[{"data":1,"prerenderedAt":73},["ShallowReactive",2],{"q-fire-112-1-fire-science-022":3},{"subject":4,"subjectSlug":5,"subjectKicker":6,"subjectShort":7,"question":8,"related":27,"sameNumber":52,"hasEssay":26},"火災學概要","fire-science","火災學 · Fire Science","火災學",{"id":9,"webId":10,"year":11,"session":12,"subject":4,"number":13,"stem":14,"options":15,"answer":20,"answerNote":21,"images":22,"imagesPending":23,"lawTimestamp":11,"explanation":24,"explanationDeep":25,"freq":12,"indexable":26},"fire-112-1-火災學概要-022","fire-112-1-fire-science-022",112,1,22,"兩種可燃性氣體以等比例混合，其燃燒下限分別為 1.5%、4.5%，則該混合氣體的燃燒下限約為多少%？",{"A":16,"B":17,"C":18,"D":19},"2.25","3.00","3.55","1.5","A",null,[],false,"本題考點：混合可燃性氣體燃燒下限的合成——勒沙特列（Le Chatelier）公式以各成分濃度除以其燃燒下限後相加取倒數，不是取算術平均。\n【正解理由】依勒沙特列公式 100／LFL(mix)＝Σ(Ci／LFLi)，Ci 為各成分在混合氣中所占百分比。等比例混合即 C1＝C2＝50%，代入得 100／LFL＝50／1.5＋50／4.5＝44.44，故 LFL＝100÷44.44＝2.25%，故選 A。\n【逐項排除】\n(A) 2.25 與勒沙特列公式結果一致，落在 1.5 與 4.5 之間並偏向下限較低的成分，為本題應選者。\n(B) 3.00 是兩下限的算術平均 (1.5＋4.5)÷2；本公式屬調和平均型，兩者僅在兩成分下限相等時才一致。\n(C) 3.55 既非調和平均 2.25 亦非算術平均 3.00，等比例混合下無算法導出此值，屬干擾項。\n(D) 1.5 是直接取較易燃成分的下限；混合後較難燃的成分會拉高整體下限，不能逕以最低者為準。\n【演算步驟】100／LFL(mix)＝C1／LFL1＋C2／LFL2；C1＝C2＝50% → 100／LFL＝50÷1.5＋50÷4.5＝33.33＋11.11＝44.44 → LFL＝100÷44.44＝2.25（體積%）。\n【記憶點】混合下限走調和平均：各自「濃度除下限」相加後再拿 100 除回去，結果必偏向易燃成分。","勒沙特列公式的推導建立在 Burgess-Wheeler 的同一個假設之上：各可燃成分在其燃燒下限濃度時對混合氣的「可燃性貢獻」相等，因此把每個成分的實際濃度除以自身下限，得到的比值可視為該成分貢獻的份額，各份額之和達到 1（以百分比計為 100）時，混合氣即恰好到達燃燒下限。寫成常用形式為 100／L＝C1／L1＋C2／L2＋…＋Cn／Ln，其中 Ci 之和為 100。同一式亦可用於燃燒上限（把 Li 換成各成分的上限），這是常被忽略的考點。\n數值特性有三個必記結論：①結果為調和平均型，必定小於或等於算術平均，且偏向下限較低（較易燃）的成分；②任一成分下限極低時會顯著拉低整體下限，故微量高危成分不可忽略；③兩成分下限相同時，調和平均與算術平均相等，混合後下限不變。以本題數值為例，若改為 8：2 混合（1.5% 者占 80%），則 100／L＝80÷1.5＋20÷4.5＝53.33＋4.44＝57.78，L＝1.73%，可見比例改變對結果的影響方向。\n適用限制同樣常考：本式對同族碳氫化合物（如甲烷、乙烷、丙烷混合）精度良好；若混合物中含惰性氣體（氮、二氧化碳）則不可直接代入，須先把惰性成分視為稀釋劑另作處理，或改用爆炸範圍圖（Coward 三角圖）判斷；含氫、乙炔等燃燒行為特殊者，或成分間有化學交互作用時，誤差亦會增大。\n延伸考點四類：①先以 Burgess-Wheeler 由燃燒熱求各成分下限，再以本式合成混合下限，兩式串接是高頻題型；②以本式反推某成分的下限（已知混合下限與比例）；③混合氣體的危險度 H＝(UFL－LFL)／LFL，可在求得混合上下限後續算；④天然氣、液化石油氣等實務混合氣的下限估算，題幹常直接給出各成分體積百分比與各自下限，代入即可，重點在於先確認各 Ci 之和是否為 100。",true,[28,32,36,40,44,48],{"webId":29,"stem":30,"number":31,"year":11,"session":12},"fire-112-1-fire-science-021","已知乙烷的完全燃燒反應如下：2C2H6 +7O2 →4CO2 +6H2O+3120 kJ，若其燃燒熱與燃燒下限的關係適用 Burgess-Wheeler 定理，則其燃燒下限約為多少%？（提示：1cal≒ 4.2 J）",21,{"webId":33,"stem":34,"number":35,"year":11,"session":12},"fire-112-1-fire-science-023","在最小發火壓力以上，可燃性氣體的最小發火能量，會隨著下列那一種條件改變而下降？",23,{"webId":37,"stem":38,"number":39,"year":11,"session":12},"fire-112-1-fire-science-020","當物質進行完全氧化燃燒時，下列何者的理論空氣需求量最高？",20,{"webId":41,"stem":42,"number":43,"year":11,"session":12},"fire-112-1-fire-science-024","於空氣中加入氧氣會改變空氣組成，進而影響可燃性氣體的某些燃燒特性，下列何者所受的影響較為明顯？",24,{"webId":45,"stem":46,"number":47,"year":11,"session":12},"fire-112-1-fire-science-019","建築物火災中，在所謂通風控制燃燒（Ventilation Controlled）情形下，可燃物燃燒速度主要受何因素決定？",19,{"webId":49,"stem":50,"number":51,"year":11,"session":12},"fire-112-1-fire-science-025","對可燃性氣體而言，下列那一種特性或條件所衍生的燃燒或爆炸風險相對較高？",25,[53,57,61,65,69],{"webId":54,"year":55,"stem":56,"number":13},"fire-113-1-fire-science-022",113,"一物體在 27℃時輻射出的能量為 E，則當溫度升高到 627℃時輻射出的能量變為多少？",{"webId":58,"year":59,"stem":60,"number":13},"fire-111-1-fire-science-022",111,"連棟式木造建築物引發火災，消防隊射水同時拆除未受火勢波及的房屋，請問其採取的滅火方法係下列何者？",{"webId":62,"year":63,"stem":64,"number":13},"fire-110-1-fire-science-022",110,"火災中所產生的輻射熱量與下列何者成正比？",{"webId":66,"year":67,"stem":68,"number":13},"fire-109-1-fire-science-022",109,"預防過氧化丁酮之火災爆炸時，下列何種原則正確？",{"webId":70,"year":71,"stem":72,"number":13},"fire-108-1-fire-science-022",108,"物體在 327℃時的輻射強度為 E，則當溫度升高到 927℃時的輻射強度為何？",1785129966542]