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Chemistry MS.

Organic Chemistry Enthalpy Of Combustion Of Alcohols ___

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Introduction Alcohols Alcohols are identified by the presence of a hydroxyl group or an -OH attached to a saturated carbon atom. They are named by replacing the suffix -e, on the parent chain, with the suffix -ol. A general formula: CnH2n+1OH.

Alcohols are classified into three groups:

1. Primary a. OH group is attached to a carbon that is bonded to one alkyl group.

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2. Secondary a. OH group is attached to a carbon that is bonded to two alkyl groups.

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3. Tertiary a. Oh Group is attached to a carbon that is bonded to three alkyl groups.

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Boiling Points The presence of a hydroxyl group means alcohols will have hydrogen bonding. Thus, alcohols will have higher boiling and melting points than hydrocarbons of similar molecular weights. Egalkanes. The boiling point increases as molecular weight increases. As Intramolecular forces are stronger.

Solubility The presence of a hydroxyl group means there is a (delta +) H and an (delta -) O atom. Therefore alcohols are polar molecules and hence are soluble in water. The solubility decreases as molecular weight increases. As the molecule becomes more non-polar.

Volatility Volatility is a measure of how easily a liquid or solid evaporates into a gas. The opposite trend to boiling point, as stronger intermolecular forces prevent alcohols from evaporating. The volatility decreases as molecular weight increases. As intramolecular forces are weaker.

Alcohols Reactions ●

Combustion ○

Complete combustion occurs when all of the products are either water or CO2. Incomplete combustion occurs when some of the products include other substances such as soot or carbon monoxide. Incomplete combustion will occur when there isn’t enough oxygen in the reaction. To combust anything, you need to overcome some initial activation energy, typically by exposing the alcohol to a flame. ■

Alcohol + CO2 → CO2 + water

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Enthalpy of combustion The standard enthalpy change of combustion ( △H^Өc) is the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions. 1 This can also be referenced as “heat of combustion”. The enthalpy of combustion is usually measured at conditions 298 K (25 C) and 101.3 kPa. This is because water will be a liquid under these conditions. The enthalpies of combustion are measured using a calorimeter, the energy that is released is calculated by n x △Hc.

Specific Heat Capacity The specific heat capacity is the amount of energy that is required to raise the temperature of one gram of a substance by 1℃. The higher the specific heat capacity, the effectiveness of a material's ability to store heat energy will increase. The heat capacity of alcohol is very high. This is due to the hydrogen bonding between its molecules. Any heat change by a substance during a chemical reaction can be calculated by: ●

q = m x C x △T ○

q is Energy, measured in Joules

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m is Mass, measured in grams

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C is the specific heat capacity, measured in Jg-1oC-1

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△ is the change of

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T is the temperature, measured in degrees celsius

To determine the enthalpy of combustion Converting the chemical energy produced by combustion to thermal energy. The thermal energy is released by a quantity of fuel that can be used to heat a measured volume of water. The change in the water temperature is used to determine the approximate amount of energy released as fuel.

Inquiry Question, Aim, Hypothesis 1. Inquiry Question Does the amount of carbons affect the enthalpy change in hydrocarbons?

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Description of the enthalpy of combustion given by :https://www.youtube.com/watch?v=wZYOGgc25yQ

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2. Aim To measure and compare the change in the enthalpy of combustion of different alcohols.

3. Hypothesis The higher the number of carbon is in the hydrocarbon chain, the greater the enthalpy change of combustion of the primary alcohol. Therefore, butan-1-ol will have a greater change in enthalpy, as it has the highest number of carbons in the hydrocarbon chain out of all the alcohols used in the experiment.

Experiment Aim To measure and compare the change in the enthalpy of combustion of different alcohols.

Material

Spirit burner

Electronic balance

Propan-1-ol

Aluminium can

Measuring Cylinder

Thermometer

Retort stand (clamp and bosshead)

Bench mat

Ruler

Stopwatch

Ethanol

Methanol

Butan-1-ol

Safety Glasses

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Risk Assessment

Hazards

Nature of hazards and risks

Control Measure

Aluminium can

Hot to touch

Avoid direct contact with the can after the experiment has been conducted, use tongs to pick up.

Alcohols

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Naked flame

Highly flammable Toxic to body and may cause irritation to skin and eyes when in contact May cause headaches, dizziness or nausea when vapour is inhaled for a prolonged period of time.

High temperature, may burn skin. Equipment may catch fire.

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Stay a safe distance from burning alcohol Wear required ppe, safety glasses Wash affected area with running water, if contact is made with burning alcohol Keep room well ventilated

Keep a safe distance away from the flame. Keep the workspace clear, and safely away from naked flame.

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Independent Variable: The four different types of alcohol.

Dependent Variable: The change in enthalpy of combustion of alcohols.

Controlled Variable: The distance between the wick and the beaker (0.5cm), Volume of water used (50mL).

Procedure Using a retort stand and clamp, set up an aluminium can 0.5-1 cm above the wick of a spirit

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burner. Pour 200mL of water into the can and record its temp. Weigh the spirit burner filled with alcohol. Record mass. Light the burner and heat the water. Stir continuously. When the water temperature rose 20 degrees, extinguish the burner and record the highest temperature reached. Measure and record the mass of the burner and unburnt alcohol. Use this info to deduce the mass of alcohol consumed. Photos:

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Results

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Calculations: ●

Energy absorbed in the can: ○

ΔH= McΔT (H2O) + McΔT (Al) = (50g x 4.18 x 20) + (16.08 x 0.90 x 20) =2379.44 J =2.4 KJ

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Molar heat of combustion (Example: Ethanol) n= m/MM = 0.5/32.04 = 0.01560549313 △Hcomb= △Htotal/n fuel = 4430.8/0.01560549313 = 283925.664

Discussion And Analysis Explanation: From the results, we can conclude that butan-1-ol emitted the highest enthalpy of combustion out of the four alcohols used within the experiment. This is due to the alcohol having a higher number of carbons within the hydrocarbon chain, therefore more energy is required to break down those bonds. The consequence of heat loss and incomplete combustion is that the temperature change as we measure and thus the enthalpies we calculate aren’t going to be as high as we expect in theory.

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Calorimetry always slightly underestimates the amount of energy released in a combustion reaction. Results remain valid when it comes to comparing the enthalpy of combustion of different alcohols as these errors were present for all the alcohols. Failure to maintain our controlled variables such as maintaining a constant distance for the flame and the can beaker may impact our ability to compare enthalpies of combustion. As the number of carbons increases, the enthalpy of combustion becomes more negative. A more negative enthalpy means more energy is released during the combustion of larger alcohols. A larger molecule has more bonds to break and so requires more energy. Other variables include the flame heating the can, the heat being lost to the surroundings, incomplete combustion resulting in reduced energy output, each of the spirit burner having a different wick, the mass of the water not being constant due to evaporation, and the aluminium can not having an airtight lid. The experiment is reliable, as each alcohol was tested three times, allowing any outliers to be determined, and if the experiment had to be repeated. The experiment is valid, as the human error was reduced as much as possible, by utilising the right equipment. The experiment was also deemed valid, as the aim was achieved. Whilst conducting the experiment there were some limitations that were faced, this includes the limited amount of resources available. Allowing only three repetitions of the experiment. One of the major limitations was the lack of alcohol variance, to further improve this inclusion of primary, secondary, and tertiary variants of alcohols are required. Improvements that can be made is the accuracy of the equipment used, this is mainly directed towards the calorimeter. Having access to a calorimeter that gives accurate internal measurements would greatly improve the experiment. Thus decreasing the experimental percentage of error between the theoretical delta h value and the experimental results. Further improvements can be made by minimizing the errors that were produced during the experiment, this includes having a sealed room, this helps by minimising the amount of heat energy lost in the surroundings. Having the same type of wick on each spirit burner, reduces the variance of the results. My hypothesis states that the higher the number of carbon atoms within the hydrocarbon chain, the greater the enthalpy change of the combustion of alcohols. This was supported throughout my results, being proven by butan-1-ol, producing the largest change in enthalpy of combustion, thus giving correctness to my hypothesis.

Conclusion To conclude, the experiment aimed to determine and compare the enthalpy of combustion and the enthalpy change of several primary alcohols including methanol, butan-1-ol, propan-1-ol, and ethanol. My hypothesis stated that the higher the number of carbon is in the hydrocarbon chain, the greater the enthalpy change of combustion of the primary alcohol. This was done by measuring the standard enthalpy change that occurs when a range of different alcohols undergo combustion using an experimental technique called calorimetry. By conducting the experiment

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three times for each alcohol, the results produced by the experiment supports my hypothesis, as butan-1-ol has the highest change in enthalpy of combustion whilst methanol produces the smallest change. Thus showing a linear relationship between the change of enthalpy of combustion of alcohols and the number of carbon atoms within the hydrocarbon.

Resources Unknown, n.d. Comparing heat energy from burning alcohols. [online] RSC Education. Available at: [Accessed 28 May 2021]. Unknown, 2018. Effect of Carbon Atoms Number in Alcohol on Combustion. [online] AUEssays.com. Available at: [Accessed 30 May 2021]. Silove, J., 2020. HSC Chemistry complete course notes. 1st ed. InStudent Publishing Pty Ltd, pp.63, 64. Unknown, n.d. 1. What are alcohols? - Alcohol, carboxylic acid and esters. [online] Sites.google.com. Available at: [Accessed 31 May 2021]. Unknown, n.d. HSC Chemistry: Enthalpy of Combustion for Alcohols - Art Of Smart. [online] Art Of Smart. Available at: [Accessed 1 June 2021]. Unknown, n.d. Enthalpy of combustion - Chemical energy - Higher Chemistry Revision - BBC Bitesize. [online] BBC Bitesize. Available at: [Accessed 2 June 2021]. Youtube. 2021. Calculating enthalpy change of combustion. [online] Available at: [Accessed 3 June 2021]. Docbrown.info. 2021. Enthalpy of combustion of alcohols data trend graph explaining trend pattern determining delta H combustion advanced A level organic chemistry revision notes doc brown. [online] Available at: [Accessed 1 June 2021]. AUEssays.com. 2021. Effect of Carbon Atoms Number in Alcohol on Combustion. [online] Available at:

[Accessed 3 June 2021].

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