How Can Energy Change During A Chemical Reaction?

You have probably seen a fire burning or burnt fuel for warmth or cooking or lite. A fire burning is one of the most noticeable examples of a chemical reaction that produces a lot of energy.

All chemical reactions involve energy changes. In some reactions, we are able to observe these energy changes as either an increase or a decrease in the overall free energy of the organization. In some reactions we meet this as a alter in the temperature. In other reactions we tin observe this change when a reaction starts to requite off light or when a reaction volition just work after light is shone on it.

The written report of energy changes (particularly heat) in chemical reactions is known as chemical thermodynamics. This is also sometimes chosen thermochemistry.

12.ane Energy changes in chemical reactions (ESBQJ)

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

When a chemical reaction occurs, bonds in the reactants break, while new bonds course in the production. The post-obit instance explains this. Hydrogen reacts with oxygen to form h2o, according to the following equation:

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

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

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

Bond energy

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

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

Figure 12.one: Graph showing the changes in energy that accept place as the distance between two atoms changes.

We can use this diagram to understand why bond breaking requires free energy and bail making releases energy. Signal X on the diagram is at the lowest energy. When a bond breaks, the atoms move apart and the altitude betwixt them increases (i.e. the cantlet moves to the correct on the \(ten\)-axis or from signal X to point 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 energy at point 10). So when a bond breaks energy is needed.

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

Looking at the example of hydrogen reacting with oxygen to course water:

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

We see that free energy is needed to break the bonds in the hydrogen molecule and to break the bonds in the oxygen molecule. And we too 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 look at the enthalpy of the system.

Enthalpy

Enthalpy is a measure out of the total energy of a chemical system for a given force per unit area, and is given the symbol H.

A chemical system is a airtight system that contains only the reactants and products involved in the reaction.

Equally we learn about exothermic and endothermic reactions we will see 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 free energy that is released when the new bonds of the products are formed. This means that in the overall reaction, energy is released as either heat or light. This type of reaction is chosen an exothermic reaction.

Exothermic reaction

An exothermic reaction is ane that releases free energy in the form of heat or low-cal.

Another way of describing an exothermic reaction is that information technology is ane in which the free energy of the products is less than the energy of the reactants, considering energy has been released during the reaction. We can correspond this using the following full general formula:

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

In other reactions, the energy that must be absorbed to suspension 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 blazon of reaction is known as an endothermic reaction.

Endothermic reaction

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

Another fashion of describing an endothermic reaction is that information technology is 1 in which the energy of the products is greater than the energy of the reactants, because energy has been absorbed during the reaction. This tin be represented by the following general formula:

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

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

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

A recommended project for formal cess is included. In this experiment learners will investigate an exothermic reaction and an endothermic reaction. This experiment is split into two parts. The first office looks at an endothermic reaction, while the 2d part looks at an exothermic reaction. You will demand polystyrene or cardboard cups, plastic lids, thermometers, vinegar, steel wool, citric acrid, sodium bicarbonate and stirring rods. There is likewise a further investigation on exothermic and endothermic reactions that learners can complete. All these experiments can exist combined into one projection in which learners investigate several unlike reactions and classify these reactions as exothermic or endothermic.

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

Endothermic and exothermic reactions - role i

Apparatus and materials

Yous will need:

• citric acid
• sodium bicarbonate
• a polystyrene cup
• a lid for the loving cup
• thermometer
• glass stirring rod
• scissors

You lot can get polystyrene cups with lids from java shops or fast food stores. Cardboard cups will also work fine. Some of the lids will have a hole for a straw, which is useful for this experiment.

Annotation that citric acrid is institute in citrus fruits such as lemons. Sodium bicarbonate is actually bicarbonate of soda (blistering soda), the baking ingredient that helps cakes to rise.

Method

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

2. Pour some citric acid \((\text{C}_{6}\text{H}_{eight}\text{O}_{vii})\) into the polystyrene cup, cover the cup with its lid and record the temperature of the solution.

3. Stir in the sodium bicarbonate \((\text{NaHCO}_{3})\), then cover the cup again.

4. Immediately record the temperature, and then accept a temperature reading every 2 minutes after that. Record your results.

The equation for the reaction that takes identify is:

\[\text{C}_{half-dozen}\text{H}_{viii}\text{O}_{7}\text{(aq)} + 3\text{NaHCO}_{iii}\text{(s)} → iii\text{CO}_{2}\text{(grand)} + 3\text{H}_{2}\text{O(}ℓ\text{)} + \text{Na}_{3}\text{C}_{six}\text{H}_{5}\text{O}_{7}\text{(aq)}\]

Results

Time (\(\text{mins}\))	0	2	4	half-dozen
Temperature (\(\text{℃}\))

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

Word and decision

• What happens to the temperature during this reaction?

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

• Why was it important 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 cup and record the temperature in the empty loving cup. You will demand to leave the thermometer in the loving cup for well-nigh 5 minutes in order to get an authentic reading.

2. Soak a piece of steel wool in vinegar for about a minute. The vinegar removes the protective coating from the steel wool then that the metal is exposed to oxygen.

3. Have the thermometer out of the cup. Keep the thermometer through the hole of the hat.

4. After the steel wool has been in the vinegar, remove it and squeeze out whatsoever vinegar that is however on the wool. Wrap the steel wool around the thermometer and place it (still wrapped round the thermometer) back into the loving cup. The cup is automatically sealed when yous practice this considering the thermometer is through the summit of the chapeau.

5. Get out the steel wool in the cup for well-nigh 5 minutes and then record the temperature. Tape your observations.

Results

You should discover that the temperature increases when the steel wool is wrapped around the thermometer.

Conclusion

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

Examples of endothermic and exothermic reactions (ESBQN)

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

Annotation that nosotros are but discussing chemical changes (recall from form 10 about physical and chemic changes). Concrete changes can also be classified equally exothermic or endothermic. When we are referring to concrete change then we talk most 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 green plants, which uses energy from the sunday to change carbon dioxide and water into food that the plant needs to survive, and which other organisms (such as humans and other animals) tin can swallow so that they too can survive. The equation for this reaction is:

     \[vi\text{CO}_{2}\text{(g)} + 6\text{H}_{ii}\text{O(ℓ)} + \text{energy} → \text{C}_{6}\text{H}_{12}\text{O}_{6}\text{(due south)} + 6\text{O}_{2}\text{(g)}\]

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

   • The thermal decomposition of limestone

     In industry, the breakdown of limestone into quicklime and carbon dioxide is very of import. Quicklime can be used to make steel from fe and likewise to neutralise soils that are too acid. However, the limestone must be heated in a kiln (oven) at a temperature of over \(\text{900}\) \(\text{℃}\) before the decomposition reaction will take place. The equation for the reaction is shown below:

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

2. Exothermic reactions

   • Combustion reactions

     The burning of fuel is an case of a combustion reaction, and we as humans rely heavily on this procedure for our energy requirements. The following equations depict the combustion of a hydrocarbon such as petrol \((\text{C}_{eight}\text{H}_{18})\):

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

     \[two\text{C}_{8}\text{H}_{18}\text{(ℓ)} + 25\text{O}_{two}\text{(one thousand)} → 16\text{CO}_{ii}\text{(m)} + xviii\text{H}_{2}\text{O(k)} + \text{oestrus}\]

     This is why we burn fuels (such every bit paraffin, coal, propane and butane) for free energy, considering the chemical changes that take identify during the reaction release huge amounts of energy, which we and then use for things like power and electricity. Y'all should also note that carbon dioxide is produced during this reaction. The chemic reaction that takes place when fuels burn has both positive and negative consequences. Although we benefit from heat, ability and electricity the carbon dioxide that is produced has a negative impact on the environment.

   • Respiration

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

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

     In the reaction above, glucose (a type of carbohydrate in the food we swallow) reacts with oxygen from the air that we exhale in, to form carbon dioxide (which we breathe out), water and energy. The free energy that is produced allows the cell to acquit out its functions efficiently. Can you come across now why you must eat food to get free energy? It is not the food itself that provides y'all with energy, just the exothermic reaction that takes place when compounds inside the food react with the oxygen y'all have breathed in!

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

Exothermic and endothermic reactions 1

Textbook Exercise 12.1

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

This is bail breaking and then free energy is absorbed.

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

This is bail forming so energy is released.

A molecule of nitrogen (\(\text{N}_{ii}\)) is formed.

A bond is formed and so free energy is released.

A molecule of carbon monoxide breaks apart.

A bond is broken and so 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 form.

Endothermic

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

Exothermic

Heat or light must be absorbed from the surroundings earlier this type of reaction takes place.

Endothermic

Source: https://www.siyavula.com/read/science/grade-11/energy-and-chemical-change/12-energy-and-chemical-change-01

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