the solution
What are 2 things you can do to make sure a titre is accurate?
1) Wash apparatus out with sodium thiosulfate to remove traces of water which could dilute the solution
2) Wash out or get a new flask when doing repeat experiments
What are 2 problems with sodium-thiosulfate solutions?
1) The solution reacts very slowly with air so needs to be made up freshly
2) Adding starch solution too soon will cause the iodine to stick to it and will not react properly
What are halogens?
They are highly reactive non-metals in group 7
What happens to the volatility of the halogens down the group and why?
-Become less volatile down the group so they become harder to vaporize
-This is because the strength of the instantaneous-induced bonds becomes stronger as the atoms get bigger and heavier
What is the appearance of Fluorine, chlorine, bromine, and iodine at RTP?
Fl- pale yellow gas
Cl- yellow-green gas
Br- red-brown liquid
I- Shiny grey solid
What is the solubility of halogens like?
- More soluble in organic compounds such as hexane
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