[{"data":1,"prerenderedAt":73},["ShallowReactive",2],{"q-fire-110-1-fire-science-025":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-110-1-火災學概要-025","fire-110-1-fire-science-025",110,1,25,"請依據可燃性氣體完全燃燒時的化學理論濃度，計算丙烷的燃燒下限？",{"A":16,"B":17,"C":18,"D":19},"1.34%","1.64%","1.84%","2.21%","D",null,[],false,"本題考點：以完全燃燒的化學理論濃度 Cst 推估燃燒下限的瓊斯(Jones)經驗公式,LEL ≈ 0.55 × Cst。\n【正解理由】化學理論濃度指燃料與理論空氣量恰好完全反應時,燃料在混合氣中所占的體積百分比。丙烷完全燃燒反應式為 C₃H₈ + 5O₂ → 3CO₂ + 4H₂O,需氧 5 莫耳,對應空氣 5 ÷ 0.21 ≈ 23.81 莫耳,故 Cst = 1 ÷(1 + 23.81)× 100% ≈ 4.03%。依瓊斯經驗公式,燃燒下限約為化學理論濃度的 0.55 倍,0.55 × 4.03% ≈ 2.21%,與丙烷實測下限約 2.1% 相當吻合,故選 D。\n【逐項排除】\n(A) 1.34%:約僅為 Cst 的 0.33 倍,係數取錯;此數值接近戊烷的下限(約 1.4%),屬碳數代錯的典型誤答。\n(B) 1.64%:對應 Cst 約 2.98%,與丙烷的 4.03% 不符,需氧莫耳數配平有誤。\n(C) 1.84%:接近丁烷的下限(約 1.8%),係把碳數多算一個、需氧誤取 6.5 莫耳所致,並非丙烷之值。\n(D) 2.21%:0.55 × 4.03% 的計算結果,正確。\n【記憶點】先配平求需氧、除以 0.21 得空氣、算出 Cst,再乘 0.55 是下限、乘 3.5 是上限。","瓊斯法則的物理意義,是可燃氣體大致在「空氣量約為理論值 1.8 倍(即 1 ÷ 0.55)的稀薄狀態」下達到火焰無法傳播的臨界,而在「燃料約為理論值 3.5 倍的過濃狀態」下因氧化劑不足而熄滅。因此 LEL ≈ 0.55 Cst、UEL ≈ 3.5 Cst 兩式必須成對記憶。以丙烷驗算上限:3.5 × 4.03% ≈ 14.1%,與實測值 9.5% 差距較大,可見此經驗式對下限的預測相當準,對上限則僅供概估——這也是命題幾乎只考下限的原因。\n\nCst 的通式為 Cst = 100 ÷(1 + 4.773 n),其中 n 為每莫耳燃料完全燃燒的需氧莫耳數,4.773 即 1 ÷ 0.21。對 CₓH_y 而言 n = x + y ÷ 4,丙烷 x=3、y=8,n = 3 + 2 = 5,與配平結果一致;甲烷 n = 2 得 Cst ≈ 9.5%,乘 0.55 約 5.2%,同樣貼近實測的 5%。若分子含氧(如乙醇 C₂H₅OH),需氧量要扣掉分子自帶的氧,n = x + y ÷ 4 − z ÷ 2,漏扣就會低估 Cst。\n\n延伸考點包括:燃燒界限隨溫度升高而放寬,故本式所得為常溫常壓值;以及與勒沙特列混合氣體公式的結合——先用瓊斯法則求出各單一成分的下限,再代入勒沙特列公式求混合氣下限,是進階題常見的兩段式解法。另一個細節是 Cst 的分母含燃料本身,若漏掉那個 1 而寫成 1 ÷ 23.81,會算得 4.2%、使下限偏高約 0.1 個百分點,恰好足以在四個接近的選項中選錯,務必留意。",true,[28,32,36,40,44,48],{"webId":29,"stem":30,"number":31,"year":11,"session":12},"fire-110-1-fire-science-024","在充足的能量下，下列何種濃度（vol%）的氫氣（H2）為燃燒界限範圍內？",24,{"webId":33,"stem":34,"number":35,"year":11,"session":12},"fire-110-1-fire-science-026","易燃性氣體受到壓縮後溫度會有升高的狀況發生，可能會發火導致火災的發生，此現象為何？",26,{"webId":37,"stem":38,"number":39,"year":11,"session":12},"fire-110-1-fire-science-023","請問 2 公斤的氫氣（H2）完全燃燒，所需要的理論空氣量為多少公斤？",23,{"webId":41,"stem":42,"number":43,"year":11,"session":12},"fire-110-1-fire-science-027","某建築物的插座電壓為 110 伏特，在其延長線上的插座同時插 880 瓦的電磁爐、110 瓦的電風扇及 1100 瓦的電熱器，請問該延長線之額定電流至少需多少安培？",27,{"webId":45,"stem":46,"number":47,"year":11,"session":12},"fire-110-1-fire-science-022","火災中所產生的輻射熱量與下列何者成正比？",22,{"webId":49,"stem":50,"number":51,"year":11,"session":12},"fire-110-1-fire-science-028","下列有關電器用品「焦耳熱」之敘述，何者錯誤？",28,[53,57,61,65,69],{"webId":54,"year":55,"stem":56,"number":13},"fire-111-1-fire-science-025",111,"近來消防機關大力推行安裝住宅用火災警報器，其安裝位置在天花板或牆壁上緣，主要考量因素為何？",{"webId":58,"year":59,"stem":60,"number":13},"fire-109-1-fire-science-025",109,"有關火羽流的敘述，下列何者錯誤？",{"webId":62,"year":63,"stem":64,"number":13},"fire-108-1-fire-science-025",108,"木材受熱分解進而燃燒的過程中，當加熱到引火溫度時，可燃性氣體釋出量迅速增加，試問木材的引火溫度約為多少℃？",{"webId":66,"year":67,"stem":68,"number":13},"fire-107-1-fire-science-025",107,"下列何者並非複燃（backdraft）發生之原因？",{"webId":70,"year":71,"stem":72,"number":13},"fire-106-1-fire-science-025",106,"依據實驗，人穿著普通作業服時，其受熱之界限值約為多少？",1785123162475]