Chemistry Topic 7- 21st Century Science

They have a Functional Group- a collection of atoms that gives it it's chemical properties.

The Functional Group for Alcohols is -OH.

Properties of Alcohols

Methane and Ethane are Gases at Room Temperature 

Methanol and Ethanol are Liquids at Room Temperature

The general formula for alcohols is:

CnH2n+1OH

When n is the number of carbon atoms.

COOH- Functional Group

Acid + Metal ---> Salt + Hydrogen

E.g. Ethanoic Acid + Magnesium ---> Magnesium Ethonoate + Hydrogen 

Acid + Soluble Hydroxide ---> Salt + Hydrogen 

E.g. Ethanoic Acid + Copper Oxide ---> Copper Ethonoate + Hydrogen 

Acid + Metal Carbonate ---> Salt + Carbon Dioxide + Water

E.g. Ethanoic Acid + Sodium Carbonate ---> Sodium Ethanoate + Carbon Dioxide+ Water.

Addition of Oxygen

Beer -----> Malt Vinegar

Wine -----> Wine Vinegar 

Carboxylic Acids react with Alcohols to form an Ester.

This reaction is carried out in the presence of a strong acid cataylst. 

Ethanoic Acid + Ethanol ---> Ethyl Ethanoate + Water

They are Naturally Occuring Esters

Fats are the Esters of

-Glycerol

-Fatty Acids

Animal Fats

Saturated Molecules

Single Carbon - Carbon Bonds  C-C

The Molecules are Unreactive

Vegetable Oils

Unsaturated Molecules

Some Double Carbon- Carbon Bonds C=C

The Molecules are Reactive

Exothermic Reactions are accompanied by a Temperature Rise. They transfer heat energy to the surroundings- They give out heat.

The energy change in an Exothermic reaction can be shown using an energy level diagram.

Energy is lost during the reaction, so the produts have less energy than the reactants.

Endothermic Reactions are accompanied by a Fall in Temperature. Heat is transferred from the surroundings -They take in heat.

The Energy change in an Endothermic Reaction can be shown using an Energy Level Diagram.

Energy is taken during the reaction, so the products have more energy than the reactants. 

In a chemical Reaction, the bonds in the reactants must be broken and new bonds must be made to form the products 

The Activation Energy is the energy needed to break bonds to start a reaction. This can be shown on an Energy Level Diagram.

In a chemical reaction...

-Making bonds is an Exothermic process

-Breaking Bonds is an Endothermic process

Chemical reactants that requite more energy to break bonds than make them are Endothermic Reactions.

Chemical Reactions that require more energy to make bonds than break them are Exothermic Reactions.

Hydrogen is burned in Oxygen to produce Water.

Hydrogen + Oxygen ---> Water

The following are bond energies for the reactants and the products 

H-H is 436kj O=O is 496kj O-H is 463kj   You can calculate the energy change. 

1. Calculate the energy used to break the bonds- 2xH-H + O=O = (2x436)+496 =1368kj

2. Calculate the energy used to make the bonds. (Water is made up of 2xO-H Bonds)- 2xH-O-H= 2x(2x463) = 1852kj

3. Energy Change= Energy used to break bonds- energy used to make bonds = 1368-1852= -484kj

This reaction is Exothermic because the energy from making the bonds is more than the energy needed to break the bonds.

Some chemical reactions are reversible, In a reversible reaction, the products can react together to produce the original reactants.

A + B <===> C + D

This means that 

- A and B can react together to produce C and D

-C and D can react together to produce A and B

A reversible reaction will reach a state of Equilibrium if it is in a closed system.

At Equilibrium

-The forward and backward reactions happen at exactly the same rate.

-The concentration of the reactants and products doesn't change over time.

Chemical Equilibriums are Dynamic. Both the forward and the backward reactions occur at the same rate so there's no overall change in concentration of the substances.

Dynamic Equilibrium may be used to explain the difference between a Strong Acid and a Weak Acid.

Some Strong Acids are...

-Hydrochloric Acid

-Nitric Acid

-Sulfuric Acid

Strong Acids ionise completely in Water. All of the acid molecules ionise so there's no equilibrium reaction.

For Example, when hydrogen chloride gas dissolves in water, all the molecules ionise to give Hydrogen ions and chloride ions

HCl + H2O ---> H+ + Cl-

Carboxylic Acids are weak acids. Weak acids only partially ionise (about 1%) in water so a dynamic Equilibrium is formed.

Qualitative Analysis is any method used to identify the chemicals in a substance.

Quantitive Analysis is any method used to determine the amount of chemical in a substance.

There are standard procedures for the collection, storage and preparation of samples for analysis. 

After a sample has been collected, it should be stored in a sterile container to prevent change or deterioration. The container should be...

-Sealed

-Labelled

-Stored in a safe place

Using a system of common practices and procedures can increase reliability since there's less room for human error. 

Chromatography is used to find out what unknown mixtures are made up of.

Substances are separated as the move between the mobile and stationary Phases.

The solvent that's used to move the solvent is called the mobile phase. Solvents can be....

-Aqueous- Water Based

-Non- Aqueous- Made from Organic liquids such as alkanes.

The medium that the solvent moves through (for example, paper) is called the stationary Phase.

For each component of the sample, a dynamic equilibrium is set up between the stationary phases and the mobile phase.

Paper Chromatography has 5 main stages:

1.If the substance to be analysed is a solid. dissolve it in a suitable solvent.

2. Place a spot of the resulting solution on to a sheet of Chromatography Paper on the pencil line, allow it to dry 

3.Place the bottom edge of the paper into a suitable solvent.

4.As the solvent rises up the paper, it dissolves the 'Spot' and carries it up the paper

5. The different chemicals in the mixture separate because their molecules have different sizes an properties.

TLC is similar to paper Chromatography but the stationary phase is a thin layer of absorbent material (E.g Silica Gel) supported on a flat, unreactive surface (e.g a glass plate.

There are several advantages of TLC over paper Chromatography:

-Faster Runs

-More even movement of the mobile phase through the stationary phase

-A choice of different absorbensies for the stationary phases.

As a result, TLC usually produces better separations for a wider range of substances.

A chromatogram is formed when the chemicals come out of a solution and bind to the stationary phase. The chromatogram can then be compared to standard chromatograms (Standard Reference materials) of known substances to identify the different chemicals.

Some Chromatograms have to be developed to show the presence of colourless substances using locating agents:

-Colourless spots can sometimes be viewed under ultraviolet (UV) Light and then marked on the plate

-The Chromatogram can be viewed by being sprayed with a chemical that reacts with the spots cause coloration.

The movement of a substance relative to the movement of the solvent front is knows as the Rf Value.

You can work out the Rf value using this formula

Rf Value= Distance Travelled by substance

 Distance Travelled by solvent

Calculating the Rf Value can help identify unknown substances

The calculated Rf value can be compared to known values for different substances.

In gas chromatography: The mobile Phase is a carrier gas, The stationary phase is a microscopic layer of liquid on an unreactive solid support and the liquid is inside glass or metal tubing, called a column.

A sample of the substance to be analysed is injected into one end of the hated column where it vaporises. The carrier gas then carries it up the column where separation takes place.

GC is able to separate the components in a mixture because of their different solubilities.

There are several advantages to using GC over TLC and Paper Chromatography, including: Greater Separating Powers, The ability to separate complex mixtures, the ability to produce quantitative data from very small samples of liquids, gases and volatile solids.

Uses of GC include: Detecting banned substances in blood samples and analysing oil spills to identify sources of pollution.