Chemistry Unit 2

Summary Notes on Chemistry Unit 2

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  • Created on: 18-05-12 08:10

Enthalpy Changes

If the enthalpy change in negative the reaction is exothermic eg. the burning of methane. 

If the enthalpy change was positive then the reaction is endothermic eg the decomposition of calcium carbonate on heating.

Enthalpy change = the heat change at a constant pressure.

Enthalpies are measured at 100kPa and 298K indirectly by measuring the heat change of the surroundings.

Standard Enthalpies:

The standard molar enthalpy of formation is the enthalpy change when one mole of compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states.

The standard molar enthalpy of combustion is the enthalpy change when one mole of compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states.

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Measuring Enthalpy Changes

We can measure the standard enthalpy change of combustion of a fuel by putting a beaker of water over the flame, this is known as a simple calorimeter. We know the specific heat capacity of the water so we can calculate enthalpy change by the equation:

q=mcT

We can improve calorimeters by insulating the flame and the water so that the final heat of the water has absorbed all the heat from the flame.

We can measure the standard enthalpy change of solution relatively easily by measuring the heat change when salts are dissolved in water. The specific heat capacity of dilute solutions can be presumed to be the same as water.

Hess's Law states that the enthalpy change for a chemical reaction is the same independent of the route from reactants to products, for enthalpy changes which are hard to measure directly, we can measure them via alternative routes.

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Bond Enthalpies

The Bond Enthalpy is the energy taken to break a bond.

Bond Dissociation Enthalpy is defined as the enthalpy change required to break a covalent bond with all species in the gaseous state

The same amount of energy is given out when this bond is made.

Mean Bond Enthalpy is the enthalpy change when one mole of one type of bond, averaged over a number of different compounds, with all species in their gaseous state.

Enthalpy change can be calculated theorhetically using bond enthalpies, however this is often not very accurate as the bonds have slightly different enthalpies in different compounds.

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Collision Theory

Most collisions between molecules do not lead to a reaction. They either don't have enough energy, or they are in the wrong orientation.

Factors which affect the rate of chemical reactions:

Temperature - this increases both the energy of the molecules and their speeds and thus the number of collisions.

Pressure/Concentration - there are more molecules in a given volume and thus more collisions.

Surface Area - when using solid reactants, increasing surface area increase the number of collisions.

Using a Catalyst - offers an alternative route with a lower activation energy so more molecules have greater than the activation energy required and will collide successfully.

Activation Energy is the minimum sum energy required by two molecules when they collide that they can react successfully.

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Catalysis

Heterogeneous catalysts are in a different phase to the the reactants usually a solid catalyst with liquid or gaseous reactants.

Homogeneous catalysts are in the same phase as the reactants.

Key Examples:

  • The Haber Process is catalysed by Iron. It produces ammonia, from nitrogen and hydrogen, which is used to make fertillisers.
  • The Ostwald Process produces NO for making nitric acid from ammonia and oxygen. It is catalysed by Platinum and Rhodium.
  • Hardening Fats by adding hydrogen across their C=C double bonds. This process is catalysed by nickel.
  • Catalytic Converters in car exhausts catalyse the reaction between CO and NO to carbon dioxide and nitrogen, they use Platinum or Rhodium as a catalyst.
  • Hydration of Ethene to Ethanol is catalysed by Phosphoric acid.
  • Esterification is catalysed by concentrated sulphuric acid.
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How Catalysts Work

Catalysis occurs in two steps:

1. The reactants form weak bonds with the catalyst (adsorbsion). This holds the gases in the right position to react together. This allows the reactants to react on the surface of the catalyst.

2. The products then break away from the catalyst (desorbsion). This frees up space on the catalyst and it can be reused.

The success of a catalyst is dependent on the strength of its bonding to the reactants. They have to adsorb strongly enough to be held for long enough to react but not too strongly, or the products will not be able to desorb and the catalyst will be "poisoned".

Zeolites are minerals that have a very open pore structure that ions or molecules can fit into. Zeolites confine molecules in small spaces, this changes their structure and reactivity. Although zeolites are found naturally the more effective catalytic ones are generally synthesised.

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Equilibria

An equilibria is achieved when the forwards and backwards reaction rates of a reversible reaction are exactly equal and thus the concentrations of the reactants and products remain constant and the macroscopic properties of the system do not change over time.

Le Chantilier's Principle states that "if a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance"

Key Examples:

Ethanol from the fermentation from sugars, the enzymes in yeast are used to catalyse the reaction.

Ethanol from the hydration of ethene, this reaction is reversible and is catalysed by the catalyst of phosphoric acid and H+ absorbed on silica, 570K and 6500kPa conditions are used, while this gives only 5% yield the gases can be separated and the reactants reused to give an overall yield of around 95%.

Methanol is produced from carbon monoxide and hydrogen gas using a copper catalyst. Conditions used are 500K and 10000kPa and produces around a 5-10% yield.

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The Halogens

Electronegativity decreases down the group as the atoms have more electrons so the outer electrons are shielded from the nuclear charge and are more easily lost. Thus atoms higher in the group can displace atoms which are lower in the group.

Melting and boiling points increase down the group as the atoms get larger and have more electrons to make stronger van der waals forces between the molecules.

Bromine is extracted from sea water by bubbling chlorine gas through the solution which displaces the bromine.

Iodine is extracted from kelp by reducing it from its ionic state with manganaese dioxide and acid.

Metal halides can be distinguished with silver ions. A solution of halide ions is acidified with nitric acid and then silver nitrate is added.

Fluoride gives no ppt.Chlorine gives a white ppt which dissolves in dilute ammonia. Bromide gives a cream ppt which only dissolves in concentrated ammonia. Iodide gives a pale yellow ppt which doesn't dissolve in any ammonia.

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Reactions of Halide ions

Halide ions react with concentrated Sulphuric Acid, reducing power increases down the group:

Sodium Chloride (solid) cannot reduce the sulphuric acid.

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Sodium Bromide (solid) can reduce the sulphuric acid to sulphur dioxide.

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Sodium Iodide (solid) can reduce the sulphuric acid to hydrogen sulfide gas via sulphur dioxide and sulphur (some of these are also produced)

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Reactions of Chlorine

Reaction with cold water:

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This is used to sterilise water, the chloric acid kills bacteria but at low concentrations is not damaging to humans.

Reaction in direct sunlight:

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Reaction with alkali:

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Group 2 - The Alkaline Earth Metals

The atoms and ions get bigger as we go down the group, due to the added electron shells.

Melting and Boiling points decrease down the group as there are more shells so the delocalised sea of electrons are more attracted to the positive ions.

Ionisation energies decrease down the group as the electrons are less attracted to the nuclear charge due to extra shells.

They react with water to give an alkaline solution of their hydroxide M(OH)2 and hydrogen.

The hydroxides get more soluble down the group (so magnesium hydroxide, milk of magnesia, is the least soluble).

The sulfates get less soluble down the group so Barium sulfate is the least soluble and thus is used as a barium meal in medicine.

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Extraction of Metals

Sulfides can be converted to oxides by heating in oxygen or "roasting" to give the oxide and sulphur dioxide.

Iron oxide is reduced in a blast furnace. Carbon reacting to give carbon dioxide then again to give carbon monoxide gives out the heat for the blast furnace and then the carbon monoxide reduces the iron oxide to iron and carbon dioxide.

Copper carbonate decomposes to copper oxide on heating, then carbon can be used to reduce the copper oxide to copper.

Aluminium oxide is reduced by electrolysis, aluminium oxide is first dissolved in molten cryolite then a current is run through it.

Titanium oxide is reduced by reacting it with carbon and chlorine to give titanium chloride and carbon monoxide then the titanium chloride is displaced by sodium under an inert argon atmosphere.

Tungsten oxide is reduced with hydrogen.

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Haloalkanes in the Atmosphere

Halogens in the atmosphere split into free radicals under UV light and attacks methane, it also catalyses the breakdown of ozone to oxygen.

Chlorine with Methane:

In Initiation the Cl molecule splits into two free radicals.

In Propagation a free radical reacts with a molecule to create a new free radical and a molecule. Either a Cl molecule reacts with a methane molecule to give HCl and a CH3 free radical or a CH3 free radical reacts with a Cl molecule to give a Cl free radical.

In Termination two free radicals react to give a molecule.

The reaction of Ozone:

This happens in two steps, firstly a Cl free radical reacts with O3 to give a ClO free radical and O2. Then the ClO free radical reacts with O3 to produce a Cl free radical and O2.

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Reactions of alkenes

Alkenes can react with...

  • Water to give an alcohol
  • Hydrogen halide to give a haloalkane
  • Halogen to give a dihaloalkane - Bromine water can be used to test for a double bond as the orange colour disappears as it is added across the double bond.
  • Concentrated sulphuric acid to give ethyl hydrogen sulfate which reacts with water to give an alcohol and sulphuric acid.
  • A hydroxide initiator to give a polymer - Low density polymers are made when a polymer is formed at high pressure and temperature and High density polymers are formed when the conditions are close to standards conditions with a Ziegler-Natta catalyst.

Polymers:

Can be recycled by melting them down and reforming them however every time they are remoulded at high temperatures some of the polymer chains are broken. They can also be recycled by heating them until the polymers break apart to monomers and can be made into new plastics.

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Alcohols

The bond angle between the C-O-H bond is 105 degrees.

Oxidation with Acidified(with dilute sulphuric acid) Potassium Dichromate

  • Tertiary alcohols cannot be oxidised.
  • Secondary alcohols can be oxidised to ketones.
  • Primary alcohols can be oxidised to aldehydes then carboxylic acids.

When the potassium dichromate is added it changes from orange to green if the alcohol is secondary or primary.

Alcohols can be dehydrated to alkenes with an aluminium oxide catalyst at about 600K. This happens via an elimination mechanism.

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