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</div><h2>SL Paper 2</h2><div class="specification">
<p class="p1">Consider the following equilibrium:</p>
<p class="p1">\[\begin{array}{*{20}{l}} {{\text{4N}}{{\text{H}}_3}{\text{(g)}} + {\text{5}}{{\text{O}}_2}{\text{(g)}} \rightleftharpoons {\text{4NO(g)}} + {\text{6}}{{\text{H}}_2}{\text{O(g)}}}&{\Delta {H^\Theta } = - 909{\text{ kJ}}} \end{array}\]</p>
</div>
<div class="specification">
<p class="p1">Nitrogen reacts with hydrogen to form ammonia in the Haber process, according to the following equilibrium.</p>
<p class="p1">\[\begin{array}{*{20}{l}} {{{\text{N}}_{\text{2}}}{\text{(g)}} + {\text{3}}{{\text{H}}_{\text{2}}}{\text{(g)}} \rightleftharpoons {\text{2N}}{{\text{H}}_{\text{3}}}{\text{(g)}}}&{\Delta {H^\Theta } = - 92.6{\text{ kJ}}} \end{array}\]</p>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Deduce the equilibrium constant expression, \({K_{\text{c}}}\), for the reaction.</p>
<div class="marks">[1]</div>
<div class="question_part_label">a.i.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Predict the direction in which the equilibrium will shift when the following changes occur.</p>
<p class="p1">The volume increases.</p>
<p class="p1">The temperature decreases.</p>
<p class="p1">\({{\text{H}}_{\text{2}}}{\text{O(g)}}\) is removed from the system.</p>
<p class="p1">A catalyst is added to the reaction mixture.</p>
<div class="marks">[4]</div>
<div class="question_part_label">a.ii.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Define the term <em>activation energy</em>, \({E_{\text{a}}}\).</p>
<div class="marks">[1]</div>
<div class="question_part_label">b.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Nitrogen monoxide, NO, is involved in the decomposition of ozone according to the following mechanism.</p>
<p class="p2">\[\begin{array}{*{20}{l}} {}&{{{\text{O}}_{\text{3}}} \to {{\text{O}}_{\text{2}}} + {\text{O}} \bullet } \\ {}&{{{\text{O}}_3} + {\text{NO}} \to {\text{N}}{{\text{O}}_2} + {{\text{O}}_2}} \\ {}&{{\text{N}}{{\text{O}}_2} + {\text{O}} \bullet \to {\text{NO}} + {{\text{O}}_2}} \\ {{\text{Overall:}}}&{{\text{2}}{{\text{O}}_3} \to {\text{3}}{{\text{O}}_2}} \end{array}\]</p>
<p class="p1">State and explain whether or not NO is acting as a catalyst.</p>
<div class="marks">[2]</div>
<div class="question_part_label">c.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Define the term <em>endothermic reaction</em>.</p>
<div class="marks">[1]</div>
<div class="question_part_label">d.i.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Sketch the Maxwell-Boltzmann energy distribution curve for a reaction with and without a catalyst, and label both axes.</p>
<div class="marks">[3]</div>
<div class="question_part_label">d.ii.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Define the term <em>rate of reaction</em>.</p>
<div class="marks">[1]</div>
<div class="question_part_label">e.i.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Iron, used as the catalyst in the Haber process, has a specific heat capacity of \({\text{0.4490 J}}\,{{\text{g}}^{ - 1}}{{\text{K}}^{ - 1}}\). If 245.0 kJ of heat is supplied to 8.500 kg of iron, initially at a temperature of 15.25 °C, determine its final temperature in K.</p>
<div class="marks">[3]</div>
<div class="question_part_label">f.</div>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question" style="padding-left: 20px;">
<p class="p1">\({\text{(}}{K_{\text{c}}}{\text{)}} = \frac{{{{{\text{[NO]}}}^4}{{{\text{[}}{{\text{H}}_2}{\text{O]}}}^6}}}{{{{{\text{[N}}{{\text{H}}_3}{\text{]}}}^4}{{{\text{[}}{{\text{O}}_2}{\text{]}}}^5}}}\);</p>
<p class="p1"><em>No mark if square brackets are omitted or are incorrect.</em></p>
<div class="question_part_label">a.i.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">right;</p>
<p class="p1">right;</p>
<p class="p1">right;</p>
<p class="p1">no change;</p>
<div class="question_part_label">a.ii.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">minimum energy needed (by reactants/colliding particles) to react/start/initiate a reaction;</p>
<p class="p1"><em>Allow energy difference between reactants and transition state.</em></p>
<div class="question_part_label">b.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">catalyst;</p>
<p class="p1">regenerated at end of reaction / <em>OWTTE</em>;</p>
<div class="question_part_label">c.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">(system) absorbs/takes in heat from surroundings / <em>OWTTE</em>;</p>
<p class="p1"><em>Allow standard enthalpy change</em>/ \(\Delta {H^\Theta }\) <em>positive.</em></p>
<p class="p1"><em>Allow bond breaking more energetic then bond formation / OWTTE.</em></p>
<p class="p1"><em>Absorbs</em><strong><em>/</em></strong><em>takes in heat alone not sufficient for mark.</em></p>
<div class="question_part_label">d.i.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1"><em>Curve showing:</em></p>
<p class="p1">general shape of Maxwell-Boltzmann energy distribution curve;</p>
<p class="p1">correct position of \({E_{\text{a}}}\) (catalysed) <strong>and </strong>\({E_{\text{a}}}\) (uncatalysed);</p>
<p class="p1"><em>labelled y-axis: </em>probability of particles (with that kinetic energy) <strong>and </strong><em>labelled x-axis: </em>(kinetic) energy;</p>
<p class="p1"><em>Allow number/fraction/proportion of particles (with kinetic energy) for y-axis label, but do not allow amount or particles.</em></p>
<p class="p2" style="text-align: center;"><img src="images/Schermafbeelding_2016-11-07_om_07.17.26.png" alt="N11/4/CHEMI/SP2/ENG/TZ0/06.d.ii/M"></p>
<p class="p1"><em>Award </em><strong><em>[2 max] </em></strong><em>if a second curve is drawn, but at a higher temperature, M2 will not be scored here.</em></p>
<div class="question_part_label">d.ii.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">change in concentration of reactant/product with time / rate of change of concentration;</p>
<p class="p1"><em>Increase can be used instead of change for product or decrease can be used </em><em>instead of change for reactant.</em></p>
<p class="p1"><em>Allow mass/amount/volume instead of concentration.</em></p>
<p class="p1"><em>Do not accept substance.</em></p>
<div class="question_part_label">e.i.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">\(q = mc\Delta T = 2.450 \times {10^5} = (8.500 \times {10^3})(0.4490)({T_{\text{f}}} - 15.25)\);</p>
<p class="p1">\({T_{\text{f}}} = 79.44{\text{ °C}}/\Delta T = 64.19{\text{ (°C}}/{\text{K)}}\);</p>
<p class="p1">\({T_{\text{f}}} = (79.44 + 273) = 352{\text{ (K)}}\);</p>
<p class="p1"><em>Award </em><strong><em>[3] </em></strong><em>for correct final answer.</em></p>
<p class="p1"><em>Accept the use of 273.15 K instead of 273 K giving final value of 352.59 K.</em></p>
<p class="p1"><em>For M1 and M2 award </em><strong><em>[1 max] </em></strong><em>for use of </em>\(q = mc\Delta T\)<em> if incorrect units of m and c are used.</em></p>
<div class="question_part_label">f.</div>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question" style="padding-left: 20px;">
<p class="p1">In part (a) of this question the \({K_{\text{c}}}\) expression was usually written correctly though the very weak students did mix up the numerator and denominator in (i), or include a + sign between substances.</p>
<div class="question_part_label">a.i.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">Candidates generally had few problems, but the reaction condition that proved to be the most the most difficult factor was the volume.</p>
<div class="question_part_label">a.ii.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">Activation energy was often clearly defined though some forgot to mention minimum.</p>
<div class="question_part_label">b.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">The best students realised that NO acted as a catalyst as it was regenerated at the end of the reaction. However many weaker students stated it was not a catalyst as it was not involved in the reaction.</p>
<div class="question_part_label">c.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">The definition of an endothermic reaction was generally well answered, however some just said it absorbs heat and forgot to mention the surroundings in their answer.</p>
<div class="question_part_label">d.i.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">Incorrect labels for the axes were often seen, as well as a very high proportion of symmetrical curves, some which did not start at the origin. Also many drew two curves. Also in some cases the catalyzed and uncatalyzed activation energies were often mixed up. The weaker students drew an enthalpy level diagram instead of a Maxwell-Boltzmann distribution.</p>
<div class="question_part_label">d.ii.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">In the definition for rate of reaction some students forgot to mention concentration.</p>
<div class="question_part_label">e.i.</div>
</div>
<div class="question" style="padding-left: 20px;">
[N/A]
<div class="question_part_label">f.</div>
</div>
<br><hr><br>