Name: heat changes in chemical equations
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 55 s
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North Carolina K-12 Science Standard Course of Study

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Chemistry

Energy

PS.Chm.7.2

Thermochemical Equations

Большинство химических реакций происходит в условиях атмосферного давления. В условиях постоянного давления изменение тепла, связанное с реакцией дельта Q, равно изменению энтальпии дельта H, также называемое энтальпией или теплотой реакции. Энтальпия реакции это разница между энтальпией продуктов и исходных веществ.Когда энтальпия продуктов больше энтальпии исходных веществ, дельта H положительна. Такие реакции поглощают тепло и являются эндотермическими. Напротив, если энтальпия исходных веществ больше энтальпии продуктов, дельта H отрицательна.Такие реакции выделяют тепло и являются экзотермическими. Для любой химической реакции величина сопутствующего изменения энтальпии зависит от стехиометрических количеств исходных веществ и продуктов, на что указывают коэффициенты сбалансированного уравнения. Сбалансированное химическое уравнение, которое включает отметки фаз и энтальпию реакции, дельта H, называется термохимическим уравнением.Рассмотрим сжигание метана основного источника топлива. Горящий метан выделяет тепло в окружающую среду. На экзотермический характер реакции указывает отрицательное изменение энтальпии в термохимическом уравнении.Уравнение горения показывает, что когда один моль газообразного метана реагирует с 2 молями газообразного кислорода с образованием 1 моля газообразного диоксида углерода и 2 молей жидкой воды, в окружающую среду выделяется 890, 8 килоджоулей тепла. Молярное соотношение между исходными веществами или продуктами и теплотой реакции можно использовать в качестве коэффициента пересчета для расчета теплообмена во время реакции. Если в газовом баллоне содержится 25, 5 кг метана, и весь метан в баллоне сгорает, какое количество тепла будет выделяться?В общем, основная задача состоит в том, чтобы преобразовать массу в моли, а затем моли в теплоту реакции. Для начала нужно умножить 25, 5 килограмма на 1000, чтобы найти массу в граммах. Затем массу метана делят на его молярную массу 16, 0 грамма на моль, чтобы получить 1594 моля метана.Наконец, используя коэффициент преобразования между молями метана и теплотой реакции, 1594 моля метана выделяют 14, 2 умноженное на десять в 6 степени килоджоулей тепла являющиеся теплотой реакции. Ответ будет отрицательный, так как реакция экзотермична, поскольку в ней выделяется тепло.

<ol>
	<li>Канагаратна, Себастьян Г. "Наглядное пособие по вычислениям энтальпии." Журнал химического образования 77, No9 (2000): 1178.</li>
	<li>Кейфер, Дэвид. «Энтальпия и второй закон термодинамики». Журнал химического образования 96, No 7 (2019): 1407-1411.</li>
	<li>Халил, Мутасим И. "Расчет энтальпии реакции матричным методом." Журнал химического образования 77, No2 (2000): 185. 
	 </li>
</ol>

Термохимические уравнения

Для химической реакции (системы), осуществляемой при постоянном давлении – с единственной работой, проведенной в результате расширения или сжатия – энтальпия реакции (также называется жара реакции, ΔHrxn) равна теплообмену с окружающей средой (qp). 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-01.png"" style=""height: 40px;"" />
 
 Изменение энтальпии является обширным свойством, и оно зависит от количества реагенты, участвующего в реакции (или количества молей реагенты). Изменение энтальпии специфически к реакции, и физические состояния реагент и вида продукции важны. Экзотермическая реакция характеризуется значением ΔHrxn, в то время как эндотермическая реакция имеет значение +ΔHrxn value. 
 
 Так как количество тепла, выделенное или поглощенное реакцией, соответствует количеству каждого вещества, потребляемого или производимого реакцией, удобно использовать термохимическое уравнение для представления изменений как вещества, так и энергии. В термохимическом уравнении изменение энтальпии реакции показано как ΔHrxn, и обычно оно приводится по уравнению для реакции. Величина ΔHrxn указывает количество тепла, связанное с реакцией, указанной в химическом уравнении. Знак ΔHrxn указывает на то, является ли реакция экзотермической или эндотермической, как написано. В следующем уравнении 1 моль газообразного водорода и 1/2 моль газообразного кислорода (при некоторой температуре и давлении) реагируют на формирование 1 моль жидкой воды (при той же температуре и давлении). 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-02.png"" style=""height: 40px;"" />
 
 Это уравнение показывает, что в окружающую среду выделяется 286 кДж тепла. Другими словами, выделяется 286 кДж тепла (реакция экзотермическая) для каждого моль водорода, потребляемого или для каждого моль воды, которая производится. Поэтому энтальпия реакции — это коэффициент пересчета, который можно использовать для расчета количества тепла, выделенного или поглощенного во время реакций, связанных с определенным количеством реагенты и продуктов. 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-03.png"" style=""height: 80px;"" />
 
 Если коэффициенты химического уравнения умножаются на какой-то коэффициент (т.е. если изменяется количество вещества), то изменение энтальпии должно быть умножено на тот же коэффициент. 
 
 (двукратное увеличение сумм) 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-04.png"" style=""height: 40px;"" />
 
 (двукратное уменьшение сумм) 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-05.png"" style=""height: 40px;"" />
 
 Чтобы показать, что изменение энтальпии реакции зависит от физических состояний реагенты и продуктов, рассмотрите возможность образования газообразной воды (или водяного пара). Когда 1 моль газообразного водорода и ½ моль газообразного кислорода реагируют на форму 1 моль газообразной воды, выделяется только 242 кДж тепла, в отличие от 286 кДж тепла, которое выделяется при формировании жидкой воды. 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-06.png"" style=""height: 40px;"" />"

Для химической реакции (системы), протекающей при постоянном давлении – с единственной работой, вызванной расширением или сжатием – ...

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heat changes in chemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property.

The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a positive one. For that reason, a thermochemical equation is useful because it shows both the chemical change and the energy change together.

In a thermochemical equation, ΔHrxn is written after the chemical equation. Its magnitude tells how much heat is linked to the reaction as written. Its sign shows whether the reaction is exothermic or endothermic in that form.

For example, 1 mole of hydrogen gas and 1/2 mole of oxygen gas can react to form 1 mole of liquid water. This reaction releases 286 kJ of heat to the surroundings. That means 286 kJ is released for every mole of hydrogen used or every mole of water made, so ΔHrxn can act like a conversion factor.

If the coefficients in the chemical equation are multiplied, the enthalpy change must be multiplied by the same factor. A two-fold increase in the amounts gives a two-fold increase in ΔHrxn, and a two-fold decrease gives a two-fold decrease. The physical state matters too. When the same reaction makes gaseous water instead of liquid water, only 242 kJ of heat are released.

Heat Changes in Chemical Equations

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Internal Energy, Heat, and Work

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thermochemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property. The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a...

constant pressure calorimetry

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enthalpies of reaction

Calculating reaction heat with Hess's law uses standard enthalpies of formation to find the enthalpy change of a reaction. When the formation values for the reactants and products are known, the overall reaction can be broken into simpler stepwise reactions. This approach helps show how the heat change for the full reaction is built from smaller parts. First, the reactants are thought of as being decomposed into their component elements. The enthalpy change for this step is proportional to the...

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Learn how ΔHrxn shows heat released or absorbed in reactions, why equation coefficients matter, and how physical state changes the enthalpy value.

Heat Changes in Chemical Equations | JoVE K12

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Heat Changes in Chemical Equations

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