File "markSceme-SL-paper3.html"
Path: /IB QUESTIONBANKS/4 Fourth Edition - PAPER/HTML/Chemistry/Topic 14/markSceme-SL-paper3html
File size: 13.78 KB
MIME-type: text/html
Charset: utf-8
<!DOCTYPE html>
<html>
<meta http-equiv="content-type" content="text/html;charset=utf-8">
<head>
<meta charset="utf-8">
<title>IB Questionbank</title>
<link rel="stylesheet" media="all" href="css/application-746ec5d03ead8d9e8b3bb7d32173f2b4e2e22a05f0c5278e086ab55b3c9c238e.css">
<link rel="stylesheet" media="print" href="css/print-6da094505524acaa25ea39a4dd5d6130a436fc43336c0bb89199951b860e98e9.css">
<script src="js/application-3c91afd8a2942c18d21ed2700e1bdec14ada97f1d3788ae229315e1276d81453.js"></script>
<script type="text/javascript" async src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-MML-AM_CHTML-full"></script>
<!--[if lt IE 9]>
<script src='https://cdnjs.cloudflare.com/ajax/libs/html5shiv/3.7.3/html5shiv.min.js'></script>
<![endif]-->
<meta name="csrf-param" content="authenticity_token">
<meta name="csrf-token" content="iHF+M3VlRFlNEehLVICYgYgqiF8jIFlzjGNjIwqOK9cFH3ZNdavBJrv/YQpz8vcspoICfQcFHW8kSsHnJsBwfg==">
<link href="favicon.ico" rel="shortcut icon">
</head>
<body class="teacher questions-show">
<div class="navbar navbar-fixed-top">
<div class="navbar-inner">
<div class="container">
<div class="brand">
<div class="inner"><a href="http://ibo.org/">ibo.org</a></div>
</div>
<ul class="nav">
<li>
<a href="../../index.html">Home</a>
</li>
<li class="active dropdown">
<a class="dropdown-toggle" data-toggle="dropdown" href="#">
Questionbanks
<b class="caret"></b>
</a><ul class="dropdown-menu">
<li>
<a href="../../geography.html" target="_blank">DP Geography</a>
</li>
<li>
<a href="../../physics.html" target="_blank">DP Physics</a>
</li>
<li>
<a href="../../chemistry.html" target="_blank">DP Chemistry</a>
</li>
<li>
<a href="../../biology.html" target="_blank">DP Biology</a>
</li>
<li>
<a href="../../furtherMath.html" target="_blank">DP Further Mathematics HL</a>
</li>
<li>
<a href="../../mathHL.html" target="_blank">DP Mathematics HL</a>
</li>
<li>
<a href="../../mathSL.html" target="_blank">DP Mathematics SL</a>
</li>
<li>
<a href="../../mathStudies.html" target="_blank">DP Mathematical Studies</a>
</li>
</ul></li>
<!-- - if current_user.is_language_services? && !current_user.is_publishing? -->
<!-- %li= link_to "Language services", tolk_path -->
</ul>
<ul class="nav pull-right">
<li>
<a href="https://06082010.xyz">IB Documents (2) Team</a>
</li></ul>
</div>
</div>
</div>
<div class="page-content container">
<div class="row">
<div class="span24">
</div>
</div>
<div class="page-header">
<div class="row">
<div class="span16">
<p class="back-to-list">
</p>
</div>
<div class="span8" style="margin: 0 0 -19px 0;">
<img style="width: 100%;" class="qb_logo" src="images/logo.jpg" alt="Ib qb 46 logo">
</div>
</div>
</div><h2>SL Paper 3</h2><div class="question">
<p class="p1">Stratospheric ozone is in dynamic equilibrium with oxygen. Give the equations that describe the formation of ozone from oxygen and its depletion in the stratosphere in the presence of ultraviolet light.</p>
<p class="p1">Formation:</p>
<p class="p1">Depletion:</p>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question">
<p><em>Formation</em></p>
<p>\({{\text{O}}_{\text{2}}} \to {\text{2O}} \bullet \) <strong>and</strong> \({{\text{O}}_2} + {\text{O}} \bullet \to {{\text{O}}_3}\);</p>
<p><em>Depletion</em></p>
<p>\({{\text{O}}_3} \to {{\text{O}}_2} + {\text{O}} \bullet \) <strong>and</strong> \({{\text{O}}_3} + {\text{O}} \bullet \to {\text{2}}{{\text{O}}_2}\);</p>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question">
<p class="p1">Part (a) produced mixed responses with some candidates writing appropriate equations, and some writing only one equation for the formation and depletion of ozone when two equations were needed for each process. A few candidates seemed to have no idea of the formula of ozone.</p>
</div>
<br><hr><br><div class="specification">
<p>Ozone prevents UV radiation emitted from the Sun reaching the surface of the Earth.</p>
</div>
<div class="question">
<p>Describe, using chemical equations, the two-step mechanism of photochemical decomposition of ozone in the Earth’s stratosphere.</p>
<p> </p>
<p>Step 1:</p>
<p> </p>
<p>Step 2:</p>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question">
<p><em>Step 1:</em></p>
<p>\({{\text{O}}_3}{\text{(g)}}\xrightarrow{{{\text{UV/}}hf{\text{/}}hv}}{{\text{O}}_2}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}}\);</p>
<p><em>UV not needed for mark (since given in question)</em>.</p>
<p><em>Step 2:</em></p>
<p>\({{\text{O}}_3}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}} \to {\text{2}}{{\text{O}}_2}{\text{(g)}}\);</p>
<p><em>Ignore state symbols.</em></p>
<p><em>Allow O instead of \(O \bullet \)</em> <em>if consistent throughout.</em></p>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question">
<p>Most of the candidates were able to score part marks for (a); marks were not gained either due to incorrectly balanced equations or incorrect Step 2 equation. In part (b) comparing \({{\text{C}}_{\text{2}}}{{\text{F}}_{\text{6}}}\) with CFCs was challenging with many not really knowing why \({{\text{C}}_{\text{2}}}{{\text{F}}_{\text{6}}}\) is used. Candidates demonstrated a poor understanding of reactivity and the strength of the fluorine to carbon bond. Many referred to CFCs as having longer lifetime and being more stable. Most candidates were able to score only one mark for this question.</p>
</div>
<br><hr><br><div class="specification">
<p class="p1">The ozone layer protects living organisms from dangerous UV radiation. In the Earth’s stratosphere, ozone is photochemically formed from oxygen by the following two-step process.</p>
<p class="p1">\[\begin{array}{*{20}{l}} {{{\text{O}}_2}\xrightarrow{{{\text{UV light}}}}2{\text{O}} \bullet } \\ {{{\text{O}}_2} + {\text{O}} \bullet \to {{\text{O}}_3}} \end{array}\]</p>
</div>
<div class="question">
<p class="p1">(a) Ozone decomposition can proceed photochemically. Describe, using chemical equations, the two-step mechanism of photochemical decomposition of ozone in the Earth’s stratosphere.</p>
<p class="p1">Step 1:</p>
<p class="p1">Step 2:</p>
<p class="p1">(b) Ozone decomposition can also be catalysed by ozone-depleting substances such as chlorofluorocarbons, CFCs. State <strong>two </strong>alternatives to CFCs.</p>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question">
<p class="p1">(a) <em>Step 1:</em></p>
<p class="p1">\({{\text{O}}_{\text{3}}} + hv \to {{\text{O}}_{\text{2}}} + {\text{O}} \bullet {\text{ }}\);</p>
<p class="p1"><em>Step 2:</em></p>
<p class="p1">\({\text{O}} \bullet + {{\text{O}}_3} \to {\text{2}}{{\text{O}}_2}\);</p>
<p class="p1"><em>Dots and radicals are not required for the mark.</em></p>
<p class="p1"><em>Accept more detailed mechanisms </em><em>(</em>\(O \bullet + {O_2} + M \to {O_3} + M*\)<em>,</em><em> etc.).</em></p>
<p class="p1">(b) hydrocarbons</p>
<p class="p1">(per)fluorocarbons</p>
<p class="p1">hydrofluorocarbons / HFCs</p>
<p class="p1">hydrochlorofluorocarbons / HCFCs</p>
<p class="p1">ammonia/ \({\text{N}}{{\text{H}}_{\text{3}}}\) / sulfur dioxide/ \({\text{S}}{{\text{O}}_2}\)</p>
<p class="p1">nitrogen/ \({{\text{N}}_2}\) / argon/Ar</p>
<p class="p1"><em>Any two correct answers scores </em><strong><em>[1]</em></strong><em>.</em></p>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question">
<p class="p1">Only a small proportion of candidates could correctly quote the two equations for the photochemical decomposition of ozone, though many could correctly identify alternatives to CFCs.</p>
</div>
<br><hr><br><div class="specification">
<p class="p1">Emissions of ozone-depleting substances such as CFCs have decreased extensively as a result of the Montreal Protocol. In the most recent assessment of ozone depletion by the United Nations Environmental Programme, scientists predict a substantial recovery of the ozone layer by 2050.</p>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Although the use of harmful CFCs is being phased out, suggest why these compounds are expected to remain in the atmosphere for the next 80–100 years.</p>
<div class="marks">[1]</div>
<div class="question_part_label">b.i.</div>
</div>
<div class="question" style="padding-left: 20px; padding-right: 20px;">
<p class="p1">Discuss <strong>one </strong>advantage and <strong>two </strong>disadvantages of using hydrocarbons as alternatives to CFCs.</p>
<p class="p2"> </p>
<p class="p1">Advantage:</p>
<p class="p2"> </p>
<p class="p2"> </p>
<p class="p2"> </p>
<p class="p1">Disadvantages:</p>
<div class="marks">[3]</div>
<div class="question_part_label">b.ii.</div>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question" style="padding-left: 20px;">
<p class="p1">CFCs have a low reactivity/strong bonds/are highly stable;</p>
<p class="p1">CFCs/radicals they produce/ Cl have high residual time in atmosphere;</p>
<p class="p1">some (developing) countries still producing/consuming CFCs / slower phase-out of CFCs;</p>
<p class="p1">harmful CFCs are still present in expanded polystyrene and old refrigerators and have yet to leak into the atmosphere;</p>
<p class="p1">slow mixing between lower atmosphere/troposphere and upper atmosphere/stratosphere;</p>
<div class="question_part_label">b.i.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p class="p1"><em>Advantage</em>:</p>
<p class="p1">do not damage the ozone layer;</p>
<p class="p1">decompose less readily (than CFCs);</p>
<p class="p1">cheaper than CFCs;</p>
<p class="p1"><em>Disadvantages</em>:</p>
<p class="p1">flammable;</p>
<p class="p1">both contribute to global warming/greenhouse gases/absorb IR radiation;</p>
<div class="question_part_label">b.ii.</div>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question" style="padding-left: 20px;">
<p class="p1">The equations required in the first part of this question were amongst the best known in the examination; the result of memorisation? A number of candidates incorrectly gave the reaction between two oxygen atoms as a significant process in ozone depletion and some discussed the way CFCs accelerate the depletion, rather than giving the natural processes asked for. Relatively few could give correct reasons for the persistence of CFCs in the atmosphere, but advantages and disadvantages of were relatively widely known.</p>
<div class="question_part_label">b.i.</div>
</div>
<div class="question" style="padding-left: 20px;">
<p>The equations required in the first part of this question were amongst the best known in the examination; the result of memorisation? A number of candidates incorrectly gave the reaction between two oxygen atoms as a significant process in ozone depletion and some discussed the way CFCs accelerate the depletion, rather than giving the natural processes asked for. Relatively few could give correct reasons for the persistence of CFCs in the atmosphere, but advantages and disadvantages of were relatively widely known.</p>
<div class="question_part_label">b.ii.</div>
</div>
<br><hr><br><div class="specification">
<p class="p1">Increasing concentrations of greenhouse gases are considered to cause global warming. Ozone depletion is another environmental concern.</p>
</div>
<div class="question">
<p class="p1">Identify a gas that is both a greenhouse gas and a cause of ozone depletion.</p>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question">
<p class="p1">specific CFC compound;</p>
<p class="p1"><em>Accept CFC/chlorofluorocarbon.</em></p>
<p class="p1"><em>Allow water vapour.</em></p>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question">
<p class="p1">Few candidates gave a CFC compound in part (a). Candidates were more familiar with the impact of global warming than they were with the impact of ozone depletion.</p>
</div>
<br><hr><br><div class="specification">
<p>The ozone layer protects us by absorbing ultraviolet (UV) radiation from the Sun during its natural formation and depletion.</p>
</div>
<div class="question">
<p>Describe, using equations, the formation and depletion of ozone in the stratosphere by natural processes.</p>
<p> </p>
<p>Formation:</p>
<p> </p>
<p> </p>
<p> </p>
<p>Depletion:</p>
</div>
<h2 style="margin-top: 1em">Markscheme</h2>
<div class="question">
<p><em>Formation:</em></p>
<p>\({{\text{O}}_2}{\text{(g)}}\xrightarrow{{{\text{UV(light)/}}hv{\text{/}}hf}}{\text{2O}} \bullet {\text{(g)}}\);</p>
<p>\({{\text{O}}_2}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}} \to {{\text{O}}_3}{\text{(g)}}\);</p>
<p><em>Depletion:</em></p>
<p>\({{\text{O}}_3}{\text{(g)}}\xrightarrow{{{\text{UV(light)/}}hv{\text{/}}hf}}{{\text{O}}_2}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}}\);</p>
<p>\({{\text{O}}_3}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}} \to {\text{2}}{{\text{O}}_2}{\text{(g)}}\);</p>
<p><em>Allow representation of radical without </em>\( \bullet \)<em> if consistent throughout mechanism.</em></p>
<p><em>UV(light)/hv/hf can be represented above arrow or mentioned in accompanying description in words.</em></p>
<p><em>Penalize omission of UV (light)/hv/hf once only.</em></p>
<p><em>Ignore state symbols.</em></p>
</div>
<h2 style="margin-top: 1em">Examiners report</h2>
<div class="question">
<p>This question related to the formation and depletion of ozone in the stratosphere by natural processes. The most common errors in (a) were failure to mention the necessity for UV light to form the oxygen radicals and inconsistent use of the dot in equations to represent radicals.</p>
</div>
<br><hr><br>