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</div><h2>HL Paper 3</h2><div class="question">
<p>In an acidic soil, nitrate ions may undergo reduction to form ammonium ions. Deduce a half-equation for the reaction.</p>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question">
<p>\({\text{NO}}_3^ - + {\text{10}}{{\text{H}}^ + } + {\text{8}}{{\text{e}}^ - } \to {\text{NH}}_4^ + + {\text{3}}{{\text{H}}_2}{\text{O}}\)</p>
<p>correct reactants;</p>
<p>correct products;</p>
<p>correct balancing;</p>
<p><em>M3 can only be scored if M1 and M2 correct</em>.</p>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question">
<p>The equation caused some difficulty for many students; electrons were omitted on the left-hand side, incorrect products were given and equations were left unbalanced.</p>
</div>
<br><hr><br><div class="specification">
<p class="p1">Enzymes are catalysts that increase the rate of all biochemical reactions, including those involved in respiration.</p>
</div>
<div class="specification">
<p class="p1">Cytochrome oxidase is a complex enzyme that catalyses the reduction of oxygen in the final stage of aerobic respiration. This enzyme is inhibited both by nitrogen(II) oxide, NO, and separately by cyanide ions, \({\text{C}}{{\text{N}}^ - }\). It has been suggested that NO acts competitively while \({\text{C}}{{\text{N}}^ - }\) acts non-competitively in inhibiting the enzyme. Experiments were carried out to test this hypothesis.</p>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">The graph below shows the effect of substrate concentration on the rate of the reaction in the absence of an inhibitor. Draw and label the results of the <strong>two</strong> experiments showing how the rate of the reaction changes in the presence of NO and in the presence of \({\text{C}}{{\text{N}}^ - }\), if the hypothesis is correct.</p>
<p class="p1" style="text-align: center;"><img src="images/Schermafbeelding_2016-08-25_om_16.22.59.png" alt="N13/4/CHEMI/HP3/ENG/TZ0/09.b.ii"></p>
<div class="marks">[2]</div>
<div class="question_part_label">b.ii.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Suggest a reason why it is more likely that NO, rather than \({\text{C}}{{\text{N}}^ - }\), acts competitively.</p>
<div class="marks">[1]</div>
<div class="question_part_label">b.iii.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">The reducing agent in the cytochrome oxidase reaction is a species that can be denoted as \({\text{X}}{{\text{H}}_{\text{2}}}\) in the reduced form. Using this notation, deduce an equation for the reaction of \({\text{X}}{{\text{H}}_{\text{2}}}\) and \({{\text{O}}_{\text{2}}}\), and outline, using oxidation numbers, why it is a redox reaction.</p>
<div class="marks">[2]</div>
<div class="question_part_label">b.iv.</div>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question" style="padding-left: 20px;">
<p class="p1"><img src="images/Schermafbeelding_2016-08-25_om_16.26.18.png" alt="N13/4/CHEMI/HP3/ENG/TZ0/09.b.ii/M"></p>
<p class="p1">line labelled NO/competitive reaching \({V_{\text{m}}}\) but with lower gradient;</p>
<p class="p1">line labelled \({\text{C}}{{\text{N}}^ - }\)/non-competitive not reaching \({V_{\text{m}}}\);</p>
<div class="question_part_label">b.ii.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">NO more likely to fit into active site / NO (structure) similar to \({{\text{O}}_2}\) / \({\text{C}}{{\text{N}}^ - }\) structure different to \({{\text{O}}_2}\) / <em>OWTTE</em>;</p>
<div class="question_part_label">b.iii.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">\({\text{2X}}{{\text{H}}_2} + {{\text{O}}_2} \to {\text{2X}} + {\text{2}}{{\text{H}}_2}{\text{O}}/{\text{X}}{{\text{H}}_2} + {{\text{O}}_2} \to {\text{X(OH}}{{\text{)}}_2}/{\text{X}}{{\text{H}}_{\text{2}}} + {{\text{O}}_{\text{2}}} \to {\text{XO}} + {{\text{H}}_{\text{2}}}{\text{O}}\);</p>
<p class="p1">oxygen changes from 0 to –2;</p>
<p class="p1"><em>Allow X<sub>2</sub></em> <em>instead of 2X.</em></p>
<p class="p1"><em>Do not allow 2– notation.</em></p>
<p class="p1"><em>Accept “X changes from –2 to 0 in X or X<sub>2</sub>“ or “X changes from –2 to +2 in </em><em>X(OH)<sub>2</sub>/XO”.</em></p>
<div class="question_part_label">b.iv.</div>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question" style="padding-left: 20px;">
<p class="p1">Option B was a very popular, and question 6 was well answered with the exception of not listing alkenyl when identifying two functional groups common to three vitamins (A, C and D). Some students did not read the question on formula of zwitterion carefully and instead have the formula of the amino acid itself without any charges. In the separation of alanine and cysteine, the first mark was well scored by many while the second mark proved to be more demanding and often candidates lost this mark as no reference was made to charges or charges inversely stated. Although the disulfide bridge was correctly identified by even weaker candidates a much few were able to identify this as a covalent bond. Structure of the triglyceride was better answered than in past sessions but drawing the ester linkage correctly was still challenging for many candidates. Although the identification of the other reactant (water) was identified by many, the one essential condition (enzyme/lipase) was done poorly. Identification of the polyunsaturated fatty acid was done well by most but the second mark on its ability to lower LDL cholesterol was missed by most. The question on enzymes and inorganic catalysts was done poorly since comparison was often missing. While some candidates were able to suggest a pair of ions in cytochrome oxidase, only stronger candidates provided both pairs. Competitive and non-competitive inhibition was generally well done; however, the reason why it is more likely that NO, rather than the cyanide ion, acts competitively was not done as well. The redox reaction of the reducing agent \({\text{X}}{{\text{H}}_{\text{2}}}\) with \({{\text{O}}_{\text{2}}}\) produced a range of possible equations but rarely did candidates scored full marks.</p>
<div class="question_part_label">b.ii.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">Option B was a very popular, and question 6 was well answered with the exception of not listing alkenyl when identifying two functional groups common to three vitamins (A, C and D). Some students did not read the question on formula of zwitterion carefully and instead have the formula of the amino acid itself without any charges. In the separation of alanine and cysteine, the first mark was well scored by many while the second mark proved to be more demanding and often candidates lost this mark as no reference was made to charges or charges inversely stated. Although the disulfide bridge was correctly identified by even weaker candidates a much few were able to identify this as a covalent bond. Structure of the triglyceride was better answered than in past sessions but drawing the ester linkage correctly was still challenging for many candidates. Although the identification of the other reactant (water) was identified by many, the one essential condition (enzyme/lipase) was done poorly. Identification of the polyunsaturated fatty acid was done well by most but the second mark on its ability to lower LDL cholesterol was missed by most. The question on enzymes and inorganic catalysts was done poorly since comparison was often missing. While some candidates were able to suggest a pair of ions in cytochrome oxidase, only stronger candidates provided both pairs. Competitive and non-competitive inhibition was generally well done; however, the reason why it is more likely that NO, rather than the cyanide ion, acts competitively was not done as well. The redox reaction of the reducing agent \({\text{X}}{{\text{H}}_{\text{2}}}\) with \({{\text{O}}_{\text{2}}}\) produced a range of possible equations but rarely did candidates scored full marks.</p>
<div class="question_part_label">b.iii.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1">Option B was a very popular, and question 6 was well answered with the exception of not listing alkenyl when identifying two functional groups common to three vitamins (A, C and D). Some students did not read the question on formula of zwitterion carefully and instead have the formula of the amino acid itself without any charges. In the separation of alanine and cysteine, the first mark was well scored by many while the second mark proved to be more demanding and often candidates lost this mark as no reference was made to charges or charges inversely stated. Although the disulfide bridge was correctly identified by even weaker candidates a much few were able to identify this as a covalent bond. Structure of the triglyceride was better answered than in past sessions but drawing the ester linkage correctly was still challenging for many candidates. Although the identification of the other reactant (water) was identified by many, the one essential condition (enzyme/lipase) was done poorly. Identification of the polyunsaturated fatty acid was done well by most but the second mark on its ability to lower LDL cholesterol was missed by most. The question on enzymes and inorganic catalysts was done poorly since comparison was often missing. While some candidates were able to suggest a pair of ions in cytochrome oxidase, only stronger candidates provided both pairs. Competitive and non-competitive inhibition was generally well done; however, the reason why it is more likely that NO, rather than the cyanide ion, acts competitively was not done as well. The redox reaction of the reducing agent \({\text{X}}{{\text{H}}_{\text{2}}}\) with \({{\text{O}}_{\text{2}}}\) produced a range of possible equations but rarely did candidates scored full marks.</p>
<div class="question_part_label">b.iv.</div>
</div>
<br><hr><br>