Chemistry C5
Ocr, Gateway Chemistry C5
- Created by: Ellie Ashton
- Created on: 04-06-11 08:51
The Mole
The mole= a name give to a certain number
One mole of atoms or molecules of any substance will have a mass in grams equal to there relative atomic mass (on the periodic table) or if its a molecule relative formula mass
Molar mass is another way of saying 'the mass of one mole' it is also measured in grams
Number of moles= mass in g / relative formula mass
The relative atomic mass of an element is the average mass on an element compared to the mass of an atom of carbon-12
Reacting Masses
Mass is conserved during a chemical reaction. this means that the mass of reactants will always equal the mass of the products
calculating mass of reactants:
- write out the balanced equation
- work out the relative formula mass of the parts you want
- apply the rule: divide to get one, then multiply to get all
Empirical formula
Empirical formula= the smallest whole number ration of an atom in a compound
to work it out:
1. list the elements in the compound
2. underneath them write the mass (given in question)
3. divide each mass by the atomic mass for that element
4. turn the numbers you get into a simple ratio
5. get the ratio in its simplest form
remember to write the empirical formula at the end
Electrolysis
Electrolysis is the breaking down of a substance using electricity
an electric current is passed through a molten or dissolved ionic compound causing it to decompose
for the electrical circuit to be complete there must be a flow of electrons
- Electrons are taken away from ions at the positive anode
- passed by an external circuit they then go to the negative cathode
- they're given to other ions in the electrolyte
as ions gain or lose electrons they become atoms or molecules and are discharged from the solution
Electrolysis in school lab
- 2 graphite or platinum electrodes connected to DC power supply and placed in the electrolyte
- positive ions go to the cathode
- negative ions go to the anode
- if molecules discharged are gases they are collected in test tubes
Discharging ions from water
In aqueous solutions as well as the ions from the solute there are hydrogen ions (H+) and hydroxide ions (OH-) from the water
- If the metal is very reactive its ions will stay in solution and H+ will be discharged at the cathode
- if the non-metal is a halide ion (group 7) it will form the halogen if not oxygen will be discharged
e.g. A solution of Aqueous potassium sulfate (K2SO4)
contains 4 ions: K+ , SO4 2- , H+ and OH-
- At cathode hydrogen is discharged: 2H+ + 2e- ----> H2
- hydroxide ions lose electrons easier than sulfate ions, so oxygen is discharged at the anode: 4OH- - 4e- ----> O2 + 2H2O
Electrolysis of molten salts
- A salt will conduct an electric current when molten
- the salt is always broken up into elements
- positive metal cations are reduced (gain electrons) to the cathode
- e.g. Pb 2+ + 2e- ---> Pb
- Negative anions are oxidised (lose electrons) to atoms at the anode
- e.g. 2Br- ---> Br2 + 2e-
Electrodes
- when a certain number of electrons flow away from the positive electrode the same number of electrons must flow onto the negative electrode
- using electrolysis of copper sulfate solution:
- negative electrode half equation: 1 mole of copper ions pick up 2 moles of electrons- makes 1 mole of copper atoms which sticks to the electrode causing mass to increase
- positive electrode half equation- 1 mole of copper atoms loses 2 moles of electrons and is released as copper ions into the solution, copper is lost from the electrode- decrease in mass
- 1 mole of copper is added to the negative electrode as 1 mole of copper is lost from the positive electrode- gain in mass is equal to the loss in mass
Electrolysis- calculating masses
The amount of product made depends on the number of electrons transferred
if you increase the number of electrons you increase the amount of substance produced
- can do this by increasing time or current
coulombs and faradays are amount of electricity
coulombs= amps time seconds (Q=It)
96000 coulombs= 1 faraday
one faraday (f) contains one mole of electrons
Electrolysis- calculating masses
1. write out the balanced half equation
2. calculate the number of faradays
- first do coulombs=amps *seconds
- number of faradays= coulombs/ 96000
3. calculate the number of moles of products
- divide the number of faradays by the number of electrons in the half equation
4. write in the relative atomic mass value to work out mass of solid products
- mass= relative atomic mass * number of moles
Concentration
Concentration is a measure of how crowded things are
- can be measure in moles per dm3
- the mole solute you dissolve in a give volume the more crowded the solute particles are and the more concentrated the solution
- 1 litre= 1000cm3 = 1dm3
concentration= number of moles / volume
remember to convert
- grams per dm3 to moles per dm3
- cm3 to dm3 divide by 1000
Concentration
It is important to get the concentration right:
- food preparation
- ingredients have to be at the right consistency so they have the right taste
- medicines
- if diluted too much may not work
- not enough may have side effects
- baby milk
- too dilute baby won't get enough nutrient
- not dilute enough could become overweight
- weedkillers
- need diluting so don't kill the plants
Recommended Daily Allowances
RDAs= amounts of nutrients (vitamins, food types etc) that an average adult should eat for a healthy diet
The mass of salt in a product is approximately 2.5 * mass of sodium it contains
Titrations
- titration's allow you to find out exactly how much acid is needed to neutralise a quantity of alkali
- Using a pipette and pipette filler add some alkali (about 25cm3) to a conical flask along with 2 or 3 drops of indicator
- fill a burette with the acid- make sure this is done below eye level
- using the burette add the acid to the alkali a bit at a time giving the conical flask a regular swirl, go slowly when you think you are reaching the colour change
- the indicator changes colour when all the alkali has been neutralised
- record the amount of acid used to neutralise
Consistent readings and the indicator
To increase the accuracy of the titration and to spot any anomalous results you need several consistent readings
- the first titration is a rough titration to get the approximate idea of where the solution changes colour
- you then need to repeat the whole thing a few times making sure you get around the same answer
Indicator used for titration's:
- universal indicator shows a range of colours but during acid-base titration's you want a sudden colour change
- want to use an indicator that does this e.g. acid--> alkali
- litmus: red---> blue
- methyl orange: red ---> yellow
- phenolphthalein: clear---> pink
pH curves
a neutralisation reaction= when an acid and an alkali react to form a salt and water
(draw in pH curves and label)
calculating concentration for titration's
moles= volume*concentration/ 1000
1. calculate moles of substance 1
2. use balanced equation- ratio of moles
3. calculate concentration of substance 2
- concentration= number of moles*1000 / volume
Gas Volumes
Collection method:
- gas syringe: give volume to nearest cm3 but can't use if reaction is too vigorous
- upturned measuring cylinder or burette-
- can use a delivery tube to bubble the gas into an upside-down measuring cylinder or gas jar filled with water
- this method is no good for collecting things like hydrogen, ammonia or chloride as they dissolve in water
- can also use an upturned burette which is slightly more accurate
can also measure the mass of gas produced:
- carry out the experiment on a mass balance
- as gas is released mass disappearing is easily measured on the balance
- most accurate because mass balance is very accurate
One mole of any gas always occupies 24dm3 at room temperature and pressure
volume= moles * 24
Reactions
Reactions stop when one reactant is used up
the reactant that is used up first in the reaction is called the limiting reactant
if you halve the amount of limiting reactant the volume of gas produced will also halve (the amount of gas produced is proportional to the amount of limiting reactant)
If you double the amount of limiting reactant it might not be the limiting reactant anymore
Faster rates or reactions are shown by steeper curves
Equilibrium
A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants: A+B <---> C + D
Reversible reactants will reach equilibrium:
- as the reactants (A+B) react their concentrations will fall- so the forward reaction will slow down- but as more products (C+D) are made their concentrations rise so the backward reaction would speed up
- after a while the forward reaction will be going at the same rate as the backward one- equilibrium
- at equilibrium both the reactions are still happening but there is no overall effect- this means that the concentrations of reactants have reached a balance and won't change
- equilibrium is only reached if the reversible reaction take place in a 'closed system' a closed system just means that none of the reactants or products can escape
Position of equilbrium
- sometimes equilibrium will lie to the right- means lots of products and not much of the reactants
- sometimes equilibrium will lie to the left- means lots of reactants but not much products
- the position of equilibrium depends on the conditions
Adding a catalyst does not change the position of equilibrium:
- they speed up both the forward and the backward reaction
- adding a catalyst means that the reaction reaches equilibrium quicker but will have the same amount of product at the end
Things that affect position of equilibrium
Temperature-all reactions are either endothermic or exothermic:
- if decrease the temperature, equilibrium will move to try and increase it, equilibrium moves in the exothermic direction- produce more heat
- if you raise the temperature, equilibrium will try and decrease it- moves in endothermic direction
Pressure- only affects gases
- increase pressure, equilibrium tries to reduce it- moves in direction where fewer moles of gas
- decrease pressure, equilibrium tries to increase it- moves in direction where more moles of gas
concentration:
- increase concentration of forward reactants, equilibrium tries to decrease it by shifting to right
- if you increase the concentration of products equilibrium tries to reduce it by shifting to left
Equilibrium tables and graphs
The haber process is a good example: N2 + 3H2 <----> 2NH3
- table: as pressure increase the proportion of ammonia increases
- graph: as temperature increase the proportion of ammonia decreases, the conditions that will give you most ammonia is high pressure and low temperature
The contact process
used to make sulfuric acid:
1. make sulfur dioxide-usually burning sulfur in air
- sulfur+oxygen ----> sulfur dioxide
- S + O2 ------> SO2
2. sulfur dioxide is then oxidised with help of catalyst to make sulfur trioxide
- sulfur dioxide + oxygen <----> sulfur trioxide
- 2SO2 + O2 <----> 2SO3
4. sulfur trioxide is used to make sulfuric acid
- sulfur trioxide + water <----> Sulfuric Acid
- SO3 + H2O -----> H2SO4
Conditions used to make SO3
Temperature:
- oxidising sulfur trioxide is exothermic (gives out heat) so to get more product you'd think the temperature should be reduced
- but reducing temperature slows reaction down so 450'c used as an optimum temperature as a compromise
Pressure
- there are 2 moles of product compared to 3 moles of reactant, so to get more product you would think pressure should be increased but increasing pressure is expensive so atmospheric pressure is used
Catalyst
- to increase rate of reaction a vanadium pentoxide catalyst is used, it doesn't change position of equilibrium
Acids
Acids release protons in water, and when they ionise they release H+ ions
Strong acids (e.g. sulfuric,hydrochloric) ionise almost completely in water
weak acids (e.g ethanoic, citric and carbonic) ionise only very slightly
- ionisation of weak acids is reversible because only a few H+ are released the equilibrium lies well to the left
The pH of an acid or alkali is a measure of concentration of H+ ions in solution- measured with universal indicator
Strong and concentrated acids
Acid strength= what proportions of the acid molecules ionise in water
concentration= how many moles of acid molecules there are in a litre of water, basically how watered down the acid is
- describes the total number of acid molecules
- more moles of acid per dm3 the more concentrated the acid is
Strong acids are better electrical conductors than weak acids
- hydrochloric acid has a much higher electrical conductivity than the same concentration of ethanoic acid- to do with the concentration
- greater concentration of ions in the strong acid means more charge can be carried
Reaction rate of strong and weak acids
Strong and Weak acids react with reactive metals and with carbonates in the same way
- difference between the reactions is the rate, ethanoic acid will react more slowly than the same concentration of hydrochloric acid
- all to do with the equilibrium of ethanoic acid
- when you put a weak acid into water it releases few H+ ions which means the concentration of H+ ions decreases so the equilibrium shifts to compensate meaning more H+ ions are released- these ions react and the equilibrium shifts as more ions are removed more are supplied
- completely different to when have a strong acid.
Amount of gas produced
Amount of gas produced depends on the amount of acid
- hydrochloric acid will react faster than ethanoic acid but the amount of product made will be the same
Weak acids
are actually useful- can be more useful that dilute strong acids
- strong acid reacts very fast- might not be useful if you want a more controllable reaction like with kettle descalers
- strong acid would react quickly with the scale and may also react with the metal kettle.
- a weak acid still removes the scale but the lower concentration of H+ ions means the reaction will be slower and easier to control
Precipitation reactions
Precipitation reactions make an insoluble substance- normally involves 2 solutions reacting together
the insoluble substance= the precipitate
most precipitate reactions involve ions, to react with each other these ions need to collide so they are able to move
this means the ionic substances have to be molten or in solution
Ionic equations of precipitate reactions
- you can tell when its a precipitation reaction because start off with 2 solutions (aq) and end up with a solid (s)
- The spectator ions= ions which were dissolved in the solution before the reaction and are still dissolved afterwards
- an ionic equation concentrates only on the useful bits of the reaction
- e.g. Ba 2+ (aq) + SO4 2- ----> BaSO4 (s)
Test for sulfates and Halides
can use precipitation reactions to identify substances
test for sulfate ions:
- add dilute HCl followed by barium nitrate
- a white precipitate of barium sulfate means the original compound was a sulfate
test for chloride, bromide or iodide
- add dilute nitric acid followed by lead nitrate
- chloride gives white precipitate of lead chloride
- bromide gives a cream precipitate of lead bromide
- iodide gives a yellow precipitate of lead iodide
Preparing insoluble salts
you must pick the right reactants
- need some ions, need to be in solution so can move about, then:
1. check each reactant and product for hazardous information
- add 1 spatula of lead nitrate to a test tube and fill it will distilled water, shake to make sure all lead nitrate is dissolved, then fo the same with 1 spatula of sodium iodide, tip the 2 solutions into a small beaker, stir and salt should precipitate out
2. put a folded piece of filter paper into filter funnel and stick in conical flask, pour contents into middle of filter paper, swill out beaker with distilled water and tip through filter paper- to get all product
3. rinse the contents of filter paper with distilled water to make sure all soluble salts washed away and scrape lead iodide onto fresh filter paper and leave it to dry
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