Chapter 3: Chemical Reactions part 3 page 1 Chemical Energies SABIS Grade 9 (Level K) ~ SABIS ® Curriculum Chemistry Online Tuition

After a long day of hiking through the mountains, it can feel good to find a pleasant spot to set up your tent and cook a hot meal. A small butane powered stove you carried in your pack is set up in a jiffy. You light the butane, which quickly begins burning and releasing heat. Soon your food is cooked and ready. You may wonder how heat can be generated from a chemical reaction. In this section, you will learn about chemical energy and how it is released or absorbed during chemical reactions.

A camping stove that uses butane as fuel can quickly heat water or food.

Energy Changes during Chemical Reactions

All chemical reactions are accompanied by a change in energy. Some reactions release energy into the surroundings and others absorb energy from the surroundings. An exothermic reaction is one that releases energy. An endothermic reaction is one that absorbs energy. Energy is generally in the form of heat but it can be also in the form of light, sound, or mechanical energy. For example, you can feel heat and see light energy being released from the exothermic reactions taking place during a fireworks show. You can feel the heat being absorbed by the endothermic reaction that occurs when you activate a cold pack from a first aid kit.

Fireworks are an example of an exothermic reaction that releases energy in the form of heat and light.

When writing chemical equations, you can indicate whether a reaction is exothermic or endothermic by adding the word energy to the equation. If the reaction is exothermic, energy is included with the products. If it is endothermic, energy is included with the reactants. Some examples are given below.

In a butane-powered stove, a chemical reaction occurs between butane and oxygen. The reaction is accompanied by a release of energy in the form of heat. The balanced chemical equation for the reaction is

Since it releases heat, this reaction is said to be exothermic. If you monitor an exothermic reaction using a thermometer to observe temperature changes before, during, and after the reaction, you would observe an increase in temperature as the reaction proceeds. The temperature measured immediately after the reaction would be higher than the temperature of the reactants just before they were mixed.

Endothermic reactions absorb heat from their surroundings. A thermometer placed in the reactants just before they are mixed would indicate a decrease in temperature as the reaction proceeds. An example of an endothermic reaction is the reaction in which ammonia breaks down to form nitrogen and hydrogen. In this case, the ammonia molecules have to gain heat in order to undergo this chemical change. The balanced chemical equation for the reaction is:

Endothermic reactions can also absorb energy in other forms. Photosynthesis (a very important reaction for living organisms) is endothermic. This reaction, however, absorbs light energy rather than heat energy. Light energy is used in this reaction to transform carbon dioxide and water into oxygen and glucose. The absorbed energy is stored in the glucose product.

Notice that we can speak more generally about endothermic processes, which includes both physical and chemical changes. For example, melting ice is a physical change that is an endothermic process

Classify each of the following reactions as exothermic or endothermic. Choose the answer.

NaOH + HCl → NaCl + H2O + energy :

HCO3 + H+ → H2O + CO2 ΔH > 0 :

A reaction in which the heat content of the reactants is greater than that of the products :

A reaction that causes the temperature of the reacting mixture to drop :

answer :

NaOH + HCl → NaCl + H2O + energy :

= exothermic

HCO3 + H+ → H2O + CO2 ΔH > 0 :

= endothermic

A reaction in which the heat content of the reactants is greater than that of the products :

= exothermic

A reaction that causes the temperature of the reacting mixture to drop :

= endothermic

Response:
That's the correct answer

Bond Energies

Where does the energy released during an exothermic reaction come from? Where does the energy absorbed during an endothermic reaction go? The answers to these questions can be found by considering bond energies.

When two atoms come together to form a bond, energy is released. Bond formation is therefore an exothermic process. This is because the bonded atoms have, generally, lower energy than the separated atoms.

From the graph below, you can see that energy would have to be put into a bond to break it. In other words, the two atoms X have more energy when they are separate than when they are bonded together. Bond energy is defined as the amount of energy necessary to break a chemical bond. Every chemical bond has a specific bond energy. The greater the bond energy, the more energy is needed to separate the two atoms. Also, the greater the bond energy is, the stronger and more stable the bond is.

The two atoms bonded together contain less energy than when they are separated.

The table below lists average bond energies for various types of covalent bonds. The unit used for measuring bond energy is kilojoule per mole, abbreviated kJ/mol. A kilojoule is equivalent to 1,000 joules. A mole is 6.02 × 1023 of any chemical substance (atoms, molecules, electrons, ions, etc.). The mole concept will be studied in detail in the next chapter. In terms of heat energy, 4.184 joules of heat energy will raise the temperature of 1 g of water by 1°C. Another unit of energy is the calorie (cal). One kilocalorie is equivalent to 4.184 kilojoules. Note that 1 “Calorie” equals 1,000 calories or 1 kilocalorie, and it is used to measure the amount of energy in food.

Bond energies are listed as averages because bond energies vary slightly depending on what other bonds are present in a molecule being decomposed. For example, a molecule of methane, CH4, has four C−H bonds. Breaking the first C−H bond requires the most energy input. The second requires a little less, the third a little less than the second, and the last C−H bond requires the least of the four. The average bond energy for C−H is calculated as the average of a wide range of bond energies measured for C−H bonds from many molecules.

Bond energies are very useful for calculating the energy change predicted for a reaction. If you know the bonds broken and the bonds formed in a particular chemical reaction, you can look up the bond energies for those bonds in a table such as the one above. Then you can calculate the energy change for the reaction by subtracting the sum of the bond energies formed in the products from the sum of the energies of the bonds broken in the reactants.

(Sum of bond energies broken) – (sum of bond energies formed) = energy change for reaction

If the energy change is negative, then the energy released from the formation of bonds in the products is greater than the energy put into the reactants to break the bonds. Therefore, this reaction is an exothermic reaction. If the energy difference is positive, the reaction would be endothermic because more energy was supplied to break apart the reactants than was released in forming the products.

Calculate the energy change for the reaction below.

3H2 + N2 → 2NH3

∆H =

What would the graph look like for a bond that is much stronger than the one shown below?

answer :

Response:
That's the correct answer

Experimental Measurement of Reaction Energy Change

The energy released or absorbed during a chemical reaction can take different forms, including heat, light, and work. In the lab, chemists focus on heat energy when they measure energy changes associated with chemical reactions. Chemists use specific conditions during their lab measurements to determine the amount of heat absorbed by or released from a chemical reaction. They define the amount of energy present in a substance as its heat content. Heat content is abbreviated with the capital letter H and is measured in joules or kilojoules. In the lab, chemists measure the change in heat content as reactants are converted to products during a chemical reaction. The change in heat content is abbreviated ΔH.

The change in heat content (ΔH) for a reaction is equal to the heat contents of the products minus the heat contents of the reactants:

ΔH = (sum Hproducts) – (sum Hreactants)

When ΔH > 0, the heat content of the products is greater than the heat content of the reactants. In this case, heat is absorbed during the reaction and the reaction is endothermic. For example, heat is absorbed when water is converted to hydrogen gas and oxygen gas. The change in heat content, ΔH, is +482 kJ:

When ΔH < 0, the heat content of the products is less than the heat content of the reactants. In this case, heat is released during the reaction and the reaction is exothermic. If you reverse the reaction above to form water from hydrogen and oxygen, it becomes exothermic. The change in heat content, ΔH, is then −482 kJ:

Heat must be put in to convert water to its elements, hydrogen and oxygen.
Heat is released during the opposite reaction when water is made from its elements.

Hess’s Law

An important discovery about energy changes in chemical reactions was made by the Swiss-born scientist Germain Hess, who became a professor of chemistry at a Russian university. There, he made careful measurements of the heat energy absorbed or released during various reactions.

Hess observed that the overall change in heat content is the same whether a reaction proceeds in one step or in a sequence of several steps. This is the basis for a principle known as Hess’s law. Hess’s law states that the sum of changes in heat content (ΔH) for a series of reaction steps will equal the change in heat content (ΔH) between the initial reactants and final products. This principle is illustrated in Figure 25.

Changes in heat content (ΔH) are the same for any set of reactants and products
even when different reaction pathways convert those reactants to products.

Figure 25 shows that ΔH between reactants A and products B is the same for either of the two reaction pathways on the graph. This means that, for the reaction pathway on the right side, the sum of the heat content changes of each step equals the net change in heat content between reactants and products: ΔH1 + ΔH2 = ΔH.

Applications of Hess’s Law

Hess’s law is also known as the law of constant heat summation. You can use Hess’s law to calculate the net heat content change for a series of reactions by summing up the individual heat content changes for each reaction in the series. For example, the following two reactions occur in a series. The product of the first reaction is a reactant in the second reaction:

You can think about chemical equations like mathematical equations that can be added. An arrow replaces the equal sign, but the process of adding equations is the same. Add the reactants of the two equations on one side of the arrow. Then add the products of the two equations on the other side of the arrow. Add the ΔH values in a third step. You should get the sum shown below reactions 1 and 2:

Now simplify the sum. First, combine the two O2 on the reactant side of the equation. Then cancel the two NO that appear on both sides of the equation.

Add the ΔH values and express the overall equation in final form:

In applying Hess’s law to sum a series of reaction equations, it is important to note that chemical equations can be written in reverse. In other words, you can rewrite the equation with the products as reactants and reactants as products. When you reverse a chemical equation, you must reverse the sign of the change in heat content for the reaction. The following equations describe the same reaction, but each is the reverse of the other:

Also, when you multiply a chemical equation by a constant value, you also multiply the heat content change by the same value. Multiplying the above equation by two gives the following equation:

You can apply this knowledge if you are given information about several individual reactions and must find the heat content change of an overall reaction. For example, suppose you need to determine the heat content change for the following reaction:

You are given the data below:

You can solve this problem using the following steps: multiply equation 1 by a factor of two, reverse equation 2, and add the resulting equations together: