12.2 Exothermic and also endothermic reactions (ESBQP)

The warmth of reaction (ESBQQ)

The heat the the reaction is represented by the prize \(Δ\textH\), where:

\<Δ \textH = E_\textprod - E_\textreact\>

In one exothermic reaction, \(Δ \textH\) is less than zero because the power of the reaction is better than the energy of the products. Energy is exit in the reaction. For example:

\<\textH_2\text(g) + \textCl_2\text(g) → 2\textHCl(g) \qquad Δ\textH

In an endothermic reaction, \(Δ \textH\) is higher than zero because the power of the reactants is much less than the energy of the products. Power is soaked up in the reaction. For example:

\<\textC (s) + \textH_2\textO(ℓ) → \textCO(g) + \textH_2\text(g) \qquad Δ\textH > 0\>

Some the the details relating come exothermic and endothermic reactions is summarised in Table 12.1.

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Type the reaction



Energy absorbed or released



Relative power of reactants and also products

Energy that reactants higher than power of product

Energy the reactants less than energy of product

Sign of ΔH

Negative (i.e. \( 0\))

Table 12.1: A to compare of exothermic and endothermic reactions.

Writing equations utilizing ΔH

ΔH has been calculation for countless different reactions and so instead of saying that ΔH is confident or negative, we can look up the worth of ΔH because that the reaction and use that worth instead.

There space two methods to write the warm of the reaction in an equation. For the exothermic reaction \(\textC(s) + \textO_2\text(g) → \textCO_2\text(g)\), we can write:

\<\textC(s) + \textO_2\text(g) → \textCO_2\text(g) \qquad Δ\textH = -\text393\text kJ·mol$^-1$\>


\<\textC(s) + \textO_2\text(g) → \textCO_2\text(g) + \text393\text kJ·mol$^-1$\>

For the endothermic reaction, \(\textC(s) + \textH_2\textO(g) → \textH_2\text(g) + \textCO(g)\), we can write:

\<\textC(s) + \textH_2\textO(g) → \textH_2\text(g) + \textCO(g) \qquad Δ\textH = \text+131\text kJ·mol$^-1$\>


\<\textC(s) + \textH_2\textO(g) + \text+131\text kJ·mol$^-1$ → \textH_2\text(g) + \textCO(g)\>

The units because that ΔH are \(\textkJ·mol$^-1$\). In other words, the ΔH value provides the quantity of power that is absorbed or released per mole of product that is formed. Systems can likewise be composed as \(\textkJ\), which then provides the full amount of energy that is exit or took in when the product forms.

The energy changes during exothermic and also endothermic reactions deserve to be plotted on a graph:

figure 12.2: The energy transforms that take place during an exothermic reaction.
figure 12.3: The energy transforms that take it place during an endothermic reaction.

We will describe shortly why we attract these graphs v a curve fairly than simply drawing a straight line from the reactants energy to the assets energy.

In the investigation on exothermic and endothermic reactions learners occupational with focused sulfuric acid. They must work in a fine ventilated room or, if possible, in a fume cupboard. This is a extremely corrosive substance and learners must manage it with care. If they spill any type of on us they must immediately wash the impacted area v plenty of running water. Either the student or your friend should educate you as soon as possible so you have the right to ensure that the student is ok. If essential the student may have to go to the bathroom to remove and rinse garments that is affected. One of two people you or an additional learner must accompany them.

Most that the salts that the learners will work-related with are hygroscopic and will conveniently absorb water native the air. This salts can cause chemical burns and should be tackled with care. If possible learners have to wear gloves to defend their hands.

Endothermic and exothermic reactions


To investigate exothermic and endothermic reactions.

Apparatus and also materials

roughly \(\text2\) \(\textg\) the calcium chloride \((\textCaCl_2)\) roughly \(\text2\) \(\textg\) of sodium hydroxide \((\textNaOH)\) approximately \(\text2\) \(\textg\) that potassium nitrate \((\textKNO_3)\) approximately \(\text2\) \(\textg\) that barium chloride \((\textBaCl_2)\) focused sulphuric acid \((\textH_2\textSO_4)\) (Be careful, this can reason serious burns) 5 check tubes thermometer
as soon as working with focused sulfuric acid always wear gloves and also safety glasses. Constantly work in a well ventilated room or in a fume cupboard.



Dissolve around \(\text1\) \(\textg\) of each of the complying with substances in \(\text5\)-\(\text10\) \(\textcm$^3$\) the water in a check tube: \(\textCaCl_2\), \(\textNaOH\), \(\textKNO_3\) and also \(\textBaCl_2\).

Observe whether the reaction is endothermic or exothermic, one of two people by emotion whether the next of the test pipe gets warm or cold, or making use of a thermometer.

Dilute \(\text3\) \(\textcm$^3$\) of concentrated \(\textH_2\textSO_4\) in \(\text10\) \(\textcm$^3$\) the water in the fifth test tube and observe even if it is the temperature changes.

Wait a couple of minutes and then carefully add \(\textNaOH\) to the diluted \(\textH_2\textSO_4\). Observe any type of temperature (energy) changes.


Record i beg your pardon of the above reactions are endothermic and which space exothermic.

Exothermic reactions Endothermic reactions

When \(\textBaCl_2\) and also \(\textKNO_3\) dissolve in water, they take in warm from the surroundings. The resolution of these salts is endothermic.

When \(\textCaCl_2\) and also \(\textNaOH\) dissolve in water, heat is released. The procedure is exothermic.

The reaction the \(\textH_2\textSO_4\) and \(\textNaOH\) is likewise exothermic.

Endothermic and exothermic reactions

In every of the following reactions, say even if it is the reaction is endothermic or exothermic, and also give a factor for her answer. Attract the resulting power graph because that each reaction.

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\(\textH_2\text(g) + \textI_2\text(g) → 2\textHI (g) + \text21\text kJ·mol$^-1$\)

Exothermic. Warm is given off and this is represented by showing \(+\) energy on the right hand side of the equation.