C5 revision

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)

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

for example....

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

concentration=no.moles÷volume

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)

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?

• temperature
• pressure (only if gases involved)
• concentration

adding a catalyst DOESN'T change the position of equilibrium , it just increases the rate of reaction!

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

Atmospheric pressure

Vanadium Pentoxide (V₂O₅) catalyst

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