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4. How can alcohols be produced and what are their properties?
4. How can alcohols be produced and what are their properties?
4.1 Primary, secondary and tertiary alcohols
4.2 Properties and bonding of alcohols
4.3 Reactions of alcohols
4.4 Enthalpy of combustion of alcohols
4.5 The production of alcohols.
4.6 Investigating products of oxidation of alcohols.
4.7 Organic fuels including biofuels.
4.1 investigate the structural formulae, properties and functional group including primary, secondary and tertiary alcohols.
Introduction:
Alcohols are one of the most important molecules in organic chemistry. They can be prepared from many different types of compounds, and they can be converted into many different types of compounds.
Recall that a functional group is either a type of bond, atom, or group of atoms which link the members of an homologous series and is important to the chemical properties of each member of the series.
Alcohols are molecules containing the hydroxyl functional group (-OH) bonded to one of the carbon atoms. If it is bonded to an end carbon, this is a primary alcohol; to a middle C of a chain, a secondary alcohol, to a C bonded to 3 other Cs, a tertiary alcohol.
The presence of the hydroxyl group strongly changes some of the physical and chemical properties of the compound, primarily due to the polarity of the C-O and O-H bonds.
Naming (extra for experts)
Many functional groups have a characteristic suffix name, and only one suffix (other than "-ene" and "-yne") may be used in a name. When the hydroxyl functional group is present together with a function of higher nomenclature priority, eg an acid, it must be located by the prefix hydroxy and an appropriate number. For example, lactic acid has the IUPAC name 2-hydroxypropanoic acid.
view videos:
- OC#16 https://www.youtube.com/watch?v=AN2gY8u9Nv4&t=7s [14.28 mins]
4.2 explain the properties within and between the homologous series of alcohols with reference to the inter-molecular and intra-molecular bonding present.
Properties
Properties
This group has two reactive covalent bonds, the C–O bond and the O–H bond. The electronegativity of oxygen (3.44) is quite a bit greater than that of carbon (2.55) and hydrogen (2.1), so the covalent bonds of this functional group are polarised: oxygen is electron rich, and both carbon and hydrogen are electrophilic (elecron loving).
Intra-molecular bonds in alkanols:
Intra-molecular bonds in alkanols are covalent bonds.
C-C and C-H are non-polar bonds, but C-O and O-H bonds are polar bonds. Differences in electronegativity between O and both C and H account for these differences and cumulatively they create polarity in the molecule.
Inter-molecular bonds in alkanols:
The alkanols have a dual nature. For one part of their structure there are only C-C or C-H bonds. These are non-polar and hence can only form dispersion forces between adjacent molecules.
However, the other part of their structure is polar due to the polarity of the C-O and O-H bond. As a result, alkanols can link together via hydrogen bonds which are much stronger than dispersion forces. This affects properties such as solubility in water and melting and boiling point.
Solvent nature
Many alcohols make good solvents as they often have a polar region (-OH) and a non-polar region (C-C region) making them at least partially soluble in a range of solvents. They also have the ability to act as solvents themselves.
Ethanol is an excellent solvent due to its highly polar nature. One end of the molecule has non-polar C-H bonds, while the other end has an hydroxyl (-OH) group. The high electronegativity of oxygen allows hydrogen bonding to take place with other molecules.
As a result, the hydroxyl end of the ethanol molecule attracts polar and ionic substances. The ethyl (C2H5) group in ethanol is non-polar, so this end attracts non-polar substances. As a result, ethanol can dissolve both polar and non-polar substances.
In industrial and consumer products, this makes ethanol the second most important solvent after water.
Ethanol is the least toxic of the alcohols (though it is poisonous in large amounts), which also makes it more suitable for use in industry and consumer products. As a result ethanol is a common solvent in cosmetics (eg. perfumes), food colourings and flavourings (eg. vanilla essence), medicinal preparations (eg. antiseptics and rubbing alcohol), some cleaning agents.
The presence of the polar covalent bond and subsequent hydrogen bonding between molecules also means they have a higher boiling point than their corresponding alkane.
Acidity
The most reactive site in an alcohol molecule is the hydroxyl group, despite the fact that the O–H bond strength is significantly greater than that of the C–C, C–H and C–O bonds.
Alcohols are very weak Brønsted acids with pKa values generally in the range of 15 - 20. The dipolar nature of the O–H bond is such that alcohols are stronger acids than alkanes (by roughly 1,030 times).
Some important properties trends in the alcohols:
- boiling point of alcohols increase as chain length, and therefore molar mass, increases due to increase in dispersion forces between molecules
- secondary and tertiary alcohols with identical molar masses to a primary alcohol will have a lower BP due to reduction in strength of hydrogen bonding between molecules
- small primary alcohols, eg methanol and ethanol, are readily soluble in water, but as chain length and molar mass increase, solubility in water decreases
- large chain alcohols are soluble in polar solvents (the alkyl dominates over the polar -OH, so as chain length and molar mass increase, solubility in non-polar solvents eg CCl4 increases.
view videos:
- OC#17 https://www.youtube.com/watch?v=lbZPnlnql-k [14.04]
TASK 4.2.1
4. (2011, Q21, 4 marks)
What features of the molecular structure of ethanol account for its extensive use as a solvent? Include a diagram in your answer.
5. (2009, Q17, 4 marks)
Water and ethanol are both used as solvents. Explain the differences and similarities in their solvent behaviour in terms of their molecular structures. Include a diagram in your answer.
view videos:
- Intro to chapter https://www.youtube.com/watch?v=i_95z8HqW-4&index=35&list=PL49FAB756081270BA [4.55]
TASK 4.2.2
PAST HSC QS (IF NO MARKS ARE INDICATED, THE QS ARE MULTIPLE CHOICE. THERE ARE CURRENTLY 20 MC QS PER PAPER, BUT IN PAST YEARS THERE WERE 15).
2012 Q31A 3
2011 Q6; Q17; Q21 4
2010 Q11
2008 Q10
2007 Q4
2004 Q11; Q13
REVIEW 4.2
12C7 Pearson Ch12 Answers.pdf12C7 Pearson Ch12 Teacher Notes.pdf4.3 write equations, state conditions and predict products to represent the reactions of alcohols, including but not limited to
a) dehydration
b) substitution with HX
c) oxidation
d) combustion
There are a number of important reactions involving alcohols which we need to investigate. These include combustion, dehydration (-H2O), substitution with a hydrogen halide (HX eg HF, HBr, HCl) and oxidation.
a) Dehydration of alcohols
Dehydration is the process of removing water molecules, or the equivalent of water molecules, from a substance.
Ethanol can be dehydrated to produce ethylene and water when heated with a concentrated sulfuric acid (H2SO4) or phosphoric acid (H3PO4) catalyst. Tertiary alcohols react quickly, secondary alcohols slowly and primary alcohols only with very strong heating.
Q: Write a chemical equation to represent the dehydration of propan-2-ol.
view videos:
- Computer simulation conversion ethylene to ethanol https://youtu.be/q_atlIFneVs [7.06]
- Modelling dehydration https://www.youtube.com/watch?v=OHilfg_c4gM [7.27]
b) Substitution Reactions involving alcohols
Alcohols can react with hydrogen halides to form an alkyl halide and water. This happens when the halogen substitutes for the hydroxyl group, and so they are called substitution reactions.
Tertiary alcohols are the most reactive when involved in substitution reactions, then secondary and primary. There is also a sequence for the halogens, with HI the most reactive and HF the least.
A positive charge can be more easily placed on a tertiary carbon than a secondary or primary carbon, since carbon atoms have more electrons around them than hydrogen atoms do to stabilise a positive charge (through the inductive effect). It is easier to form t-butyl chloride or isobutylene from t-butyl alcohol by way of the t-butyl cation than it is, for example, to form 2-chlorobutane or butenes from 2-butanol by way of the sec-butyl cation. The varying reactivities of alcohols are thus seen to have the same underlying explanation as Markovnikov's rule.”
Q: Write a chemical equation to represent the substitution reaction involving hydrogen iodide and propan-2-ol.
c) Oxidation Reactions
Primary and secondary alcohols are readily attacked by oxidising agents. Tertiary alcohols do not react. Usual reagents for the oxidation of alcohols are acidified solutions of KMnO4 or K2Cr2O7, written as KMnO4/H+ for example.
- Primary alkanols are oxidised initially to alkanals and then to alkanoic acids.
- Secondary alkanols are oxidised to alkanones.
- Tertiary alkanols do not undergo oxidation reactions
Primary alcohols may be converted directly into alkanoic acids when strongly heated with acidified KMnO4 solution.
CH3CH2OH + hot acid. KMnO4 → CH3COOH
Under cold conditions, or without excess KMnO4, the alkanal will be produced as an intermediate product.
CH3CH2OH + cold KMnO4 → CH3CHO
As these are true oxidation reactions, we can write them as redox couples:.
Q: Methanol can be oxidised to CO2 and H2O in strong oxidising conditions. Write the equation.
The H2O, H+ and e- are then cancelled/adjusted (as with algebra rules).
The oxidation of ethanol by acidified potassium dichromate(VI)
Potassium dichromate(VI) solution acidified with dilute sulphuric acid is used to oxidise ethanol, CH3CH2OH, to ethanoic acid, CH3COOH.
The oxidising agent is the dichromate(VI) ion, Cr2O72-. This is reduced to chromium(III) ions, Cr3+.
Ethanol to ethanoic acid half-equation:
Balance the oxygens by adding a water molecule to the left-hand side:
Add hydrogen ions to the right-hand side to balance the hydrogens:
And finally balance the charges by adding 4 electrons to the right-hand side to give an overall zero charge on each side:
The dichromate(VI) half-equation:
Balance the chromiums.
Balance the oxygens by adding water molecules . . .
. . . and the hydrogens by adding hydrogen ions:
Now all that needs balancing is the charges. Add 6 electrons to the left-hand side to give a net 6+ on each side.
Combining the half-reactions to make the ionic equation for the reaction
What are the multiplying factors for the equations? The simplest way of working this out is to find the smallest number of electrons which both 4 and 6 will divide into - in this case, 12. That means that you can multiply one equation by 3 and the other by 2.
Note: Don't worry too much if you get this wrong and choose to transfer 24 electrons instead. All that will happen is that your final equation will end up with everything multiplied by 2. Your examiners might well allow that.
Now you will find that there are water molecules and hydrogen ions occurring on both sides of the ionic equation. You can simplify this to give the final equation.
- protonated means H+ from the H2SO4 is added to it
view videos:
TASK 4.3.1
d) Combustion of alcohols
The combustion reaction for alcohol is dependent on the amount of oxygen which is present. If there is insufficient oxygen, which is often the case for spirit burners, the fuel won’t combust completely and carbon will be a by-product. As chain length increases, the tendency towards incomplete combustion increases.
Complete combustion of ethanol produces CO2 (carbon dioxide) and H2O
Incomplete combustion of ethanol produces CO (carbon monoxide - yellow flame) and some C (black deposit of soot). Less oxygen, more soot produced.
View videos:
TASK 4.3.2
1. (2010, Q23, 3 marks)
a) Write a balanced chemical equation for the complete combustion of 1-butanol. (1 mark)
2. (2010, Q11)
An organic liquid, when reacted with concentrated sulfuric acid, produces a compound that decolourises bromine water.
What is the formula of the organic liquid?
A) C6H12
B) C6H14
C) C6H11OH
D) C5H11COOH
Write equations to show: (Davis, Disney, & Smith, 2018, p. 339)
a) The dehydration of 1-propanol
b) The dehydration of 3-methyl-1-pentanol
c) The reaction of 2-butanol with hydrogen bromide
d) The reaction of 2-methyl-2-pentanol with hydrogen chloride
e) The oxidation of 2-propanol
f) The oxidation of 2-methyl-3-hexanol
g) The complete combustion of octan-1-ol
4.4 conduct a practical investigation to measure and reliably compare the enthalpy of combustion for a range of alcohols.
view videos:
Molar Heat of Combustion (delta Hc) refers to the heat released when one mole of a certain compound undergoes complete combustion with oxygen at a constant pressure of exactly one atmosphere (100 kPa) and at 25°C with the final products being carbon dioxide gas and liquid water.
Heats of combustion are quoted as positive numbers while the enthalpy changes of combustion reactions (ΔH) are quoted as negative numbers. This is because combustion reactions are always exothermic.
Heats of combustion are expressed in kilojoules per mole (kJ/mol. or kJ mol.-1).
The accepted value for the molar heat of combustion of ethanol is 1360 kJ mol.-1.
To process the results from an experiment designed to calculate the enthalpy of combustion of an alcohol, substitute the experimental results into the formula below to determine the enthalpy change:
ΔH = -q, where q = mCΔT
ΔH = enthalpy change in joules
m = mass of water
C = thermal capacity (4.2 for water)
ΔT = change in temperature in degrees Celsius
Determine the number of moles of ethanol combusted (number of moles = mass/MM, n = m/MM), and divide the enthalpy change in kilojoules by this number to determine the experimental value of the molar heat of combustion of ethanol in kilojoules per mole.
Note: Due to heat loss to surroundings, the experimental value of the molar heat of combustion of ethanol will often be significantly lower than the accepted value.
If we conduct the same experiment for another alkanol, such as methanol, and the difference between the experimental value and accepted value was found, this difference could be used to calibrate the experimental results for ethanol and produce a more accurate experimental result.
The process described above can be applied to any alkanol and can be modified slightly in order to find the heat of combustion in kilojoules per gram instead of kilojoules per mole.
EPrac 4.4.1 Enthalpy of Combustion for Alcohols
Aim
The aim of this experiment is to determine and compare the heat of combustion values for methanol, ethanol and propanol by measuring and processing calorimetry data.
Method
1. Warning: Wear safety glasses during this experiment!
2. Warning: This experiment involves flammable fuels. Your teacher will review fire accident procedures before you start.
3. Warning: Ensure that your equipment is placed safely and is not a danger to neighbouring students.
4. Warning: Methanol is toxic. Avoid skin contact.
5. Before starting this experiment, you need to identify which data to collect in order to calculate the heat of combustion values. For guidance, study the Data processing section before you start. Transfer exactly 100.0 mL (m = 100.0 g) of water into a 100 mL conical flask. Immerse the thermometer. Stop and think what to do next.
6. Obtain a spirit burner filled with methanol. Stop and think what to do before you light the burner.
7. Clamp the neck of the flask over the burner and adjust the height a few centimetres above the wick. Ignite the wick. Heat the flask until the water temperature rises by about 20ºC.
8. Blow out the flame. Gently stir the flask with the thermometer. Stop and think about what temperature reading needs to be recorded.
9. Remove the spirit burner. Stop and think about what data need to be measured and recorded.
10. Empty the flask and shake dry.
11. Repeat steps 1–10 for spirit burners filled with ethanol and propanol (or whichever alcohols are available).
TASK 4.4.1
4.5 investigate the production of alcohols: general
Synthesis of alcohols
There are a number of different ways in which alkanols can be synthesised. A couple of examples are shown, with fermentation examined in more detail later in the module.
Synthesis of methanol
Methanol is prepared industrially by the catalysed (copper/zinc oxide) reaction between hydrogen and carbon monoxide at 260oC and 10MPa.
CO(g) + 2H2(g) → CH3OH(g)
It is also possible for the carbon monoxide to be converted to carbon dioxide prior to being hydrogenated. This will also release water as a byproduct.
Synthesis of ethanol
Ethanol is most commonly prepared through fermentation of glucose. This reaction is facilitated by yeast as a form of anaerobic respiration.
C6H12O6(aq) → C2H5OH(aq) + 2CO2(g)
Ethanol, and many other alkanols, can also be synthesised through the hydration of an alkene, such as ethene. Hydration is the process of adding water molecules, or the equivalent of water molecules, to a substance. Ethylene can be hydrated to produce ethanol when heated with a dilute sulfuric acid (H2SO4) catalyst:
4.5 investigate the production of alcohols, including
a) substitution reactions of halogenated organic compounds
The addition of water to a haloalkane can also facilitate a substitution reaction. The hydroxy group replaces the halogen to form an alkanol. This is because the C-halogen bond is highly polar and comparatively unstable compared to a C-H bond. This bond becomes more stable as we rise through the halogens, to the extent that the C-F bond is more stable than the C-H bond. This means a fluoroalkane will not undergo substitution to form an alkanol. Write a chemical equation to represent the reaction between 2-bromobutane and water. One of the products will be an alcohol.
view videos:
- Production of ethanol https://www.youtube.com/watch?v=y8UrDvLBsJw&index=43&list=PL49FAB756081270BA [7.02]
View videos:
TASK 4.5.1
4.5b investigate the production of alcohols, including fermentation
PRACTICAL 4.5 Fermentation Reaction
Introduction
Fermentation is an anaerobic metabolic process that occurs in micro-organisms (fungi and bacteria) and produces energy by breaking down carbohydrates into simpler molecules. Fermentation was one of the first chemical processes to have been utilised (at least 5500 years ago) and its secrets intrigued a few great chemists (Lavoisier, Liebig, Pasteur). It wasn’t until 1939 that the complex biochemistry of this deceptively simple reaction was finally worked out. Today, fermentation is a vital process used in diverse industries (brewing, baking, pharmaceutical).
Planning Guidelines
1. Choose appropriate glassware to contain the fermenting solution. Think about size and weight limitations. If your vessel is too large it will be unweighable. If your vessel is too small, the mass changes during fermentation will be negligible.
2. Prepare a 5% glucose solution. The volume of solution you prepare is determined by the size of the fermenting vessel. Add 1 g of dried yeast to the glucose solution and mix thoroughly. Add a pinch of Na2HPO4 or NaH2PO4 as a yeast nutrient.
3. Should your fermentation vessel be tightly capped? Study the fermentation equations given on the following pages for technical guidance.
4. How often should you monitor and record the changing mass during fermentation? Should you plot a graph of mass versus time? Note: As a rough estimate, your fermentation will continue for at least 24 hours.
View videos:
TASK 4.5.2
4.6 investigate the products of the oxidation of primary and secondary alcohols
Oxidation is an important chemical reaction for alcohols as it can be used to help us identify whether an alcohol is a primary, secondary or tertiary alcohol.
This is the same sort of oxidation we looked at in Redox.
We need to select a strong oxidising agent, such as potassium permanganate or potassium dichromate. If we acidify these solutions, we can write both the oxidation and reduction half reactions which are taking place (remember a strong oxidant CAUSES oxidation (electron loss), so is reduced itself (gains electrons). In the two examples below, potassium is a spectator ion and does not take part in the reaction.
8H+(aq) + MnO4-(aq) + 5e- → 4H2O(l) + Mn2+(aq)
14H+(aq) + Cr2O72-(aq) + 6e- → 7H2O(l) + 2Cr3+(aq)
Oxidation of Primary Alcohols
Depending on the quantity of the oxidising agent, primary alcohols may go through more than one oxidation step.
The first step is the formation of an alkanal. Ethanol has been chosen as an example with the permanganate ion as the oxidising agent.
5C2H5OH(aq) + 6H+(aq) + 2MnO4-(aq) → 5C2H4O(aq) + 2Mn2+(aq) + 8H2O(l)
I can investigate the products of the oxidation of primary and secondary alcohols.
The second step is the formation of an alkanoic acid from the alkanal. Ethanal has been chosen as an intermediate with the permanganate ion again acting as the oxidising agent.
5C2H4O(aq) + 6H+(aq) + 2MnO4-(aq) → 5CH3COOH(aq) + 2Mn2+(aq) + 3H2O(l)
NB. If the primary alcohol is methanol, this could further oxidise to carbon dioxide and water.
Oxidation of Secondary Alcohols
Secondary alkanols can be oxidised by the same oxidising agents to form a ketone. Propan2-ol has been chosen as an example with the permanganate ion as the oxidising agent.
5C3H7OH(aq) + 6H+(aq) + 2MnO4-(aq) → 5C3H6O(aq) + 2Mn2+(aq) + 8H2O(l)
Oxidation of Tertiary Alcohols
Tertiary alcohols do not undergo oxidation. This is a very useful, negative, test often used in their identification.
View videos:
TASK 4.6.1
Repeat for KMnO4 if the half reaction is
MnO4 – + 8H+ + 5e– gives Mn2+ + 4H2O 1.51 V
TASK 4.6.2
4.7 compare and contrast fuels from organic sources to biofuels, including ethanol
Our reliance on fuels has enormous implications both economically and environmentally. The current reliance on organic sources - fossil fuels (those fractions extracted from coal, crude oil and natural gas) is having a major impact economically (cost and control) and environmentally (high carbon dioxide emissions and global warming).
The need for alternative souces of fuels instead of the compounds presently obtained from the petrochemicals
- Renewable Resource: A resource that can be regenerated and hence can last indefinitely (eg sunlight = solar energy).
- Non-renewable Resource: A resource that cannot be regenerated and hence cannot last indefinitely (eg fossil fuels)
- Petroleum is a non-renewable resource and the earth’s supply of this substance will eventually be consumed.
- Projections predict that the world’s supply of petroleum will only extend into the early decades of this century, although this will depend upon changes in the rate of consumption.
- The world is heavily dependent on compounds obtained from the petrochemical industry (notably for fuels and plastics), and when the world’s supply has been exhausted, alternative sources of these compounds will be required.
- Alternative sources that are renewable would be favourable as their use would prevent:
- facing the same problem of exhaustion that petroleum will be facing soon.
- causing the release of more carbon into the carbon pool in the form of carbon dioxide (which causes climate change)
· compare and contrast fuels from organic sources to biofuels, including ethanol
1. Complete a table to identify 3 advantages and 3 disadvantages associated with the use of ethanol as an alternative fuel to petroleum. [12-14, 11/12-4]
2. Compare ethanol with octane per mole and per gram in terms of
a) oxygen requirements for complete combustion
b) energy production [12-14, 11/12-5]
3. Using (1) and (2), evaluate ethanol’s effectiveness as an alternative fuel to petroleum.[11/12-5, 11/12-7]
One idea to counter our heavy reliance on fossil fuels is to use biofuels. Bioenergy Australia defines biofuels as “liquid fuels that have been derived from other materials such as waste plant and animal matter”. (Bioenergy Australia (Forum) Ltd, 2016)
Ethanol
The natural plant polymer cellulose is a source of ethanol (via fermentation)
Sources of cellulose include:
- Specifically grown crops
- Agricultural residue
- Forestry residue
- Solid council waste
Alternative sources that are renewable could be favourable as their use would prevent the problem of a finite and exhaustible fuel that petroleum will soon be facing.
This is no new idea. Ethanol already appears in our petrol as E10 a 10% mix of ethanol and petrochemicals, and has done so for 30 years. However the proportion of ethanol has not increased in Australia since that time as car engines need significant modifications if they are to use higher ratios of ethanol. Despite the fact that this has been done in some parts of the world, it has and is not in Australia.
There are both advantages and disadvantages associated with the use of ethanol as an alternative fuel to petroleum.
Advantages
Unlike petroleum, ethanol is a renewable resource. Ethanol burns more cleanly in air than petroleum, producing less carbon (soot) and carbon monoxide. The use of ethanol as opposed to petroleum could reduce carbon dioxide emissions, provided that a renewable energy resource was used to produce crops required to obtain ethanol and to distil fermented ethanol.
Disadvantages
Ethanol has a lower heat of combustion (per mole, per unit of volume, and per unit of mass) than petroleum. Large amounts of arable land are required to produce the crops required to obtain ethanol, leading to problems such as soil erosion, deforestation, fertiliser run-off and salinity. Major environmental problems would arise out of the disposal of waste products from fermentation (called fermentation liquors). Typical current engines would require modification to use high concentrations of ethanol.
Q. What has prompted the need for the addition of ethanol?
Q. How does ethanol compare with octane in terms of oxygen requirements for complete combustion and energy production per mole and per gram?
View videos:
- OC#23 https://www.youtube.com/watch?v=jOH2bPS-6zA [8.40]
- Introduction https://www.youtube.com/watch?v=EwJLzjedZKU&index=21&list=PL49FAB756081270BA [3.52]
READ POM 2 Chemistry Contexts 2 Ch 2 https://drive.google.com/drive/u/1/folders/1CqBwsw9AA__fbPcDBxAtRqI7n4WXfwd7
VIEW PowerPoint (below)
View Prezzi
- Molar Heat of Combustion: The heat liberated when one mole of a certain compound undergoes complete combustion with oxygen at a constant pressure of exactly one atmosphere (100 kPa) and at 25°C with the final products being carbon dioxide gas and liquid water.
- Heats of combustion are quoted as positive numbers while the enthalpy changes of combustion reactions (ΔH) are quoted as negative numbers, as combustion reactions are always exothermic.
- Heats of combustion are typically stated in kilojoules per mole (kJ/mol. or kJ mol.-1).
Ethanol
- The accepted value for the molar heat of combustion of ethanol is 1360 kJ mol.-1.
- The following steps allow the calculation of an experimental value for the molar heat of combustion of ethanol:
- Measure and record the mass of a burner containing ethanol.
- Measure 100 mL of water into a beaker and measure the temperature of the water.
- Place the beaker of water directly above the burner and light it.
- Allow the burner to heat the water for one minute, then extinguish it.
- Immediately measure and record the mass of the burner and the temperature of the water.
- Calculate the change in mass and the change in temperature.
- Substitute the experimental results into the formula below to determine the enthalpy change:
ΔH = -mCΔT
where:
- ΔH = enthalpy change in joules
- m = mass of water
- C = thermal capacity (4.2 for water)
- ΔT = change in temperature in degrees Celsius
- Determine the number of moles of ethanol combusted (number of moles = mass/FM), and divide the enthalpy change in kilojoules by this number to determine the experimental value of the molar heat of combustion of ethanol in kilojoules per mole.
- Due to heat loss to surroundings, the experimental value of the molar heat of combustion of ethanol will be significantly lower than the accepted value.
- If the same experiment was conducted for another alkanol, such as methanol, and the difference between the experimental value and accepted value was found, this difference could be used to calibrate the experimental results for ethanol and produce a more accurate experimental result.
- The process described above can be applied to any alkanol, and can be modified slightly in order to find the heat of combustion in kilojoules per gram instead of kilojoules per mole.
Octane
Octane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH3(CH2)6CH3. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain. One of these isomers, 2,2,4-trimethylpentane (isooctane) is used as one of the standard values in the octane rating scale.
Octane is a component of gasoline (petrol). As with all low-molecular-weight hydrocarbons, octane is volatile and very flammable.
Standard enthalpy of combustion of octane is between −5.53 and−5.33 MJ mol−1
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