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data-target="#modal-feedback" style="margin-bottom: 10px"><i class="fa fa-send"></i> Feedback</button> </div> </div> <div class="col-md-9" id="main-column"> <h1 class="page_title"> Measuring energy changes <a href="#" class="mark-page-favorite pull-right" data-pid="887" title="Mark as favorite" onclick="return false;"><i class="fa fa-star-o"></i></a> </h1> <ol class="breadcrumb"> <li><a href="../../../chemistry.html"><i class="fa fa-home"></i></a><i class="fa fa-fw fa-chevron-right divider"></i></li><li><a href="../358/energetics.html">Energetics</a><i class="fa fa-fw fa-chevron-right divider"></i></li><li><span class="gray">Measuring energy changes</span></li> <span class="pull-right" style="color: #555" title="Suggested study time: 60 minutes"><i class="fa fa-clock-o"></i> 60'</span> </ol> <article id="main-article"> <p> <img alt="" src="../../images/test-images/digtherm-1.png" style="width: 160px; height: 196px; float: left;">All physical and chemical processes are accompanied by an energy change. This section explains how the enthalpy change of a chemical reaction can be determined from the effect that it has on the temperature of the surroundings. <strong>Use revision cards 6-10 to become familiar with the relevant calorimetry techniques</strong>, particularly their limitations. <strong>Use the practice questions to ensure that you are confident processing the raw data</strong>, writing out the calculations in full to avoid making careless mistakes with units, and stating any assumptions that you make.</p> <hr class="hidden-separator"> <div class="panel panel-has-colored-body panel-turquoise"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Key concepts</p> </div> </div> <div class="panel-body"> <div> <div class="panel-body"> <div> <p>Ensure you are confident using the terms below and learn the asterisked* definitions</p> <p>enthalpy (<em>H</em>)*, system, surroundings, exothermic, endothermic, standard enthalpy change (<span calibri="" light="" style="font-size:12.0pt;font-family:"></span>Δ<em>H</em><sup><s>o</s></sup>)*, specific heat capacity</p> <div class="tib-flashcard"><a class="show-flashcards btn btn-success btn-xs-block btn-block " data-levels="1" data-mode="Normal" data-topics="619" data-subject-id="7" data-n-flashcards="6" style="text-align:center">Show flashcards</a></div><hr> <div class="panel panel-has-colored-body panel-has-border panel-turquoise"> <div class="panel-heading"><a class="expander" href="#"><span class="fa fa-plus"></span></a> <div> <p>Understanding energy changes</p> </div> </div> <div class="panel-body"> <div class="video-embed vimeo"><iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" mozallowfullscreen="" webkitallowfullscreen="" height="420" width="100%" src="https://player.vimeo.com/video/350976874"></iframe></div> </div> <div class="panel-footer"> <div> </div> </div> </div> </div> </div> </div> </div> <div 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</div> <div class="panel-body"> <div> <div class="panel-body"> <div> <div class="tib-quiz" data-stats="7-156-887"><div class="label label-default q-number">1</div><div class="exercise shadow-bottom"><div class="q-question"><p>Which of the following are true statements in thermochemistry?</p><p><strong>1: </strong>Heat is a form of energy.</p><p><strong>2: </strong>Temperature represents a measure of the average kinetic energy of particles in a system.</p><p><strong>3: </strong>The energy of a system cannot change.</p></div><div class="q-answer"><p><label class="radio"> <input type="radio"> <span>1, 2 and 3</span></label> </p><p><label class="radio"> <input class="c" type="radio"> <span>1 and 2 only</span></label> </p><p><label class="radio"> <input type="radio"> <span>1 and 3 only</span></label> </p><p><label class="radio"> <input type="radio"> <span>2 and 3 only</span></label> </p></div><div class="q-explanation"><p>Heat is a form of energy. It is considered to be a measure of the (kinetic) energy of particles transferred between two systems/system and surroundings.</p><p>Temperature represents a measure of the average kinetic energy of particles in a system. Not all the particles will have the same kinetic energy (there will be a distribution of kinetic energies) but temperature represents an average.</p><p>The energy of a system <strong>can </strong>change. Energy can move from one system to another or from a system to surroundings. For example in an exothermic reaction heat energy moves from system to surroundings.</p><p>Thus 1 and 2 only is the correct answer.</p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">2</div><div class="exercise shadow-bottom"><div class="q-question"><p>Which of the following best represents 'standard state' in thermochemistry?</p></div><div class="q-answer"><p><label class="radio"> <input type="radio"> <span>pure substance in its standard state at 298K</span></label> </p><p><label class="radio"> <input type="radio"> <span>pure gaseous substance at 100kPa and 298K </span></label> </p><p><label class="radio"> <input type="radio"> <span>pure gaseous substance at 298K</span></label> </p><p><label class="radio"> <input class="c" type="radio"> <span>pure substance in its standard state at 100kPa and 298K</span></label> </p></div><div class="q-explanation"><p>Strictly speaking standard states do not include temperature! However, chemists tend to assume a temperature of 298K, so that is included here, and should be included in any answer given.</p><p>Standard state<strong> does </strong>assume a standard pressure of 100kPa (this used to be 1 atmosphere - 101.325kPa - but is now 100kPa). Substances are also assumed to be pure and in the correct state (standard state) at 100kPa (and 298K).</p><p>Thus 'pure substance in its standard state at 100kPa and 298K' is the correct answer.</p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">3</div><div class="exercise shadow-bottom"><div class="q-question"><p>If a chemical reaction is endothermic in the forward direction, which of these statements is true?</p></div><div class="q-answer"><p><label class="radio"> <input type="radio"> <span>The energy of the products is higher than the energy of the reactants and ΔH is negative.</span></label> </p><p><label class="radio"> <input type="radio"> <span>The energy of the products is lower than the energy of the reactants and ΔH is negative.</span></label> </p><p><label class="radio"> <input type="radio"> <span>The energy of the products is lower than the energy of the reactants and ΔH is positive.</span></label> </p><p><label class="radio"> <input class="c" type="radio"> <span>The energy of the products is higher than the energy of the reactants and ΔH is positive.</span></label> </p></div><div class="q-explanation"><p>If a reaction is endothermic, then heat energy (enthaply) is transferred <strong>from</strong> the surroundings <strong>to</strong> the system. Thus the system gains energy, ΔH is positive, and the energy of the products is higher than the energy of the reactants:</p><p>On the left; exothermic - on the right; endothermic.</p><p><img alt="" src="../../images/energetics-thermochemistry/exo-endo.png" style="width: 486px; height: 270px;"></p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">4</div><div class="exercise shadow-bottom"><div class="q-question"><p>A chemical reaction is carried out in aqueous solution (water), and the temperature of the solution rises by several degrees in temperature. Which of these statements is true with respect to the reaction?</p></div><div class="q-answer"><p><label class="radio"> <input type="radio"> <span>The energy of the products is higher than the energy of the reactants and ΔH is negative.</span></label> </p><p><label class="radio"> <input class="c" type="radio"> <span>The energy of the products is lower than the energy of the reactants and ΔH is negative.</span></label> </p><p><label class="radio"> <input type="radio"> <span>The energy of the products is higher than the energy of the reactants and ΔH is positive.</span></label> </p><p><label class="radio"> <input type="radio"> <span>The energy of the products is lower than the energy of the reactants and ΔH is positive.</span></label> </p></div><div class="q-explanation"><p>If the temperature of the solution has increased, then the system has released energy to the surroundings. The reaction is therefore <strong>exothermic</strong>. Thus the system loses energy, ΔH is negative, and the energy of the products is lower than the energy of the reactants:</p><p>On the left; exothermic - on the right; endothermic.</p><p><img alt="" src="../../images/energetics-thermochemistry/exo-endo.png" style="width: 486px; height: 270px;"></p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">5</div><div class="exercise shadow-bottom"><div class="q-question"><p>Which of the following would be correctly included in the definition of a <em>standard enthalpy of formation</em>?</p><p><strong>1: </strong>One mole of the compound is formed...</p><p><strong>2: </strong>...from its elements...</p><p><strong>3: </strong>...in the gaseous state.</p></div><div class="q-answer"><p><label class="radio"> <input type="radio"> <span>2 and 3 only</span></label> </p><p><label class="radio"> <input type="radio"> <span>1, 2 and 3</span></label> </p><p><label class="radio"> <input type="radio"> <span>1 and 3 only</span></label> </p><p><label class="radio"> <input class="c" type="radio"> <span>1 and 2 only</span></label> </p></div><div class="q-explanation"><p>Standard enthalpy of formation is a definition that needs to be learned:</p><p>Standard enthalpy of formation is the enthalpy change when <strong>one mole </strong>of a compound is formed <strong>from its elements </strong>in their <strong>standard states</strong> (at 100kPa and 298K).</p><p>Thus 1 and 2 only is the correct answer.</p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">6</div><div class="exercise shadow-bottom"><div class="q-question"><p>Which of the equations below represents the <em>standard enthalpy of combustion of ethane</em>?</p></div><div class="q-answer"><p><label class="radio"> <input type="radio"> <span>C<sub>2</sub>H<sub>6(g) </sub>+ 2½O<sub>2(g) </sub>→ 2CO<sub>(g)</sub> + 3H<sub>2</sub>O<sub>(l)</sub></span></label> </p><p><label class="radio"> <input class="c" type="radio"> <span>C<sub>2</sub>H<sub>6(g)</sub> + 3½O<sub>2(g)</sub> → 2CO<sub>2(g)</sub> + 3H<sub>2</sub>O<sub>(l)</sub></span></label> </p><p><label class="radio"> <input type="radio"> <span>C<sub>2</sub>H<sub>6(g) </sub>+ 3½O<sub>2(g) </sub>→ 2CO<sub>2(g)</sub> + 3H<sub>2</sub>O<sub>(g)</sub></span></label> </p><p><label class="radio"> <input type="radio"> <span>2C<sub>2</sub>H<sub>6(g) </sub>+ 7O<sub>2(g)</sub> → 4CO<sub>2(g)</sub> + 6H<sub>2</sub>O<sub>(l)</sub></span></label> </p></div><div class="q-explanation"><p>Standard enthalpy of combustion is a definition that needs to be learned:</p><p>Standard enthalpy of combustion is the enthalpy change when <strong>one mole </strong>of a compound combusts <strong>completely </strong>(in excess oxygen) with all reactants and products in their <strong>standard states</strong> (at 100kPa and 298K).</p><p>Thus C<sub>2</sub>H<sub>6(g)</sub> + 3½O<sub>2(g)</sub> → 2CO<sub>2(g)</sub> + 3H<sub>2</sub>O<sub>(l)</sub> is the correct answer.</p><p>Errors shown in the other answers include:</p><p>2C<sub>2</sub>H<sub>6(g) </sub>+ 7O<sub>2(g)</sub> → 4CO<sub>2(g)</sub> + 6H<sub>2</sub>O<sub>(l) </sub>two moles of ethane (rather than <strong>one mole</strong>);</p><p>C<sub>2</sub>H<sub>6(g) </sub>+ 3½O<sub>2(g) </sub>→ 2CO<sub>2(g)</sub> + 3H<sub>2</sub>O<sub>(g) </sub>water in the gaseous state (rather than liquid - its <strong>standard state</strong>);</p><p>C<sub>2</sub>H<sub>6(g) </sub>+ 2½O<sub>2(g) </sub>→ 2CO<sub>(g)</sub> + 3H<sub>2</sub>O<sub>(l) </sub>carbon monoxide produced (rather than carbon dioxide - to show <strong>complete</strong> combustion).</p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">7</div><div class="exercise shadow-bottom"><div class="q-question"><p>A student investigated a combustion reaction. The student used a spirit burner to burn some fuel and heat 200cm<sup>3</sup> of water. During the experiment the water temperature increased from 22.0°C to 38.4°C.</p><p>What is the approximate (given to 1 decimal place) amount of heat energy (in kJ) absorbed by the water?</p></div><div class="q-answer"><p><label class="radio"> <input class="c" type="radio"> <span>13.7</span></label> </p><p><label class="radio"> <input type="radio"> <span>13710.4</span></label> </p><p><label class="radio"> <input type="radio"> <span>22070.4</span></label> </p><p><label class="radio"> <input type="radio"> <span>22.0</span></label> </p></div><div class="q-explanation"><p>Using Q=mcΔT and a specific heat capacity of water of 4.18 J K<sup>−1</sup> g<sup>−1</sup> (given in the data book).</p><p>Q (heat energy) = mass × specific heat capacity × temperature change (these three values are for whatever substance is absorbing the heat energy; in this case, water. Water has a density of 1 g cm<sup>−3</sup> (so 200cm<sup>3</sup> is 200g water).</p><p>38.4 − 22.0 = 16.4°C (A temperature <strong>change</strong> of 16.4°C is the same as 16.4K).</p><p>Q = 200 × 4.18 × 16.4 = 13710.4J = 13.7 <strong>kJ</strong> to 1d.p</p><p>Thus the answer is 13.7</p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">8</div><div class="exercise shadow-bottom"><div class="q-question"><p>A student investigated the enthalpy change for solid ammonium chloride (NH<sub>4</sub>Cl) dissolving in distilled water. The student added 5.59g of ammonium chloride to 100cm<sup>3</sup> of water and measured a temperature decrease of 5.0°C.</p><p>What is the approximate enthalpy change for this dissolving process for one mole of ammonium chloride (in kJ mol<sup>−1</sup>)?</p></div><div class="q-answer"><p><label class="radio"> <input class="c" type="radio"> <span>+20</span></label> </p><p><label class="radio"> <input type="radio"> <span>+20000</span></label> </p><p><label class="radio"> <input type="radio"> <span>−20000</span></label> </p><p><label class="radio"> <input type="radio"> <span>−20</span></label> </p></div><div class="q-explanation"><p>Using Q=mcΔT and a specific heat capacity of water of 4.18 J K<sup>−1</sup> g<sup>−1</sup> (given in the data book).</p><p>Q (heat energy) = mass × specific heat capacity × temperature change (these three values are for whatever substance is absorbing the heat energy; in this case, water. Water has a density of 1 g cm<sup>−3</sup> (so 100cm<sup>3</sup> is 100g water). A temperature <strong>change</strong> of 5°C is the same as 5K.</p><p>Q = 100 × 4.18 × 5 = 2090J</p><p>(for the 5.59g of NH<sub>4</sub>Cl used)</p><p>Molar mass of NH<sub>4</sub>Cl = 14.01 + (1.01×4) + 35.45 = 53.50 g mol<sup>−1</sup></p><p>Moles of NH<sub>4</sub>Cl used = mass / molar mass = 5.59 / 53.50 = 0.104485981</p><p>For one mole of ammonium chloride the enthalpy change will therefore be:</p><p>2090 / 0.104485981 = 20003J mol<sup>−1</sup> approximately</p><p>Which is 20 kJ mol<sup>−1</sup> approximately</p><p>And the enthalpy change is <strong>endothermic </strong>because the temperature dropped, so the sign is positive.</p><p>Thus the answer is +20</p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">9</div><div class="exercise shadow-bottom"><div class="q-question"><p>Assume that the standard enthalpy change of neutralisation for the reaction between hydrochloric acid and sodium hydroxide (one mole of each) is −57.6 kJ mol<sup>−1</sup>.</p><p>If a student adds 500cm<sup>3</sup> of 1.00 mol dm<sup>−3</sup> HCl<sub>(aq)</sub> to 500cm<sup>3</sup> of 1.00 mol dm<sup>−3</sup> of NaOH<sub>(aq) </sub>and measures the temperature change of the total volume, what might they expect the temperature change to be?</p></div><div class="q-answer"><p><label class="radio"> <input class="c" type="radio"> <span>an increase of 6.9K</span></label> </p><p><label class="radio"> <input type="radio"> <span>an increase of 13.8K</span></label> </p><p><label class="radio"> <input type="radio"> <span>a decrease of 13.8K</span></label> </p><p><label class="radio"> <input type="radio"> <span>a decrease of 6.9K</span></label> </p></div><div class="q-explanation"><p>Using Q=mcΔT and a specific heat capacity of water of 4.18 J K<sup>−1</sup> g<sup>−1</sup> (given in the data book).</p><p>Q (heat energy) = mass × specific heat capacity × temperature change (these three values are for whatever substance is absorbing the heat energy; in this case, the solutions are predominently water. Water has a density of 1 g cm<sup>−3</sup> (so 1000cm<sup>3</sup> is 1000g water).</p><p>Moles of HCl (and also NaOH) = concentration (mol dm<sup>−3</sup> )× volume (dm<sup>3</sup>) = 1.00 × 0.500 = 0.500 mol</p><p>(Reaction is 1:1 molar ratio)</p><p>0.5 of a mole of each are used, therefore the enthalpy change will be:</p><p>−57.6 × 0.5 = −28.8 kJ = −28800J</p><p>This value is negative, so the reaction is <strong>exothermic </strong>and the temperature change will be an <strong>increase</strong>.</p><p>Q=mcΔT</p><p>28800 = 1000 × 4.18 × ΔT</p><p>ΔT = 28800 / (1000 × 4.18) = 6.889952153K</p><p>Thus the answer is <strong>an increase of 6.9K</strong></p></div><div class="actions"><span class="score" data-score="0"></span><button class="btn btn-default btn-sm btn-xs-block text-xs-center check"><i class="fa fa-check-square-o"></i> Check</button></div></div><div class="label label-default q-number">10</div><div class="exercise shadow-bottom"><div class="q-question"><p>A student investigated a combustion reaction. The student used a spirit burner to burn 0.40 mol of fuel and heat 200cm<sup>3</sup> of water in a 140g copper calorimeter. During the experiment the temperature of the water and the copper calorimeter increased from 21.4°C to 46.8°C.</p><p>The specific heat capacity of copper is 0.385 J K<sup>−1</sup> g<sup>−1</sup>, and for water 4.18 J K<sup>−1</sup> g<sup>−1</sup> (given in the data book).</p><p>What is the approximate enthalpy change of combustion for this fuel (in kJ mol<sup>−1</sup>)?</p></div><div class="q-answer"><p><label class="radio"> <input type="radio"> <span>−53.1</span></label> </p><p><label class="radio"> <input class="c" type="radio"> <span>−56.5</span></label> </p><p><label class="radio"> <input type="radio"> <span>−9.0</span></label> </p><p><label class="radio"> <input type="radio"> <span>−90.3</span></label> </p></div><div class="q-explanation"><p>Using Q=mcΔT</p><p>Q (heat energy) = mass × specific heat capacity × temperature change (these three values are for whatever substance is absorbing the heat energy; in this case, water and also copper. Water has a density of 1 g cm<sup>−3</sup> (so 200cm<sup>3</sup> is 200g water).</p><p>The temperature <strong>change</strong> (46.8 − 21.4 = 25.4) of 25.4°C is the same as 25.4K.</p><p>For the water:</p><p>Q = 200 × 4.18 × 25.4 = 21234.4J</p><p>And for the copper calorimeter:</p><p>Q = 140 × 0.385 × 25.4 = 1369.06J</p><p>Total Q = 21234.4 + 1369.06 = 22603.46J</p><p>(for the 0.40 mol of fuel burnt)</p><p>For one mole of fuel the enthalpy change will therefore be:</p><p>22603.46 / 0.40 = 56508.65J mol<sup>−1</sup></p><p>Which is 56.5 kJ mol<sup>−1</sup> approximately</p><p>And the enthalpy change is <strong>exothermic</strong>, so the sign is negative.</p><p>Thus the answer is −56.5</p><p>(An answer of −53.1 is obtained by using only the heat energy absorbed by the water)</p><p>(An answer of −90.3 is obtained by using a mass of 340g of water)</p><p>(An asnwer of −9.0 is obtained by multiplying by 0.40 (mol) instead of dividing)</p></div><div class="actions"><span class="score" 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