IB Chemistry HL 1.3 Energy from Fuels

[Maximum number: 2]

Urea, $\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}$ , is excreted by mammals and can be used as a fertilizer.

The combustion of urea produces water, carbon dioxide and nitrogen.

Formulate a balanced equation for the reaction.

$2\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}(\mathrm{~s})+3 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 4 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+2 \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{~N}_{2}(\mathrm{~g})$

[Maximum number: 1]

Ethanol is obtained by the hydration of ethene, $\mathrm{C}_{2} \mathrm{H}_{4}$ .

Suggest two possible products of the incomplete combustion of ethene that would not be formed by complete combustion.

[Maximum number: 5]

Ethanol is a biofuel that can be mixed with gasoline.

(a)

Write the equation for the complete combustion of ethanol.

[ 1 ]

$\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{l})+3 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{CO}_{2}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

(b)

Outline the evidence that relates global warming to increasing concentrations of greenhouse gases in the atmosphere.

[ 2 ]

Any two of:

«showing strong» correlation between «atmospheric» $\ce{CO2}$ concentration/greenhouse gas concentration and average «global/surface/ocean» temperature

lab evidence that greenhouse gases/ $\mathrm{CO}_{2}$ absorb(s) infrared radiation «advanced» computer modelling

ice core data

tree ring data

ocean sediments / coral reefs / sedimentary rocks data

Do not accept "global warming" for "average temperature".

Do not accept "traps/reflects heat" OR "thermal energy".

Evidence must be outlined and connected to data.

Accept references to other valid greenhouse gases other than carbon dioxide/ $\mathrm{CO}_{2}$ , such as methane/ $\mathrm{CH}_{4}$ or nitrous oxide/ $\mathrm{N}_{2} \mathrm{O}$ .

(c)

Explain, including a suitable equation, why biofuels are considered to be carbon neutral.

[ 2 ]

biofuel raw material/sugar/glucose formed by photosynthesis

OR

biofuel raw material/sugar/glucose uses up carbon dioxide during its formation OR

biofuel from capturing gases due to decaying organic matter formed from photosynthesis

$6 \mathrm{CO}_{2}(\mathrm{~g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{aq})+6 \mathrm{O}_{2}(\mathrm{~g})$

Accept arguments based on material coming from plant sources consuming carbon dioxide/carbon for M1.

[Maximum number: 2]

Ethanol is a biofuel that can be mixed with gasoline.

Outline the evidence that relates global warming to increasing concentrations of greenhouse gases in the atmosphere.

Any two of:

«showing strong» correlation between «atmospheric» $\ce{CO2}$ concentration/greenhouse gas concentration and average «global/surface/ocean» temperature

lab evidence that greenhouse gases/ $\mathrm{CO}_{2}$ absorb(s) infrared radiation «advanced» computer modelling

ice core data

tree ring data

ocean sediments / coral reefs / sedimentary rocks data

Do not accept "global warming" for "average temperature".

Do not accept "traps/reflects heat" OR "thermal energy".

Evidence must be outlined and connected to data.

Accept references to other valid greenhouse gases other than carbon dioxide/ $\mathrm{CO}_{2}$ , such as methane/ $\mathrm{CH}_{4}$ or nitrous oxide/ $\mathrm{N}_{2} \mathrm{O}$ .

[Maximum number: 2]

Ethanol is a biofuel that can be mixed with gasoline.

Explain, including a suitable equation, why biofuels are considered to be carbon neutral.

biofuel raw material/sugar/glucose formed by photosynthesis

OR

biofuel raw material/sugar/glucose uses up carbon dioxide during its formation OR

biofuel from capturing gases due to decaying organic matter formed from photosynthesis

$6 \mathrm{CO}_{2}(\mathrm{~g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{aq})+6 \mathrm{O}_{2}(\mathrm{~g})$

Accept arguments based on material coming from plant sources consuming carbon dioxide/carbon for M1.

[Maximum number: 5]

As well as being burnt, methanol can also be used to provide electricity through a fuel cell. A schematic diagram of such a fuel cell, that depends on the transfer of hydrogen ions between the electrodes, is shown below.

(a)

Deduce half-equations for the reactions at the two electrodes and hence the equation for the overall reaction.

Anode (negative electrode):

Cathode (positive electrode):

Overall:

[ 3 ]

Anode:
$\mathrm{CH}_{3}\mathrm{OH}(aq)+\mathrm{H}_{2}\mathrm{O}(l) \rightarrow \mathrm{CO}_{2}(aq)+6\mathrm{H}^{+}(aq)+6e^{-}$

Cathode:
$\mathrm{O}_{2}(aq)+4\mathrm{H}^{+}(aq)+4e^{-} \rightarrow 2\mathrm{H}_{2}\mathrm{O}(l)$

Overall:
$2\mathrm{CH}_{3}\mathrm{OH}(aq)+3\mathrm{O}_{2}(g) \rightarrow 2\mathrm{CO}_{2}(aq)+4\mathrm{H}_{2}\mathrm{O}(l)$

(b)

Identify one factor that affects the voltage of a cell and a different factor that affects the current it can deliver.

Voltage:

Current:

[ 2 ]

[Maximum number: 1]

Nitrous acid, $\mathrm{HNO}_{2}$ , is a weak acid which can be used to make acidic buffers.

The overall reaction for the synthesis of ethyl ethanoate from ethane is:

2 \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})+\frac{3}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{~g})+2 \mathrm{HCl}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

Calculate the standard entropy change, $\Delta S^{\theta}{ }_{r}$ , in $\mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ , for this reaction. Use section 13 of the data booklet and $\mathrm{S}^{\theta}$ of ethyl ethanoate $=259 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ .

« 70+259-(161+160)=» 8 « $\mathrm{J} \mathrm{K}^{-1} \mathrm{~mol}^{-1}$ »

[Maximum number: 2]

Nitrous acid, $\mathrm{HNO}_{2}$ , is a weak acid which can be used to make acidic buffers.

The overall reaction for the synthesis of ethyl ethanoate from ethane is:

2 \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})+\frac{3}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{~g})+2 \mathrm{HCl}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

Calculate the Gibbs energy change, $\Delta G^{\theta}$ in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for this reaction under standard conditions. Use the value of $-4 \mathrm{~kJ} \mathrm{~mol}^{-1}$ for $\Delta H^{\theta}{ }_{\mathrm{r}}$ and your answer from (d)(iv). If you did not obtain an answer for (d)(iv) use $-10 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ , although this is not the correct answer. Use sections 1 and 4 of the data booklet.

conversion to common units
$\begin{aligned} \left\langle\Delta G^{\theta}=-4 \mathrm{~kJ} \mathrm{~mol}^{-1}-298 \mathrm{~K}^{-} 0.008 \mathrm{~kJ} \mathrm{~K}^{-1} \mathrm{~mol}^{-1} »\right. -6.4 « \mathrm{~kJ} \mathrm{~mol}^{-1} » \text { ↓ } \end{aligned}$

M1 is for conversion to common units M2 is for correct value.

Award [2] for correct final answer. If $-10 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$ is used, the answer will be -1.0 « $\mathrm{kJ} \mathrm{mol}^{-1}$ ».

[Maximum number: 2]

Nitrous acid, $\mathrm{HNO}_{2}$ , is a weak acid which can be used to make acidic buffers.

The overall reaction for the synthesis of ethyl ethanoate from ethane is:

2 \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})+\frac{3}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{~g})+2 \mathrm{HCl}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

Explain the effect, if any, on the spontaneity of this reaction if the temperature was increased. Use section 1 of the data booklet.

the $-\mathrm{T} \Delta S$ expression becomes more negative / so Gibbs energy/ $\Delta \mathrm{G}$ becomes more negative

increase in spontaneity «as temperature is increased»

Marking guidance:

Accept feasibility in place of spontaneity.

If $-10 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$ is used, the answer will be opposite (decrease in spontaneity, -TAS becomes more positive, $\Delta G$ becomes less negative).

Do not accept "spontaneous" without reference to increases.

[Maximum number: 2]

Sulfur trioxide is an important compound in industry.

$\quad \mathrm{SO}_{3}(\mathrm{~g})$ is made using the contact process.

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad \Delta H^{\ominus}<0

Calculate the value of the equilibrium constant for the formation of $\mathrm{SO}_{3}(\mathrm{~g})$ at 773 K . Use sections 1 and 2 of the data booklet. If you did not obtain an answer to (d)(viii), use $-25.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ , although this is not the correct answer.

$\begin{aligned} <-25.8 \times 10^{3}=-8.31 \times 773 \times \ln K> \ln K=<\frac{-25.8 \times 10^{3}}{-8.31 \times 773}=-4.016 K=55.5 \end{aligned}$

Award [2] for correct final answer.

Accept answers in the range 55-60.

Award [2] for K=49.0 if $-25.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ was used.

IB Chemistry HL 1.3 Energy From Fuels Questions

Exam points

Covered syllabus points

Covered syllabus points

Question 1

Question 2

Question 3

Question (a)

Question (b)

Question (c)

Question 4

Question 5

Question 6

Question (a)

Question (b)

Question 7

Question 8

Question 9

Question 10