Chemistry C2

AQA Additonal Science Chemistry for taking in Summer 2011

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  • Created by: Pippa
  • Created on: 19-05-11 15:57

Chemical Bonding.

Ionic Bonding is when one element gives another one or more electrons.
- This happens to make them have a full outer shell.

- The particles become charged ions.
- As they are oppositely charged, they attract in an IONIC LATTICE.
- They have high melting and boiling points.
- When liquid, they conduct electricity - molten or in solution.

Covalent Bonding is when two elements share electrons in order to have a full outer shell.
 ---------> Simple Molecular Substances
- Low melting and boiling points as the intermolecular bonds are very weak.
- Covalent intermolecular bonds are very strong.
- No ions so don't conduct electricity.
---------> Giant Covalent Substances
- Atoms are bonded by strong covalent bonds.
- Very high melting points.
- Don't conduct electricity, even when molten. Insoluble in water.

Metallic Bonding involves charged metal nuclei, and a sea of delocalised electrons.
- The electrons are free to move so conduct electricity and heat.
- They are malleable as they can slide over each other.
 

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New Materials and Relative Formula Mass.

Smart Materials behave differently depending on the conditions. They can change colour, expand or contract, be malleable, for example.

Nanomaterials are very small, so have strange properties. They can be used as catalysts as they have such a big surface area, and to build surfaces with specific properties. 

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Relative Formula Mass is the sum of all the atomic masses of the elements in the compound.

%age mass of an             =      atomic mass x no. of atoms of that element
element in a compound                     Relative Formula Mass

%age atom economy       =      total Relative Formula Mass of useful products
                                                   total Relative Formula Mass of reactants 

%age yield                       =                   actual yield (grams)        x 100
                                                           predicted yield (grams) 

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The Mole.

A mole  is simply a very large number

 One Mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass of that compound. 

Number of Moles       =     Mass in g (of element of compound)
        Relative Formula Mass 

Number of Moles       =     Volume (in Litres) x Concentration of the Solution (in M)

To find the empirical formula, find the number of moles of each element, and look for the simplest ratio. 

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Rates of Reaction.

Rate of Reaction depends on:

1. Temperature: At higher temperatures, particles move around faster so more collisions take place. They have more energy so more reactions have higher than the activation energy, so more successful collisions take place.

2. Concentration (or pressure): At higher concentrations, there are more particles in a small space so more collisions take place.

3. Surface Area: With a larger surface area, there is more space for the collisions to take place, so the more collisions take place.

4. Catalysts: Catalysts reduce the activation energy, so there is a larger number of successful collisions.
--> they work best when they have a large surface area.
--> used in industry as lower temperatures can be used and they don't get used up. However they are expensive and have to be cleaned. You need different catalysts for different reactions, which is expensive, and they can be 'poisoned' by impurities and stop working.

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Measuring Rates of Reaction.

1. Precipitation: This is when the product of the reaction is a precipitate, which makes the solution cloudy - so observe a marker through the solution and measure how quickly it takes to disappear.
---> very subjective. 

2. Change in Mass: This is when the mass decreases as a gas is given off, which can be measures on a mass balance.
---> very accurate, but releases gas into the room.

3. Volume of Gas given off: The gas released by a reaction is measured in a gas cyclinder. 
---> quite accurate, but doesn't work for vigorous reactions.

An Exothermic Reaction gives out energy - eg. combustion, neutralisation and many oxidation reactions.

An Endothermic Reaction takes in energy - eg. thermal decomposition reactions. 

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Reversible Reactions.

A reaction where both the reactants react to make the products and the products react to make the reactants.

Dynamic Equilibrium: Where the reaction is happening at the same rate on both side so the overall effect is nil.

The Position of Equilibrium depends on:
---> Temperature: if increased - moves towards the endothermic reaction.
                             if decreased - moves towards the exothermic reaction.
---> Pressure: if increased - moves towards the reaction producing less moles.
                       if decreased - moves towards the reaction producing more moles.

The Haber Process produces Ammonia, where the forward reaction is exothermic.
                       N2 + NH3 -------------> 2NH3

--> High pressures are used, which favour the forward reaction.
--> An Iron Catalyst is used.

--> A temperature of 450 degrees is a compromise between rate of reaction and maximum yield.
 

 

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Acids and Alkalis.

Acids are solutions containing H+ ions.
Alkalis are bases dissolved in water, which contain OH- ions.

* Acid + Base ---> Salt + Water.
* Metal + Acid ---> Salt + Hydrogen.
* Metal Oxide + Acid ---> Salt + Water.
* Metal Hydroxide + Acid ---> Salt + Water.
* Metal Carbonate + Acid ---> Salt + Carbon Dioxide + Water. 

Making Insoluble Salts (precipitation reactions)
---> All sodium, pottssium and ammonium salts are soluble.
---> All chlorides except silver and lead are soluble.
---> All nitrates are soluble.
---> All sulphates except barium and lead are soluble.
---> Most Carbonates are soluble.

Soluble salts can also be made by reacting an insoluble base with an acid and evaporating off the water. Salts can also be produced by displacement. 

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Electrolysis.

Electrolysis needs an electrolyte (dissolved salt/ molten substance), an anode (+ve) and a cathode (-ve). 

In molten substances: Negatively charged ions are oxidised at the anode.
                                   Positively charged ions are reduced at the cathode.

In Salt Solutions: These also have H+ and OH- ions from the water.
              --->
The least reactive positively charged ion is oxidised at the cathode.
              --->  The SIMPLE negatively charged ion is oxidised at the anode. If                                the solution contains more complicated ions, oxygen is released:                              4OH- -----> 2H20 + 02 +4e- 

Electrolysis of Brine: (sodium chloride solution)
Cathode: Hydrogen, used in the haer process and making margarine.
Anode: Chlorine, used in swimming pools and disinfectants.
Solution: OH- and Na+ ions - Sodium Hydroxide, used in soap.

Purification of Copper: 
Cathode: Made of pure copper, where copper ions are reduced to form copper.
Anode: Made of impure copper, where copper is oxidised and migrates to cathode. Leads to sludge building up (impurities).
Electrolyte: Copper Sulfate Solution. 

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