1.7 Energy changes in chemical reactions  (Page 3/5)

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 Type of reaction Exothermic Endothermic Energy absorbed or released Released Absorbed Relative energy of reactants and products Energy of reactants greater than energy of product Energy of reactants less than energy of product Sign of $\Delta$ H Negative Positive
Writing equations using $\Delta$ H

There are two ways to write the heat of the reaction in an equation

For the exothermic reaction $C\left(s\right)+{O}_{2}\left(g\right)\to C{O}_{2}\left(g\right)$ , we can write:

$C\left(s\right)+{O}_{2}\left(g\right)\to C{O}_{2}\left(g\right)$ $\Delta$ H = -393 kJ.mol ${}^{-1}$ or

$C\left(s\right)+{O}_{2}\left(g\right)\to C{O}_{2}\left(g\right)$ + 393 kJ.mol ${}^{-1}$

For the endothermic reaction, $C\left(s\right)+{H}_{2}O\left(g\right)\to {H}_{2}\left(g\right)+CO\left(g\right)$ , we can write:

$C\left(s\right)+{H}_{2}O\left(g\right)\to {H}_{2}\left(g\right)+CO\left(g\right)$ $\Delta$ H = +131 kJ.mol ${}^{-1}$ or

$C\left(s\right)+{H}_{2}O\left(g\right)$ + 131 kJ.mol ${}^{-1}$ $\to CO+{H}_{2}$

The units for $\Delta$ H are kJ.mol ${}^{-1}$ . In other words, the $\Delta$ H value gives the amount of energy that is absorbed or released per mole of product that is formed. Units can also be written as kJ, which then gives the total amount of energy that is released or absorbed when the product forms.

Investigation : endothermic and exothermic reactions

Apparatus and materials:

Approximately 2 g each of calcium chloride (CaCl ${}_{2}$ ), sodium hydroxide (NaOH), potassium nitrate (KNO ${}_{3}$ ) and barium chloride (BaCl ${}_{2}$ ); concentrated sulfuric acid (H ${}_{2}$ SO ${}_{4}$ ) (Be Careful, this can cause serious burns) ; 5 test tubes; thermometer.

Method:

1. Dissolve about 1 g of each of the following substances in 5-10 cm ${}^{3}$ of water in a test tube: CaCl ${}_{2}$ , NaOH, KNO ${}_{3}$ and BaCl ${}_{2}$ .
2. Observe whether the reaction is endothermic or exothermic, either by feeling whether the side of the test tube gets hot or cold, or using a thermometer.
3. Dilute 3 cm ${}^{3}$ of concentrated H ${}_{2}$ SO ${}_{4}$ in 10 cm ${}^{3}$ of water in the fifth test tube and observe whether the temperature changes.
4. Wait a few minutes and then carefully add NaOH to the H ${}_{2}$ SO ${}_{4}$ . Observe any energy changes.
5. Record which of the above reactions are endothermic and which are exothermic.

Results:

• When BaCl ${}_{2}$ and KNO ${}_{3}$ dissolve in water, they take in heat from the surroundings. The dissolution of these salts is endothermic .
• When CaCl ${}_{2}$ and NaOH dissolve in water, heat is released. The process is exothermic .
• The reaction of H ${}_{2}$ SO ${}_{4}$ and NaOH is also exothermic .

Examples of endothermic and exothermic reactions

There are many examples of endothermic and exothermic reactions that occur around us all the time. The following are just a few examples.

1. Endothermic reactions
• Photosynthesis Photosynthesis is the chemical reaction that takes place in plants, which uses energy from the sun to change carbon dioxide and water into food that the plant needs to survive, and which other organisms (such as humans and other animals) can eat so that they too can survive. The equation for this reaction is: $6{\mathrm{CO}}_{2}+12{\mathrm{H}}_{2}\mathrm{O}+\mathrm{energy}\to {\mathrm{C}}_{6}{\mathrm{H}}_{12}{\mathrm{O}}_{6}+6{\mathrm{O}}_{2}+6{\mathrm{H}}_{2}\mathrm{O}$ Photosynthesis is an endothermic reaction because it will not happen without an external source of energy, which in this case is sunlight.
• The thermal decomposition of limestone In industry, the breakdown of limestone into quicklime and carbon dioxide is very important. Quicklime can be used to make steel from iron and also to neutralise soils that are too acid. However, the limestone must be heated in a kiln at a temperature of over ${900}^{0}C$ before the decomposition reaction will take place. The equation for the reaction is shown below: ${\mathrm{CaCO}}_{3}\to \mathrm{CaO}+{\mathrm{CO}}_{2}$
2. Exothermic reactions
• Combustion reactions - The burning of fuel is an example of a combustion reaction, and we as humans rely heavily on this process for our energy requirements. The following equations describe the combustion of a hydrocarbon such as methane (CH ${}_{4}$ ): $\mathrm{Fuel}+\mathrm{Oxygen}\to \mathrm{Heat}+\mathrm{Water}+\mathrm{Carbon}\mathrm{Dioxide}$ ${\mathrm{CH}}_{4}+2{\mathrm{O}}_{2}\to \mathrm{Heat}+2{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{CO}}_{2}$ This is why we burn fuels for energy, because the chemical changes that take place during the reaction release huge amounts of energy, which we then use for things like power and electricity. You should also note that carbon dioxide is produced during this reaction. Later we will discuss some of the negative impacts of $C{O}_{2}$ on the environment. The chemical reaction that takes place when fuels burn therefore has both positive and negative consequences.
• Respiration Respiration is the chemical reaction that happens in our bodies to produce energy for our cells. The equation below describes what happens during this reaction: ${\mathrm{C}}_{6}{\mathrm{H}}_{12}{\mathrm{O}}_{6}+6{\mathrm{O}}_{2}\to 6{\mathrm{CO}}_{2}+6{\mathrm{H}}_{2}\mathrm{O}+\mathrm{energy}$ In the reaction above, glucose (a type of carbohydrate in the food we eat) reacts with oxygen from the air that we breathe in, to form carbon dioxide (which we breathe out), water and energy. The energy that is produced allows the cell to carry out its functions efficiently. Can you see now why you are always told that you must eat food to get energy? It is not the food itself that provides you with energy, but the exothermic reaction that takes place when compounds within the food react with the oxygen you have breathed in!

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