Chemistry C7 Notes

HydroCarbons are made up of only carbon and hydrogen atoms,

The 'spine' of a hydrocarbon is made up of a chain of carbon atoms. There is a group of hydrocarbons called the alkanes.

In an alkane the carbon atoms are joined together by a single carbon-carbon bonds. So, all the carbon atoms are linked to four carbon or hydrogen atoms by single bonds.

This means that all their bonds are single and the hydrocarbon is saturated.

Methane, CH(4)       

Propane, C(3)H(8)

Butane C(4)H(10)

Alcohols form a homologous series with the functional groip -OH. The presence of the  -OH gives the alcohols there characteristic properties. The general formula for alcohols is C(n)H(2n+1)OH, where n is the number of carbon atoms.

The two simplest alcohols are methanol and ethanol:

Methanol, CH(3)OH               

Ethanol, C(2)H(5)OH       

Ethanol can be used as a solvent, a fuel or a component in alcoholic drinks.

Methanol is an  important chemical feedstock (i.e. a raw material used for an industrial process) Methanol can be used in the manufacture of fuels, adhesives, foams, cosmetics and solvents.

Alcohols contaion a hydrocarbon chain and an -OH group, so we can compare there physical properties to those of alkanes and water.

       Boiling Point   Melting Point    Density (g/cm"3")

Alcohol     78            -117             0.79

Water      100             0             1.0

Alkane       -89         -183            0.546

From the table, it can be seen that...

-The hydrocarbon chane behaves like the alkane i.e. it is less dense than water because the long hydrocarbon chains do not mix with water.

-The -OH group behaves like water which explians the higher than the expected boiling point.

Alcohols burn in air to produce carbon dioxide and water.

They produce these substances because of the presence of the hydrocarbon chain.

The following equation shows what happens when an alcohol burns in the air.

c(2)H(5)OH(I) + 3O(2) -------> 3H(2)O(G) + 2CO(2)(G)

Alcohols react with sodium to prodice a salt and hydrogen gas. It is  the presence of the functional group -OH that allows this reaction to occur as in the example below.

Ethanol + Sodium ---> Sodium Ethoxide + Hydrogen

-Sodium floats on water

-Sodium sinks in alcohol

-Sodium doesn't react with alkane 

Carboxylic acids form a homologous series with the functional group -C00H. The presence of the -COOH gives the carboxylic acids there characteristic properties.

The two simplest carboxylic acids are methanoic acid and ethanoic acid:

Methanoic acid, HCO(2)H

Ethanoic Acid CH(3)CO(2)H

Vinegar is a dilute solution of Ethanoic acid.

Carboxylic acids are found in many substances and some have unpleasant smells and tastes. For example they are responsible for....

-The aroma of a sweaty training shoe.

-The taste of rancid butter.

Carboxylic acids are weak acids. Like all acids, they can react with metals, alkalies and carbonates to produce carboxylic acid salts. For example:

Carboxylic acid and a metal:

Ethanoic acid + Sodium ----> Sodium Ethanoate + Hydrogen

Carboxylic acid and a alkali

Ethanoic acid + Sodium hydroxide -----> Sodium Ethanoate + Water

Carboxylic acid and a carbonate

Ethanoic acid + Sodium carbonate -----> Sodium Ethanoate + Water + Carbon                                                                                                                        Dioxide

Fats and oils are naturally occurring esters. Living organisms make them to use as an energy store.

Fats are esters of.....

-Glycerol which is an alcohol with three -OH Groups

- Fatty acids which are carboxylic acids with very long hydrocarbon chains

Saturated Fats

Animal fats, such as lard and fatty meat, are mostly saturated molecules. This means they have single carbon carbon bonds and the molecules are non-reactive.

Unsaturated Fats

Vegetable oils, such as olive oil and sunflower oil, are mostly unsaturated molecules. This means that they contain some double carbon carbon bonds. The presence of these bonds means the molecules are reactive.

Carboxylic acids react with alcohols to form esters, as in the following example:

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

This reaction is carried out in the presence of a catalyst, i.e. concentrated sulfuric acid.

Esters have distinctive smells that are responsible for the smells and flavours of fruits. Due to their sweet smell, they are often used in the manufacture of perfumes, fragrances and food products (for artificial flavours such as raspberry, pear and cherry)

Esters are also found in products such as solvents in adhesives and plasticizers because they contain hydrocarbon chains.  

1) Ethanol ad excess Ethanoic acid are heated under the reflux in the presence of concentrated sulfuric acid.

2) The ester is removed by distillation. (Ethyl Ethanoate boils at 77'C)

3) The distillate is transferred to a separating funnel where it is purified. A solution of sodium carbonate is added and the mixture is shaken up. This mixture will react with any remaining acid and extract it into the aqueous phase. The aqueous phase is then run off leaving the ester in the funnel.

4)The product is transferred to a conical flask and anhydrous calcium chloride is added to remove any remaining water molecules. The calcium chloride is removed later by filtration.

(This will be placed in a power point with diagrams and labels - I'll add the name so you can search it when its done)

Exothermic reactions are accompanied by a temperature rise. They transfer heat energy to the surroundings i.e. they give out heat. Combustion of carbon is an exothermic reaction.

Carbon + Oxygen ----> Carbon Dioxide + Heat Energy

C + O(2) ----> CO(2)

It is not only reactions between fuels and oxygen that are exothermic. Neutralising alkalis with acids and many oxidisation reactions also 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 products have less energy than the reactants. 

Endothermic reactions are accompanied by a fall in temperature. Heat is transferred from the surrounding i.e. they take in heat. Dissolving ammonium nitrate crystals in the water is an example of an endothermic reaction.

ammonium nitrate + Water ----> Ammonium nitrate solution - Heat energy

NH(4)NO(3)(s) + H(2)O(i) ------> NH(4)NO(3)(aq)

Thermal decomposition is also an example of an endothermic reaction.

The energy change in an endothermic reaction can be shown using an energy level diagram. Energy is taken in during the reaction so the products have more energy than the reactants.

In a chemical reaction, new substances are produced. In order for this to happen, the bonds in the reactants must be broken and new bonds formed in order to make new products.

The activation energy is the energy needed to start a reaction, i.e. to break old bonds. This can also be shown on an energy level diagram.

Breaking a chemical bond requires a lot of energy this is and endothermic process. When a new chemical bond is formed, energy is given out this is an exothermic process.

If more energy is required to break old bonds than is released when the new bonds are formed, the reaction is endothermic.

Energy --> Reactants --> Products --> Energy

If more energy is released when the new bonds formed than is needed to break the old bonds the reaction is exothermic.

Hydrogen is burned in oxygen to produce water.

Hydrogen + Oxygen --> Water

2H(2)(g) + O(2)(g) --> 2H(2)O(g)

The following bond energies for the reactants and products

H-H = 436kj       O=O =496kj      O-H = 463kj

Calculate the change?

Breaking bonds =  1368KJ

Making Bonds = 1852KJ

Energy Change = -484KJ

The reaction is exothermic because the energy from making the bonds in the product is more than the energy need to break the reactant bonds.

Some chemical reactions are reversible, i.e. the products can react together to produce the original components.

A + B <-----> C + D

This means that A and B can react together to make C and D. As well as C and D can react together to create A and B.

For example, solid ammonium chloride decomposes when heated to produce ammonia and hydrogen chloride gas, both of which are colourless. Hydrogen chloride gas and ammonia react to produce white clouds of ammonium chloride.

Ammonium Chloride <--> Ammonia + Hydrogen Chloride

NH(4)Cl(s) <---> NH(3)(g) + HCl(g)

A reversible reaction will reach a state of equillibium if it is in a closed system (a system where no reactants are added and no products are taken away)

At equilibrium the reaction appears to have stopped. However, neither the forward reaction (from left to right) nor the backward reaction (right to left) are complete as both reactants and products are present at the same time. The concentration of the reactants the products does not change.

The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction

Although a reversible reaction might not go to completion, it could still be used efficiently in an industrial process, e.g. the haber process for the industrial manufacture of ammonia.

One equilibrium is achieved, the concentration the reactants and products does not change. The equilibrium can be approached from either direction, i.e. the reactant side or the product side.

Chemical equilibriums are dynamic/ both the forward and backward reactions are still occurring, but at the same rate. Therefore, there is no overall change in the concentration of the substances.

STRONG ACIDS - Hydrochloric acid, nitric acid and sulfuric acid are all examples of strong acids. When the acid is formed there is 100% ionisation of the molecules.

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

HCl(G) + H(2)O(i) ---> H(+)(aq) + Cl(-)(aq)

WEAK ACIDS - Carboxylic acids are weak acids. in a dilute solution of Ethanoic acid there is only about 1% ionisation, i.e. only 1 in every 100 molecules ionise. A dynamic equilibrium is formed/

For example, the following formula shows the ionisation of a weak acid in water.

CH(2)COOH(aq) + H(2)O(l) ---> CH(3)COO(-)(aq) + H(+)(aq)

Therefore, weak acids have a higher pH than strong acids (a number closer to 7) as the concentration of H(+) ions in the solution is much lower.

there are two types of analytical procedures:

- Qualitative methods

- Quantitative methods

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

Using a system of common practice and procedures, such as ensuring that samples are not contaminated, can increase reliability since there is less room for human error. Different people can also repeat a tests on the same sample and can produce the same result.

Qualitative analysis is any method used to identify the chemicals in a substance. For example, using an indicator to find out if acids are present or using thin layer of chromatography.

Quantitative analysis is any method used to determine the amount of chemical in a substance. For example, carrying out an acid base titration to find out how much acid is present.

Many of the analytical methods you have learned are based on samples in solutions.

When collecting data, it is very important that the samples are representative of the bulk of the material under test.

This is achieved by collecting multiple samples at rendom. After a sample as been collected, it should be stored in a sterile container to prevent change or deterioration.

The container should be sealed, labelled and stored in a safe place.

Chromatography is a technique used to find out what unknown mixtures are made up of. Substances are separated by the movement of a mobile phase though a stationary phase.

1) If the substance to be analysed is a solid, dissolve it in a suitable solvent (the substance used will depend in the solubility of the substance)

2) Place a spot of the resulting solution onto a sheet of chroatography paper on the pencil line and allow it to dry.

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

4) The solvent rises up the paper, dissolving the 'spot' and carrying it, in solution, up the paper.

5) The different chemicals in the mixture become separated because their molecules have different sizes and properties. The molecules that bind strongly to the paper, travel a shorter distance than the molecules that bind weakly to the paper.

The solvent that is used to move the solution is called the mobile phase.

A range of aqueous and non aqueous solvents may be used. Aqueous solvents are water based, whereas non aqueous solvents are made from organic liquids such as alkalines.

The medium that it moves though is called the stationary phase. in this case the paper is the stationary phase.

A chromatogram is formed when the chemicals come out of the solution and bind to the paper i.e. they move between the mobile and stationary phase.

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

Different molecules in the sample mixture travel different distances according to how strongly they are attracted to the molecules in the stationary phase. in relation to their attraction to the solvent chemicals.

TLC is similar to paper chromatography. However, the stationary phase is a thin layer of absorbent material (e.g. silia gel, alumia or celluse) Supported on a flat, non-reactive surface e.g. glass, metal or plastic plates. 

As a result, thin layer chromatography usually produces better seperations for a wider range of substances.

Some chromatograms have to be developed to show the presence of colourless substances.

-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 to cause colouration. 

In paper and thin layer chromatography, the movement of a substance relative to the movement of the solvent front is known as the R, value:

R, Value = distance travelled by the substance / Distance travelled by solvent.

Calculating the R, value can aid in the identification of unknown substances.

For example:

R, = distance travelled by the substance / Distance travelled by solvent.

    = 3cm/6cm

    = 0.5cm

In gas-liquid chromatography, or simply gas chromatography (GC), the mobile phase is a carrier gas, usually an inert gas such as helium or nitrogen. The stationary phase is a microscopic layer of liquid on an non-reactive solid support. 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 heated column where it vaporises. The carrier gas then carries it up the column where separation takes place. 

GC has a greater separating power than TLC or Paper chromatography, and can separate complex mixtures.

The size of each peak in the chromatogram, produced be GC shows the relative amount of each chemical in the sample.

Many methods of quantitative analysis use solutions. The concentration of a solution is the quantity of solid dissolved in the liquid. The concentration of a solution is measured in g/dm(3).

The formula below is used to calculate the concentration.

Concentration (g/cm/(3) = Mass(g) / Volume(dm(2))

Example

£.6g copper sulfate is dissolved in 80cm(2) water.

What is the concentration of the solution?

Concentration = Mass / Volume

= 80cm(2) / 1000 gives you it in Dm (3)

= 3.6q / 80cm(3) = 45g/dm(3)

The concentrations of standard solutions are known accurately. Therefore, these solutions can be used to measure the concentration of other solutions. A standard procedure is used to make up the solution.

For example, the following method is used to make up a standard solution of copper sulphate:

1) Weigh out 5g copper sulfate in a beaker.

2) Transfer the solid copper sulfate into a volumetric flask using using a short stem funnel. Wash the funnel and beaker with distilled water. Pour the washings into the volumetric flask (this will ensure that all of the solid has been transferred)

3) Add distilled water to the flask until it is about three quarters full. Place the stopper in the top and gently shake until all the solid has dissolved.

4) Place the flask on a level surface and fill it up with water intil the level of solution reaches the 100cm(2) mark.

1) Fill a burette with the alkali and take initial reading of the volume.

2) Accurately weigh out a 4g sample of soluid acid and dissolve it in 100cm(3) of distilled water.

3) Use a pipette to measure 25cm(3) of the aqueous acid and put it into a conical flask. Add a few drops of indicator to the conical flask. Place the flask on a white tile under the burette.

4)Add the alkali from the burette to the acid in the flask drop by drop. Swirl the flask to ensure it mixes well. Near the end of the reaction, the indicator will start t change the alkali colour. Keep swirling and adding the alkali until the indicator is completely pink and neutralised.

5)Record the volume of the alkali added by subtracting the amount in the burette at the end of the reaction.

6) Repeat the whole procedure until you have stable results or an average.

When asked to interpret the result of a titration experiment you may be given all of the information needed to carry out the calculation.

You will be given the titration formula and you need to be able to substitute the correct numbers and work out the final answer.

Concentration on acid = volume X concentration of NaOH

                                                        volume HCl

The validity of an experiment can depend on the accuracy of the results.

Inaccurate results can be the result of errors of measurements or mistakes. Mistakes are errors that are introduced when the person undertaking the experiment does something incorrectly, for example misreading a scale.

There are two general sources of measured uncertainty: systematic errors and random errors.

Accuracy describes how close a result is to the true or 'actual' value. Precision is a measure of the spread leads to a greater uncertainty.

The degree of uncertainty is often assessed by working out the average results and stating the range.

Systematic errors mean that repeat measurements are consistently to high or low. This could result from a incorrectly calibrated flask

For example, a burette reading's could be plus or minus 0.5cm(3). We could make this mistake because of the meniscus, we should take readings from the base of the meniscus. 

If the burette is used at a different temperature from the temperature it was calibrated at then a systematic error may be introduced.

Random errors mean that repeat measurements give different values. For example, repeat measurements can introduce random errors because the meniscus is not on the calibration line.

The end point of a titration can be determined by the use of a pH meter or light sensor. It can also be detected by using the naked eye, but this method may introduce random errors.

The chemical industry synthesizes chemicals on different scales according to there original values.

Bulk chemicals are made on a large scale for example, ammonia, sulfuric acid, sodium hydroxide, phosphoric acid.

Fine chemicals are made on a small scale for example, drugs, food additives, fragrances.

New chemical products or processes are the result of extensive programme of research and development, for example, researching catalysts for new processes.

Products have to be thoroughly tested to ensure that they are effective and safe to use

Governments have a duty to protect he people and environment from any dangers that could occur as a result of procedures involving chemicals.

They impose strict regulations in order to control:

-chemical process

-The storage of chemicals

-The transportation of chemicals

-The research and development of chemicals.

In the UK, the health and safety executive (HSE) is responsible for the regulation of risks to health and safety arising from the extraction, manufacture and use of chemicals need to be labelled with the standard hazard symbols.

More recently, legislation has been passed to encourage companied to reduce the amount of pollution they produce.

The chemical industry carries out research and development to ensure that its processes are sustainable i.e. they meet the needs of present generation without comprimising future generations.

Sustainability of any chemical process depends on the principles of green chemistry.

In recent years, there has been alot of research and development into catalysts. A large reaction can be created using only a small amount of energy and as the catalyst remains unchanged, it can be used over and over again. This makes the process more sustainable.

Catalysts reduce the activation energy needed for a reaction - this makes the reaction go faster. This can be illustrated using an energy level diagram.