Energy Changes In Chemical Reactions Exothermic And Endothermic Processes Pdf

energy changes in chemical reactions exothermic and endothermic processes pdf

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What happens when you take a basketball, place it halfway up a playground slide, and then let it go? The basketball rolls down the slide. What happens if you do it again? Does the basketball roll down the slide?

Energy Changes in Chemical Reactions

This page deals with the basic ideas about energy changes during chemical reactions, including simple energy diagrams and the terms exothermic and endothermic. Obviously, lots of chemical reactions give out energy as heat. Getting heat by burning a fuel is a simple example, but you will probably have come across lots of others in the lab.

Other reactions need a continuous supply of heat to make them work. Splitting calcium carbonate into calcium oxide and carbon dioxide is a simple example of this. Any chemical reaction will involve breaking some bonds and making new ones. Energy is needed to break bonds, and is given out when the new bonds are formed.

It is very unlikely that these two processes will involve exactly the same amount of energy - and so some energy will either be absorbed or released during a reaction. You will find this discussed in more detail in the page about bond enthalpies. Notice that in an exothermic change, the products have a lower energy than the reactants. The energy that the system loses is given out as heat. The surroundings warm up. This time the products have a higher energy than the reactants. The system absorbs this extra energy as heat from the surroundings.

This shows that kJ of heat energy are evolved when equation quantities of carbon and oxygen combine to give carbon dioxide.

The mol -1 per mole refers to the whole equation in mole quantities. You always think of the energy change during a reaction from the point of view of the reactants. The reactants carbon and oxygen have lost energy during the reaction. When you burn carbon in oxygen, that is the energy which is causing the surroundings to get hotter. In this case, kJ of heat are absorbed when 1 mole of calcium carbonate reacts to give 1 mole of calcium oxide and 1 mole of carbon dioxide.

You can tell that energy is being absorbed because of the plus sign. A simple energy diagram for the reaction looks like this:. The products have a higher energy than the reactants. Energy has been gained by the system - hence the plus sign. Whenever you write values for any energy change, you must always write a plus or a minus sign in front of it.

You are likely to come across statements that say that something is energetically more stable than something else. For example, in the next page in this section you will find that I have said that oxygen, O 2 , is more energetically stable than ozone, O 3. What does this mean? The lower down the energy diagram something is, the more energetically stable it is.

If ozone converted into ordinary oxygen, heat energy would be released, and the oxygen would be in a more energetically stable form than it was before. Similarly, if you mix petrol gasoline and air at ordinary temperatures when you are filling up a car, for example , why doesn't it immediately convert into carbon dioxide and water?

It would be much more energetically stable if it turned into carbon dioxide and water - you can tell that, because lots of heat is given out when petrol burns in air. But there is no reaction when you mix the two. For any reaction to happen, bonds have to be broken, and new ones made. Breaking bonds takes energy.

There is a minimum amount of energy needed before a reaction can start - activation energy. If the molecules don't, for example, hit each other with enough energy, then nothing happens. We say that the mixture is kinetically stable , even though it may be energetically unstable with respect to its possible products. So a petrol and air mixture at ordinary temperatures doesn't react, even though a lot of energy would be released if the reaction took place. Petrol and air are energetically unstable with respect to carbon dioxide and water - they are much higher up the energy diagram.

But a petrol and air mixture is kinetically stable at ordinary temperatures, because the activation energy barrier is too high. If you expose the mixture to a flame or a spark, then you get a major fire or explosion. The initial flame supplies activation energy. The heat given out by the molecules that react first is more than enough to supply the activation energy for the next molecules to react - and so on. The moral of all this is that you should be very careful using the word "stable" in chemistry!

If this is the first set of questions you have done, please read the introductory page before you start. Energy changes during chemical reactions Obviously, lots of chemical reactions give out energy as heat. Simple energy diagrams A reaction in which heat energy is given off is said to be exothermic. A reaction in which heat energy is absorbed is said to be endothermic.

You can show this on simple energy diagrams. For an exothermic change: Notice that in an exothermic change, the products have a lower energy than the reactants. For an endothermic change: This time the products have a higher energy than the reactants. Expressing exothermic and endothermic changes in numbers Here is an exothermic reaction, showing the amount of heat evolved: This shows that kJ of heat energy are evolved when equation quantities of carbon and oxygen combine to give carbon dioxide.

How do you know that heat is evolved? That is shown by the negative sign. And here is an endothermic change: In this case, kJ of heat are absorbed when 1 mole of calcium carbonate reacts to give 1 mole of calcium oxide and 1 mole of carbon dioxide. A simple energy diagram for the reaction looks like this: The products have a higher energy than the reactants. Energetic stability You are likely to come across statements that say that something is energetically more stable than something else.

If you plot the positions of oxygen and ozone on an energy diagram, it looks like this: The lower down the energy diagram something is, the more energetically stable it is. So why doesn't ozone immediately convert into the more energetically stable oxygen?

Questions to test your understanding If this is the first set of questions you have done, please read the introductory page before you start.

2.3.2: Energy Changes in Chemical Reactions

Why is it important to know how much energy is transferred in an exothermic or endothermic reaction? Why are there energy changes when a chemical reaction takes place? Why do reactions give out heat energy to the surroundings exothermic reaction and other reactions absorb heat energy endothermic reactions. Do physical state changes involve energy changes? Examples of exothermic energy changes and endothermic energy changes in chemical reactions are described and explained.

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7.3: Exothermic and Endothermic Reactions

In grade 10 learners learnt about physical and chemical changes. In this chapter learners will learn about the energy changes that occur in chemical reactions. The concepts of exothermic and endothermic reactions are introduced. Learners will also learn about activation energy. The following list summarises the concepts covered in this chapter.

This page deals with the basic ideas about energy changes during chemical reactions, including simple energy diagrams and the terms exothermic and endothermic. Obviously, lots of chemical reactions give out energy as heat. Getting heat by burning a fuel is a simple example, but you will probably have come across lots of others in the lab.

Temperature changes in exothermic and endothermic reactions

In this investigation, students classify chemical reactions as exothermic or endothermic. Next, students explore the relationship between an observed change in temperature and the classification of a change as chemical or physical. After students explore one example of an endothermic change and one example of an exothermic change, they are then asked to explore the connection between energy changes and chemical reactions. To do this, students may need some guidance to arrive at the idea that temperature changes may also accompany dissolving. Students will have an easier time devising a fair test if they are well versed in the definitions of physical changes and chemical changes. Students should propose an experiment to you before they test their hypothesis. To observe a temperature change during a physical change, students should devise a procedure such as:.

Are you loving this? Not loving this? Please consider taking a moment to share your feedback with us. Students will conduct two chemical reactions. In the first, the temperature will go down endothermic and in the second, the temperature will go up exothermic.

This site uses cookies from Google and other third parties to deliver its services, to personalise adverts and to analyse traffic. Information about your use of this site is shared with Google. By using this site, you agree to its use of cookies. Read our policy. Energy is at the heart of chemical energetics, so supporting students in linking observation of temperature change with the energy transfers taking place is important. Giorgia leads a team at a small company that is helping to fight food waste by developing smart labels technology. Discover the importance of chemistry in everyday eating experiences with this collection of edible experiments.


The rate at which a chemical reaction takes place may be very important, both in in exothermic and endothermic reactions can be represented as shown.


Due to the absorption of energy when chemical bonds are broken, and the release of energy when chemical bonds are formed, chemical reactions almost always involve a change in energy between products and reactants. By the Law of Conservation of Energy, however, we know that the total energy of a system must remain unchanged, and that oftentimes a chemical reaction will absorb or release energy in the form of heat, light, or both. The energy change in a chemical reaction is due to the difference in the amounts of stored chemical energy between the products and the reactants.

Due to the absorption of energy when chemical bonds are broken, and the release of energy when chemical bonds are formed, chemical reactions almost always involve a change in energy between products and reactants. By the Law of Conservation of Energy, however, we know that the total energy of a system must remain unchanged, and that oftentimes a chemical reaction will absorb or release energy in the form of heat, light, or both. The energy change in a chemical reaction is due to the difference in the amounts of stored chemical energy between the products and the reactants. This stored chemical energy, or heat content, of the system is known as its enthalpy.

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Беккер с отвращением оглядел комнату. Грязь, в раковине мутная коричневатая вода. Повсюду разбросаны грязные бумажные полотенца, лужи воды на полу.

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