What is the mole?
The mole is just a name given to a certain number
"a mole"= 6.023x10^23 or 602 300 000 000 000 000 000 000
number of moles= Mass in g (of the element/compound) ÷ Mr (of element/compund)
Reacting masses and Empirical formula
things always react in the same ratios... mass is conserved during a chemical reaction, so this just means the mass of reactants will always equal the mass of products
"Empirical formulas are just the simplest ratio in a compound"
empirical formulas are calculated from masses or percentages
this is how you work iut the empirical formula
1. list all the elements in the compund
2. thenwrite their experemental masses or percentages
3. divide by the Ar for that particular element
4. turn into a simple ratio by multiplying or dividing them by a certain number
5. get the ratio in its simplest form, this will be the empirical formula
find the empirical formula of iron oxide when 44.8g of iron reacts with 19.2g of oxygen. (Ar for iron=56 Ar for oxygen=16)
1. Fe O
2. 44.8 19.2
3. 44.8÷56=0.8 19.2÷16=1.2
4. x10= 8 12
5. ÷4= 2 3
.....so the empirical formula is Fe2O3
Electrolysis is the breaking down of a substance using electricity
cathode= negative anode= positive
graphite or platinum electodes connected to DC power supply placed in electolyte
the cathode attracs positive ions and the anode attracts negative ions
in the aqueous solution there are hydrogen (H⁺) and hydroxide ions (OH⁻)
reaction at the Cathode: 2H⁺ + 2e⁻ --> H₂
reaction at the Anode: 4OH⁻ - 4e⁻ --> O₂ = 2H₂O
in molten salts theres only one source of ions, a slat will conduct an electric current when molten. Positive metal Cations are reduced ( they gain electrons) and Negative anions are oxidised (they lose electrons)
"Concentration is a measure of how crowded things are"
the more solute you dissolve in a given volume, the more crowded the solute molecules are and the more concentrated the solution.
1 Litre... = 1000 cm3 also 1dm3
converting grams per dm3 to moles per dm3
so for instance say you have a solution of sulphuric acid of 0.04mol/dm3 what is the concentration in grams per dm3?
so work out the relative formula mass for the solute- H₂SO₄
H=1, S=32, O=16
so H₂SO₄ = (1x2)+32+(16x4)=98
so in 1 dm3:
mass in grams= moles x relative formula mass
= 0.04x98= 3.92g/dm3
you can use sodium contene to estimate the amount of salt
example... if a slice of bread contained 0.2g of sodium-how much salt would it contain? ( Mr of sodium= 23, Mr of sodium chloride=58.5)
1. find ratio of sodium chlorides Mr to sodium's: 58.5÷23= 2.543...
2. x the amount of sodium: 2.543...x0.2= 0.5086...= 0.5g
Titrations are used to find out the concentration
titrations allow you to find out exactly how much of an acid is needed to neutralise a quantity of an alkali (vice versa)
you use single indicators for titrations NOT universal indicator.
Litmus- red in acid, blue in alkali
Methyl orange- red in acid, yellow in alkali
Phenolphthalein- white in acid, pink in alkali
pH curves show pH against volume of acid or alkali added
acid+alkali ---> salt+water (this is a neutralisation reaction)
Gas Volumes and Following Reactions
the collection method depends on the gas
1. Gas syringe
2. upturned measuring cylinder or burette
One mole of gas has a volume of 24dm3 (at room temperature (25 degrees C and 1 atmosphere) )
reactions stop when one reactant is used up!
when marble chips are (CaCO3) are dropped into a beaker of hydrochloric acid, you can obviously tell that there is a reaction taking place because there are lots of bubbles given off. After a while the fizzing slows down and the reaction will stop.
The reactant thats used up first is called the limiting reactant (the amount of gas is directly proportional to the amount of limiting reactant)
"A reversible reaction is a one where the products of the reaction can themselves react to produce the original products"
A + B D C + D
Reversible reactions will reach equilibrium, at equilibrium both reactions are still happening but theres no overall effect- this is called dynamic equilibrium.
Equilibrium will only happen if the reversible reaction occurs in a closed system (the reactants or products cant escape)
what three things can change the position of equilibrium?
- pressure (only if gases involved)
adding a catalyst DOESN'T change the position of equilibrium , it just increases the rate of reaction!
The Contact process
The contact process is used to make sulfuric acid
1. first stage is to make sulfur dioxide, usually by burning sulfur in the air
sulfur + oxygen ---> sulfur dioxide
S + O₂ ---> SO₂
2. then sulfur dioxide is oxidised to make sulfur trioxide (SO₃)
Sulfur dioxide + oxygen D sulfur trioxide
2SO₂ + O₂ D 2SO₃
3. then sulfur trioxide is used to make sulfuric acid
Sulfur trioxide + water ---> sulfuric acid
SO₃ + H₂0 ---> H₂SO₄
What conditions are needed?
well you have to chose the conditions very carefully, the reaction in step 2 is reversible, so the conditions can be controlled to get more product.
Temperature- optimum temp. 450⁰C
Vanadium Pentoxide (V₂O₅) catalyst
Strong and Weak Acids
Acids release protons in water
acids ionise- they release hydrogen ions H⁺
HCl --> H⁺ + Cl⁻
H₂SO₄ ---> 2H⁺ + SO₄ 2-
acids can be strong or weak
- strong acids (sulfuric, hydrochloric, nitric) dissociate completely (ionise) in water.
- weak acids (ethanoic, citric, carbonic) only partially dissociate
for example... strong acid: HCl ---> H⁺ + Cl⁻
weak acid: CH₃COOH D H⁺ + CH₃COO⁻ (reversible reaction)
The more moles of acid per dm3, the more concentrated it, so you can have a dilute but strong acid and a concentrated weak acid
strong acids=better electrical conductors than weak acids because due to the greater concentration of ions
Strong acids react faster than weak acids,
amount of gas produced in a reaction depends on the amount of acid, obviously.
Precipitation reactions make an insoluble substance
2 solutions react=insoluble substance
the insoluble substance= precipitate, turns substance cloudy
test for sulfates ( SO₄ 2-) and halides ( Cl-, Br-, I-)
to test for chloride, bromide and iodide ions you just add dilute nitric acid (HNO3) then some lead nitrate Pb(NO3)2.
Chloride= white precipitate of lead chloride - Pb2+ + 2Cl- ---> PbCl2
Bromide= cream precipitate of lead bromide- Pb2+ + 2Br- ---> PbBr2
Iodide= yellow precipitate of lead iodide- Pb2+ + 2I- ---> PbI2