12-2 Review and Reinforcement Heat and Enthalpy Changes Answers

Y'all have probably seen a burn down called-for or burnt fuel for warmth or cooking or calorie-free. A burn down called-for is one of the near noticeable examples of a chemical reaction that produces a lot of free energy.

All chemical reactions involve free energy changes. In some reactions, we are able to observe these energy changes as either an increment or a decrease in the overall energy of the system. In some reactions nosotros run into this as a alter in the temperature. In other reactions nosotros can observe this change when a reaction starts to give off lite or when a reaction volition merely work after light is shone on information technology.

The report of free energy changes (particularly heat) in chemical reactions is known every bit chemic thermodynamics. This is also sometimes called thermochemistry.

12.1 Energy changes in chemical reactions (ESBQJ)

What causes the energy changes in chemical reactions? (ESBQK)

When a chemical reaction occurs, bonds in the reactants break, while new bonds form in the product. The following instance explains this. Hydrogen reacts with oxygen to grade water, according to the following equation:

\[2\text{H}_{2}\text{(g)} + \text{O}_{ii}\text{(g)} → two\text{H}_{2}\text{O}\text{(g)}\]

In this reaction, the bond between the two hydrogen atoms in the \(\text{H}_{2}\) molecule will pause, as volition the bond between the oxygen atoms in the \(\text{O}_{two}\) molecule. New bonds will form between the two hydrogen atoms and the unmarried oxygen atom in the water molecule that is formed equally the product.

For bonds to intermission, free energy must be absorbed. When new bonds form, energy is released. The energy that is needed to break a bond is called the bond energy or bond dissociation energy. Bond energies are measured in units of \(\text{kJ·mol$^{-1}$}\).

Bond energy

Bond energy is a measure of bail strength in a chemic bail. It is the corporeality of free energy (in \(\text{kJ·mol$^{-one}$}\)) that is needed to break the chemical bail between two atoms.

Remember when we discussed bonding (chapter 3) we used the following energy diagram:

Figure 12.1: Graph showing the changes in free energy that accept identify equally the distance between 2 atoms changes.

We can utilise this diagram to empathise why bail breaking requires energy and bond making releases energy. Bespeak Ten on the diagram is at the everyman free energy. When a bond breaks, the atoms move apart and the distance between them increases (i.eastward. the atom moves to the right on the \(ten\)-axis or from point Ten to signal A). Looking at the diagram nosotros see that when this happens, the energy increases (i.eastward. the energy at point A is greater than the free energy at point X). Then when a bail breaks energy is needed.

When a bond forms the atoms motion closer together and the altitude between them decreases (i.due east. the atom moves to the left on the \(10\)-axis or from indicate A to indicate X). Looking at the diagram we meet that when this happens, the energy decreases (i.due east. the free energy at point X is less than the free energy at point A). So when a bond forms energy is released.

Looking at the case of hydrogen reacting with oxygen to form water:

\[two\text{H}_{2}\text{(thousand)} + \text{O}_{2}\text{(one thousand)} → 2\text{H}_{2}\text{O}\text{(g)}\]

We see that energy is needed to interruption the bonds in the hydrogen molecule and to suspension the bonds in the oxygen molecule. And we also see that energy is released when hydrogen and oxygen bail to form water. When we look at the entire reaction and consider both bond breaking and bond forming we need to expect at the enthalpy of the arrangement.

Enthalpy

Enthalpy is a measure of the total free energy of a chemical system for a given pressure, and is given the symbol H.

A chemical organization is a closed organisation that contains only the reactants and products involved in the reaction.

As we learn about exothermic and endothermic reactions we volition run across more on the concept of enthalpy.

Exothermic and endothermic reactions (ESBQM)

In some reactions, the energy that must be absorbed to break the bonds in the reactants, is less than the energy that is released when the new bonds of the products are formed. This ways that in the overall reaction, energy is released as either estrus or light. This type of reaction is called an exothermic reaction.

Exothermic reaction

An exothermic reaction is one that releases energy in the grade of heat or light.

Some other way of describing an exothermic reaction is that it is 1 in which the energy of the products is less than the energy of the reactants, because energy has been released during the reaction. We can represent this using the following full general formula:

\[\text{Reactants} → \text{Products} + \text{Energy}\]

In other reactions, the energy that must be absorbed to interruption the bonds in the reactants, is more than the energy that is released when the new bonds in the products are formed. This means that in the overall reaction, free energy must be absorbed from the surroundings. This type of reaction is known equally an endothermic reaction.

Endothermic reaction

An endothermic reaction is one that absorbs free energy in the form of heat or light.

Another way of describing an endothermic reaction is that it is one in which the energy of the products is greater than the free energy of the reactants, because energy has been captivated during the reaction. This can be represented by the following full general formula:

\[\text{Reactants} + \text{Free energy} → \text{Products}\]

The difference in energy (E) between the reactants and the products is known as the heat of the reaction. It is also sometimes referred to as the enthalpy change of the system. This is represented using \(\Delta \text{H}\)

\(\Delta\) is read equally delta and means a change in. You may recall this symbol from physics.

A recommended projection for formal cess is included. In this experiment learners volition investigate an exothermic reaction and an endothermic reaction. This experiment is split into 2 parts. The first part looks at an endothermic reaction, while the second function looks at an exothermic reaction. You will demand polystyrene or paper-thin cups, plastic lids, thermometers, vinegar, steel wool, citric acid, sodium bicarbonate and stirring rods. There is also a further investigation on exothermic and endothermic reactions that learners tin can complete. All these experiments can be combined into one project in which learners investigate several different reactions and allocate these reactions every bit exothermic or endothermic.

This serial of experiments starts with an endothermic reaction between citric acid and sodium bicarbonate. The 2nd experiment in the series looks at the exothermic reaction betwixt steel and oxygen in the air. The last part of the series is given equally an investigation into various exothermic and endothermic reactions.

Endothermic and exothermic reactions - part 1

Apparatus and materials

You lot will need:

• citric acrid
• sodium bicarbonate
• a polystyrene cup
• a chapeau for the cup
• thermometer
• glass stirring rod
• scissors

You can get polystyrene cups with lids from coffee shops or fast food stores. Paper-thin cups will also work fine. Some of the lids will have a hole for a harbinger, which is useful for this experiment.

Note that citric acid is found in citrus fruits such equally lemons. Sodium bicarbonate is actually bicarbonate of soda (baking soda), the blistering ingredient that helps cakes to rise.

Method

1. If your lid does not have a hole for a straw, then cut a minor hole into the lid.

2. Pour some citric acrid \((\text{C}_{half dozen}\text{H}_{8}\text{O}_{vii})\) into the polystyrene cup, embrace the cup with its hat and record the temperature of the solution.

3. Stir in the sodium bicarbonate \((\text{NaHCO}_{3})\), then embrace the cup once more.

4. Immediately record the temperature, and and then take a temperature reading every two minutes after that. Record your results.

The equation for the reaction that takes place is:

\[\text{C}_{half dozen}\text{H}_{viii}\text{O}_{7}\text{(aq)} + 3\text{NaHCO}_{3}\text{(s)} → 3\text{CO}_{two}\text{(g)} + 3\text{H}_{2}\text{O(}ℓ\text{)} + \text{Na}_{three}\text{C}_{6}\text{H}_{five}\text{O}_{7}\text{(aq)}\]

Results

Fourth dimension (\(\text{mins}\))	0	ii	4	half dozen
Temperature (\(\text{℃}\))

Plot your temperature results on a graph of time (\(10\)-axis) against temperature (\(y\)-axis).

Discussion and conclusion

• What happens to the temperature during this reaction?

• Is this an exothermic or an endothermic reaction? (Was energy taken in or given out? Did the temperature increment or decrease?)

• Why was it of import to keep the cup covered with a lid?

Endothermic and exothermic reactions - part two

Apparatus and materials

• Vinegar
• steel wool
• thermometer
• polystyrene cup and plastic lid (from previous experiment)

Method

1. Put the thermometer through the plastic lid, cover the loving cup and record the temperature in the empty loving cup. You will need to go out the thermometer in the cup for virtually v minutes in order to go an accurate reading.

2. Soak a piece of steel wool in vinegar for well-nigh a minute. The vinegar removes the protective blanket from the steel wool and so that the metal is exposed to oxygen.

3. Take the thermometer out of the cup. Keep the thermometer through the pigsty of the lid.

4. After the steel wool has been in the vinegar, remove it and squeeze out any vinegar that is still on the wool. Wrap the steel wool around the thermometer and identify it (still wrapped circular the thermometer) back into the loving cup. The cup is automatically sealed when you practice this considering the thermometer is through the top of the lid.

5. Leave the steel wool in the cup for nigh 5 minutes and so tape the temperature. Record your observations.

Results

Yous should notice that the temperature increases when the steel wool is wrapped around the thermometer.

Conclusion

The reaction betwixt oxygen and the exposed metallic in the steel wool is exothermic, which means that free energy is released and the temperature increases.

Examples of endothermic and exothermic reactions (ESBQN)

In that location are many examples of endothermic and exothermic reactions that occur around us all the fourth dimension. The following are merely a few examples.

Notation that we are only discussing chemical changes (recall from grade 10 about physical and chemic changes). Physical changes can as well be classified every bit exothermic or endothermic. When we are referring to concrete change then we talk about exothermic or endothermic processes. Evaporation is an endothermic process while condensation is an exothermic process.

1. Endothermic reactions

   • Photosynthesis

     Photosynthesis is the chemic reaction that takes place in dark-green plants, which uses energy from the dominicus to change carbon dioxide and water into food that the establish needs to survive, and which other organisms (such as humans and other animals) can swallow so that they also tin can survive. The equation for this reaction is:

     \[6\text{CO}_{2}\text{(m)} + half-dozen\text{H}_{2}\text{O(ℓ)} + \text{free energy} → \text{C}_{6}\text{H}_{12}\text{O}_{6}\text{(s)} + 6\text{O}_{ii}\text{(g)}\]

     Photosynthesis is an endothermic reaction. Energy in the form of sunlight is absorbed during the reaction.

   • The thermal decomposition of limestone

     In industry, the breakup of limestone into quicklime and carbon dioxide is very important. Quicklime tin can be used to brand steel from atomic number 26 and also to neutralise soils that are too acid. Nonetheless, the limestone must be heated in a kiln (oven) at a temperature of over \(\text{900}\) \(\text{℃}\) before the decomposition reaction volition take place. The equation for the reaction is shown below:

     \[\text{CaCO}_{three}\text{(s)} → \text{CaO(south)} + \text{CO}_{2}\text{(yard)}\]

2. Exothermic reactions

   • Combustion reactions

     The called-for of fuel is an case of a combustion reaction, and nosotros as humans rely heavily on this process for our energy requirements. The following equations describe the combustion of a hydrocarbon such as petrol \((\text{C}_{8}\text{H}_{18})\):

     fuel + oxygen \(→\) estrus + water + carbon dioxide

     \[ii\text{C}_{8}\text{H}_{18}\text{(ℓ)} + 25\text{O}_{2}\text{(g)} → sixteen\text{CO}_{two}\text{(one thousand)} + 18\text{H}_{2}\text{O(g)} + \text{estrus}\]

     This is why we fire fuels (such as paraffin, coal, propane and butane) for free 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 besides notation that carbon dioxide is produced during this reaction. The chemic reaction that takes place when fuels burn has both positive and negative consequences. Although nosotros do good from heat, power and electricity the carbon dioxide that is produced has a negative bear on on the environment.

   • Respiration

     Respiration is the chemical reaction that happens in our bodies to produce energy for our cells. The equation beneath describes what happens during this reaction:

     \[\text{C}_{6}\text{H}_{12}\text{O}_{6}\text{(s)} + 6\text{O}_{2}\text{(k)} → half dozen\text{CO}_{2}\text{(g)} + half dozen\text{H}_{2}\text{O}\text{(ℓ)} + \text{energy}\]

     In the reaction above, glucose (a blazon of sugar in the food nosotros swallow) reacts with oxygen from the air that we exhale in, to form carbon dioxide (which nosotros breathe out), h2o and energy. The energy that is produced allows the cell to carry out its functions efficiently. Tin you lot see now why you lot must eat food to go energy? Information technology is not the food itself that provides you lot with free energy, merely the exothermic reaction that takes place when compounds within the food react with the oxygen you lot have breathed in!

Lightsticks or glowsticks are used by divers, campers, and for decoration and fun. A lightstick is a plastic tube with a glass vial within it. To activate a lightstick, you bend the plastic stick, which breaks the glass vial. This allows the chemicals that are inside the glass to mix with the chemicals in the plastic tube. These two chemicals react and release energy. Another part of a lightstick is a fluorescent dye which changes this energy into calorie-free, causing the lightstick to glow! This is known as phosphorescence or chemiluminescence.

Exothermic and endothermic reactions i

Textbook Practice 12.1

The bond between hydrogen and chlorine in a molecule of hydrogen chloride breaks.

This is bail breaking and so free energy is absorbed.

A bond is formed betwixt hydrogen and fluorine to course a molecule of hydrogen fluoride.

This is bond forming and and so energy is released.

A molecule of nitrogen (\(\text{Northward}_{2}\)) is formed.

A bail is formed and so energy is released.

A molecule of carbon monoxide breaks apart.

A bond is cleaved and then energy is absorbed.

Reactants react to give products and energy.

Exothermic

The energy that must be absorbed to break the bonds in the reactants is greater than the energy that is released when the products class.

Endothermic

The energy of the products is found to be greater than the energy of the reactants for this type of reaction.

Exothermic

Heat or lite must be absorbed from the surround before this type of reaction takes identify.

Endothermic

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