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CHEMISTRY, M7 2016 VCE 24 MC

Methanol is a liquid fuel that is often used in racing cars. The thermochemical equation for its complete combustion is

\(\ce{2CH3OH(l) + 3O2(g)\rightarrow 2CO2(g) + 4H2O(l) \quad \quad \ \ \ \Delta H = –1450 kJ mol^{–1}}\)

Octane is a principal constituent of petrol, which is used in many motor vehicles. The thermochemical equation for
the complete combustion of octane is

\(\ce{2C8H18(l) + 25O2(g)\rightarrow 16CO2(g) + 18H2O(l) \quad \quad \Delta H = –10\ 900 kJ mol^{-1}}\)

The molar mass of methanol is 32 g mol\(^{-1}\) and the molar mass of octane is 114 g mol\(^{–1}\). Which one of the following statements is the most correct?

  1. Burning just 1.0 g of octane releases almost 96 kJ of heat energy.
  2. Burning just 1.0 g of methanol releases almost 23 kJ of heat energy.
  3. Octane releases almost eight times more energy per kilogram than methanol.
  4. The heat energy released by methanol will not be affected if the oxygen supply is limited.
Show Answers Only

\(B\)

Show Worked Solution

Consider option B:

→ 1 mole of \(\ce{CH3OH}\) produces 725 kJ of heat energy

→ \(\ce{MM (CH3OH)} = 32.0\ \text{grams}\)

→ Heat energy of 1 gram \(\ce{CH3OH} = \dfrac{725}{32.0} = 22.7\) kJ

\(\Rightarrow B\)

♦ Mean mark 40%.

CHEMISTRY, M7 2023 HSC 15 MC

The table gives the heat of combustion of three different alcohols at 25°C.
 

\( Alcohol \) \( Heat \ of \ combustion \)
\( \text{(Kj  g} ^{-1}) \)
\( \text{Methanol} \) \(22.68\)
\( \text{Ethanol} \) \(29.67\)
\( \text{Butan-1-ol} \) \(36.11\)
 
Which of the following gives the best approximation for the molar heat of combustion of propan-1-ol, expressed in kJ g\(^{-1} \)?
 
  1. \(\left(\dfrac{22.68+29.67+36.11}{3}\right)\)
  2. \(\left(\dfrac{29.67+36.11}{2}\right)\)
  3. \(\left(\dfrac{22.68+29.67}{2}\right)\)
  4. \(\left(\dfrac{3 \times 36.11}{4}\right)\)
Show Answers Only

\(B\)

Show Worked Solution

→ The boiling point of straight-chained alcohols increases with their chain length.

→ Therefore the boiling point of Propan-1-ol will exist between Ethanol and Butan-1-ol

\(\Rightarrow B\)

CHEMISTRY, M7 2023 HSC 25

A student used the apparatus shown to investigate the combustion of octan-1-ol.
 

 
The following results were obtained by the student.

Mass of water heated = 205 g
Initial temperature of water = 23.7°C
Final temperature of water = 60.4°C

The following data are given.

Molar enthalpy of combustion of octan-1-ol = – 5294 kJ mol\(^{-1} \)
Molar mass of octan-1-ol = 130.23 g kJ mol\(^{-1}\)
  1. Assuming that no energy released by this combustion is lost to the surroundings, calculate the mass of octan-1-ol burnt.  (3 marks)

  1. Explain ONE advantage of using a biofuel compared to fossil fuels.  (2 marks)
Show Answers Only

a.   0.774 grams

b.    Advantage of biofuel vs fossil fuel

→ Combustion of biofuels derived from plants will have a lower greenhouse impact as the carbon dioxide released during combustion will replace that used in photosynthesis, unlike fossil fuels.

Other answers could include:

→ Biofuels are biodegradable and therefore pose a much reduced environmental threat than fossil fuels which are not.

→ Biofuels are more sustainable than fossil fuels as they are produced from renewable resources.

Show Worked Solution

a.   Find the heat absorbed by the water ((\q\)):

\(q\) \(=mc \Delta T \)  
  \(=205 \times 4.18 \times 36.7 \)  
  \(=31\ 488.23\ \text{J}\)  
  \(=31.448\ \text{kJ}\)  

 
\(\text{octan-1-ol}\ \Rightarrow \ce{C8H18O} \)

\(\ce{MM(C8H18O) = 12.01 \times 8 + 1.008 \times 18 + 16 = 130.224} \)

\(\ce{n\text{(octan-1-ol)}}\) \(= \dfrac{-31.448\ \text{kJ}}{-5294\ \text{kJ mol}^{-1}} \)  
  \(=5.94 \times 10^{-3}\ \text{mol} \)  

 

\(\ce{m\text{(octan-1-ol)}}\) \(= 5.94 \times 10^{-3} \times 130.224 \)  
  \(= 0.774\ \text{g} \)  

 
b.    Advantage of biofuel vs fossil fuel

→ Combustion of biofuels derived from plants will have a lower greenhouse impact as the carbon dioxide released during combustion will replace that used in photosynthesis, unlike fossil fuels.

Other answers could include:

→ Biofuels are biodegradable and therefore pose a much reduced environmental threat than fossil fuels which are not.

→ Biofuels are more sustainable than fossil fuels as they are produced from renewable resources.

CHEMISTRY, M7 EQ-Bank 22

Calculate the mass of methanol that must be burnt to increase the temperature of 325 g of water by 65°C, if exactly half of the heat released by this combustion is lost to the surroundings.

The heat of combustion of methanol is 726 kJ mol ¯1.  (3 marks)

Show Answers Only

7.79 g

Show Worked Solution

\(\ce{Using $q = mc\Delta T$}\):

\(\ce{$q$ = 325 \times 10^{-3} \times 4.18 \times 10^{3} \times 65 = 88 302.5 J = 88.3025 kJ}\)

\(\ce{Methanol heat combustion = 726 kJ mol^{-1} (given)}\)

\[\ce{n(CH3(OH)) = \frac{88.3025}{726} }\]

\(\ce{MM(CH3(OH)) = 12.01 + 3 \times 1.008 + 16 + 1.008 = 32.042 }\)

\[\ce{m(CH3(OH)) = \frac{88.3025}{726} \times 32.042 }\]

\(\text{Since 50% of heat is lost → twice as much methanol is needed}\)

\[\ce{m(CH3(OH)) initial = 2 \times \frac{88.3025}{726} \times 32.042 = 7.79 g} \]

CHEMISTRY, M7 2016 HSC 23

A spirit burner containing ethanol was used to heat water in a conical flask for three minutes to measure the molar heat of combustion of ethanol.

The results from the investigation are shown.
 

  1. On the grid, draw a line graph to represent the data contained in the table.   (3 marks)
     

  1. The following values were also recorded during the investigation:
  2.       Initial mass of spirit burner = 236.14 g
  3.       Final mass of spirit burner = 235.56 g
  4.       Calculated experimental molar heat of combustion of ethanol = – 827 kJ mol ¯1.
  5. Using information from the previous page and the above values, determine the mass of water that was in the conical flask.   (3 marks)

Show Answers Only

a.   
     

b.  0.199 kg

Show Worked Solution

a.   


 

b.   Using the graph:

\(\triangle T = 31.0-18.5=12.5°\text{C} \)

\( \text{m(Ethanol burnt)} = 236.14-235.56=0.58\ \text{g} \)

\[ \ce{n(Ethanol) = \frac{m}{MM} = \frac{0.58}{2 \times 12.01 + 6 \times 1.008 + 16.00} = 0.0126 mol} \]

\(\ce{Energy released ($q$) = 0.0126 \times 827 = 10.420 kJ}\)

\[q = -mC\triangle T \ \ =>\ \ m= -\frac{q}{C\triangle T} \]

`:.m\text{(water)}` `=(10\ 420)/(4.18 xx 10^3 xx 12.5)` `=0.00019927\ text{g}`
  `=0.199\ text{kg}`  

Mean mark (b) 53%.

CHEMISTRY, M7 2015 HSC 20 MC

The table shows the heat of combustion of four straight chain alkanols.
 

What is the mass of water that could be heated from 20°C to 45°C by the complete combustion of 1.0 g of heptan-1-ol?

  1. 0.032 kg
  2. 0.044 kg
  3. 0.36 kg
  4. 0.38 kg
Show Answers Only

`D`

Show Worked Solution

`q=mC DeltaT \ \ =>\ \ m=q/(C DeltaT)`

`m` `=(4638 xx 10^3)/((4.18 xx 10^3) xx (45-20))`  
  `=44.38\ text{J per 116.2 g heptan-1-ol}`  
  `=44.38/116.2`  
  `=0.38\ text{kg}`  

 
`=>D`


♦ Mean mark 51%.

CHEMISTRY, M7 2017 HSC 28b

The molar heat of combustion \(\left(\Delta H_c\right)\) for ethanol is 1360 kJ mol ¯1.

Calculate the energy generated per kg of \(\ce{CO2}\) released by the combustion of ethanol.   (3 marks)

Show Answers Only

`15\ 450\ text{kJ}`

Show Worked Solution

\(\ce{C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(g)} \)

\(\ce{m(CO2) = 12.01 + 2 \times 16.00 = 44.01 g mol^{-1}}\)

\[\ce{n(CO2) = \frac{1000}{44.01} = 22.72 mol}\]

\[\ce{n(C2H5OH) = \frac{22.72}{2} = 11.36 mol}\]

`:.\ text{Energy per kg CO}_2` `=11.36 xx 1360`  
  `=15\ 450\ text{kJ}`  

♦ Mean mark 48%.

CHEMISTRY, M7 2015 HSC 10 MC

Which of the equations correctly describes incomplete combustion?

  1. `\text{C}_2\text{H}_5\text{OH}(l)` + `2\text{O}_2(g)` `\rarr`  `2\text{CO}(g)` + `3\text{H}_2\text{O}(l)`
  2. `\text{C}_2 \text{H}_5\text{OH}(l)` + `(7)/(2)\text{O}_2(g)` `\rarr` `2\text{CO}_2(g)` + `3\text{H}_2\text{O}(l)`
  3. `\text{C}_2\text{H}_5\text{OH}(l)` + `3\text{O}_2(g)` `\rarr` `2\text{CO}_2(g)` + `3\text{H}_2\text{O}(l)`
  4. `\text{C}_2\text{H}_5\text{OH}(l)` + `2\text{O}_2(g)` `\rarr` `\text{C}(s)` + `\text{CO}(g)` + `3\text{H}_2\text{O}(l)`
Show Answers Only

`A`

Show Worked Solution

→ Incomplete combustion produces carbon and/or carbon monoxide (eliminate B and C).

→ Option D is not balanced (oxygen atoms do not equate)

`=>A`


♦ Mean mark 44%.

CHEMISTRY, M7 2017 HSC 9 MC

The following equipment was set up to measure the heat of combustion of an alkanol.
 

   
  

Black deposits were observed on the bottom of the conical flask and the heat of combustion measured was lower than the theoretical value.

Which of the following equations could account for these observations?

  1. `2 text{C}_(2) text{H}_(6)(g)+7 text{O}_(2)(g) rarr 4 text{CO}_(2)(g)+ 6 text{H}_(2)text{O}(g)`
  2. `text{C}_(3) text{H}_(8) text{O}(g)+4 text{O}_(2)(g) rarr text{CO}_(2)(g)+ text{CO}(g)+4 text{H}_(2) text{O}(g)`
  3. `2 text{C}_(4) text{H}_(10) text{O} (g)+3 text{O}_(2)(g) rarr 8 text{C}(s)+2 text{H}_(2)(g)+8 text{H}_(2) text{O}(g)`
  4. `2 text{C}_(2) text{H}_(6) text{O}(g)+4 text{O}_(2)(g) rarr 2 text{CO}_(2)(g)+2 text{C}(s)+6 text{H}_(2) text{O}(g)`
Show Answers Only

`D`

Show Worked Solution

The black deposit is carbon (soot).

By Elimination:

A: Ethane is not an alkanol (eliminate A).

B: No soot is formed in this reaction (eliminate B).

C: This reaction is incorrect as no hydrogen gas is produced (eliminate C).

`=>D`

CHEMISTRY, M7 2019 HSC 23

The following apparatus was used in an experiment to determine the molar enthalpy of combustion of ethanol.
 


 

  1. Calculate the experimental molar enthalpy of combustion `(Delta_(c) H)` of ethanol when 0.370 g ethanol was used to raise the water temperature from 18.5°C to 30.0°C.   (4 marks)

  2. Upon replication, the molar enthalpy of combustion obtained in the experiment was consistently much lower than the accepted value.
  3. Explain ONE change that could be made to the experiment that would improve the accuracy of the obtained value.   (2 marks)

Show Answers Only

a.  `-628\ text{kJ mol}^(-1)`

b.   Improvement to experiment accuracy (one of many possible answers):

→ The molar enthalpy of combustion measured in the experiment was not very accurate due to heat being lost to the surroundings.

→ To improve the accuracy, it would be helpful to move the spirit burner closer to the beaker to reduce heat loss.

Show Worked Solution
a.    `text{n(ethanol)}` `= text{m}/text{MM}`
    `=0.370/46.068`
    `=0.008032\ text{mol}`

 
`q=mC DeltaT = 105\ text{g} xx 4.18\ text{J g}^(-1)\ text{K}^(-1) xx (30-18.5)\ text{K} = 5047.35\ text{J}`

`(Delta_(c) H)` `=-q/n`  
  `=- 5047.35/0.008032`  
  `=-628\ 405\ text{J mol}^(-1)`  
  `=-628\ text{kJ mol}^(-1)\ \ text{(3 sig fig)}`  

 

b.   Improvement to experiment accuracy (one of many possible answers):

→ The molar enthalpy of combustion measured in the experiment was not very accurate due to heat being lost to the surroundings.

→ To improve the accuracy, it would be helpful to move the spirit burner closer to the beaker to reduce heat loss.

CHEMISTRY, M7 2020 HSC 24

Biodiesel, an alternative fuel to diesel, may be produced from vegetable oil. The chemical reaction which converts oils from biomass into biodiesel is shown. `text{R}_1`, `text{R}_2` and `text{R}_3` are alkyl chains which may vary from 10 to 22 carbons in length.
  


  

  1. Which functional group is present in both the oil and the biodiesel?   (1 mark)

  2. Explain why biodiesel `(text{C}_14 text{H}_30 text{O}_2)` produces less soot than diesel `(text{C}_18 text{H}_38)` when combusted under the same conditions. Support your answer with balanced chemical equations.   (3 marks)

  3. The energy densities of biodiesel and diesel are 38 MJ kg ¯1 and 43 MJ kg ¯1 respectively. The densities of biodiesel and diesel are 0.90 kg L¯1 and 0.83 kg L¯1 respectively.

    When 60.0 L of diesel is combusted in a typical engine, 2141 MJ of energy is released.

    What volume of biodiesel would be required to produce the same amount of energy?   (2 marks)

  4. Explain TWO advantages and TWO disadvantages of using bioethanol (ethanol produced from biomass) as an alternative to a fossil fuel.    (4 marks)

Show Answers Only

a.   Ester functional group.
 

b.   \( \ce{C14H30O2(l) + 41/2 O2(g) → 14 CO2(g) + 15 H2O(l)}\)

\( \ce{C18H38(l) + 55/2 O2(g) → 18 CO2(g) + 19 H2O(l)}\)

→ Soot is produced when a fuel source undergoes incomplete combustion due to insufficient oxygen.

→ Since biodiesel already contains oxygen atoms within its structure, it would require less oxygen to undergo complete combustion compared to diesel, and is therefore less likely to produce soot.
 

c.   `text{m(biodiesel)} = 2141 / 38 = 56.3\ text{kg}`

`text{V(biodiesel)} = 56.3 / 0.90 = 63\ text{L  (nearest L)}`
 

d.    Advantages of bioethanol (two examples needed only):

→ Bioethanol is sustainable because it is produced from renewable resources, whereas petrol is produced from nonrenewable crude oil reserves.

→ Additionally, bioethanol is biodegradable whereas petrol isn’t. As a result, bioethanol would pose less of an environmental threat in comparison to petrol.

→ Ethanol produces less airborne particulates that are associated with lung cancer.
 

Disadvantages of bioethanol (two examples needed only):

→ Bioethanol requires a large amount of arable land in order to grow crops to produce bioethanol. Thus, it would lead to soil erosion and environmental pollution.

→ More energy is also required to produce bioethanol because of the requirement for labour, fertilisation, and distillation of ethanol from fermentation.

→ If fossil fuels are used as the energy source within the manufacturing process of biofuel, it will not achieve carbon neutrality and will contribute to global warming.

Show Worked Solution

a.   Ester functional group.
 

b.   \( \ce{C14H30O2(l) + 41/2 O2(g) → 14 CO2(g) + 15 H2O(l)}\)

\( \ce{C18H38(l) + 55/2 O2(g) → 18 CO2(g) + 19 H2O(l)}\)

→ Soot is produced when a fuel source undergoes incomplete combustion due to insufficient oxygen.

→ Since biodiesel already contains oxygen atoms within its structure, it would require less oxygen to undergo complete combustion compared to diesel, and is therefore less likely to produce soot.


♦♦ Mean mark (a) 39%, (b) 49%.

c.   `text{m(biodiesel)} = 2141 / 38 = 56.3\ text{kg}`

`text{V(biodiesel)} = 56.3 / 0.90 = 63\ text{L  (nearest L)}`

d.    Advantages of bioethanol (two examples needed only):

→ Bioethanol is sustainable because it is produced from renewable resources, whereas petrol is produced from nonrenewable crude oil reserves.

→ Additionally, bioethanol is biodegradable whereas petrol isn’t. As a result, bioethanol would pose less of an environmental threat in comparison to petrol.

→ Ethanol produces less airborne particulates that are associated with lung cancer.
 

Disadvantages of bioethanol (two examples needed only):

→ Bioethanol requires a large amount of arable land in order to grow crops to produce bioethanol. Thus, it would lead to soil erosion and environmental pollution.

→ More energy is also required to produce bioethanol because of the requirement for labour, fertilisation, and distillation of ethanol from fermentation.

→ If fossil fuels are used as the energy source within the manufacturing process of biofuel, it will not achieve carbon neutrality and will contribute to global warming.


♦ Mean mark (d) 55%.

CHEMISTRY, M7 2022 HSC 29

The enthalpies of combustion of four alcohols were determined in a school laboratory.

The results are shown in the table.
 

  1. Plot the results, including a curved line of best fit, to estimate the enthalpy of combustion of butan-1-ol.   (3 marks)

  1. The published value for the enthalpy of combustion of pentan-1-ol is closer to `-3331\ text{kJ}\ text{mol}^(-1)`.
  2. Justify ONE possible reason for the difference between the school's results and published values.   (2 marks)
Show Answers Only

a.    

From interpolating the graph, the enthalpy of combustion of butan-1-ol is –2120 kJ mol ¯1.

b.   Heat loss to the surroundings.

→ The school’s results are lower in magnitude than the published values because heat is lost to the surroundings, making the measured change in temperature smaller.

Other possible answers:

→ Incomplete combustion

→ Temperature change will be reduced if combustion is incomplete as less heat is released. Enthalpy of combustion will be lower as a result.

Show Worked Solution

a.    

From interpolating the graph, the enthalpy of combustion of butan-1-ol is –2120 kJ mol ¯1.

b.   Heat loss to the surroundings.

→ The school’s results are lower in magnitude than the published values because heat is lost to the surroundings, making the measured change in temperature smaller.

Other possible answers:

→ Incomplete combustion

→ Temperature change will be reduced if combustion is incomplete as less heat is released. Enthalpy of combustion will be lower as a result.


♦ Mean mark (b) 48%.