Why reactions are endothermic or exothermic

If we put energy into a molecule, we can cause its bonds to break, separating a molecule into individual atoms. Bonds between certain specific elements usually have a characteristic energy, called the bond energy, that is needed to break the bond.

The same amount of energy was liberated when the atoms made the chemical bond in the first place. The term bond energy is usually used to describe the strength of interactions between atoms that make covalent bonds. For atoms in ionic compounds attracted by opposite charges, the term lattice energy is used. For now, we will deal with covalent bonds in molecules. Although each molecule has its own characteristic bond energy, some generalizations are possible. For example, although the exact value of a C—H bond energy depends on the particular molecule, all C—H bonds have a bond energy of roughly the same value because they are all C—H bonds.

When a chemical reaction occurs, the atoms in the reactants rearrange their chemical bonds to make products. The new arrangement of bonds does not have the same total energy as the bonds in the reactants. Therefore, when chemical reactions occur, there will always be an accompanying energy change.

Energy changes in chemical reactions are usually measured as changes in enthalpy. In this process, one adds energy to the reaction to break bonds, and extracts energy for the bonds that are formed.

The energy changes can be tabulated and calculated as follows:. Because the bonds in the products are stronger than those in the reactants, the reaction releases more energy than it consumes.

This excess energy is released as heat, so the reaction is exothermic. Hence, we can re-write the reaction with the heat released kcal on the product side of the equation, as follows:. Step 1- First look at the equation and identify which bonds exist on in the reactants bonds broken.

Using the bond energies given in the chart above, find the enthalpy change for the thermal decomposition of water:. Endothermic and exothermic reactions can be thought of as having energy as either a reactant of the reaction or a product.

Endothermic reactions require energy, so energy is a reactant. In the course of an endothermic process, the system gains heat from the surroundings and so the temperature of the surroundings decreases gets cold. A chemical reaction is exothermic if heat is released by the system into the surroundings. Because the surroundings is gaining heat from the system, the temperature of the surroundings increases.

Exothermic Reaction : When methane gas is combusted, heat is released, making the reaction exothermic. This information can be shown as part of the balanced equation in two ways. First, the amount of heat released can be written in the product side of the reaction. The energy kcal is produced, hence the reaction is exothermic. The energy 5. A simple and familiar example is the combustion of methane gas CH 4.

We could write "heat" as one of the products on the right products side of the reaction if we wished. The term enthalpy , H , is used by chemists to describe how heat flows into or out of a system. It doesn't tell us whether the reaction will occur or how fast.

Endothermic reactions are usually not a great safety hazard. However, because the reaction draws heat from its surroundings, the reaction container may become cold and cause condensation or ice to form.

This can be a safety hazard if the materials involved react with water. Get your Class D and other specialty extinguishers from Safety Emporium. Many common chemicals undergo exothermic reactions. For example, simply dissolving sodium hydroxide NaOH in water produces enough heat that if this is not done carefully it could melt a plastic container!

Our entry on oxidation has an example of an even more exothermic reaction. It may be necessary to provide an introduction to explain the conventions of energy-level diagrams.

This collection of over practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures.

Use this infographic with your 16—18 students and get them working with thermochemistry, intermolecular forces and free energy. Use this practical to investigate how solutions of the halogens inhibit the growth of bacteria and which is most effective. Site powered by Webvision Cloud. Skip to main content Skip to navigation.

No comments. Try this related experiment For 14—16 year old students, the additional class practical and teacher demonstration featured at the bottom of this page provides a further opportunity to practise classifying reactions as exothermic or endothermic, using test tubes instead of polystyrene cups.

Test tube reactions not suitable for 11—14 years In this practical, students carry out three test tube reactions and use their hands on the base of the test tube to detect whether the process gives out or takes in energy, classifying them as exothermic or endothermic. Wear eye protection throughout. The anhydrous salt can be regenerated by heating in a hot oven. Under no circumstances must the zinc powder be allowed to come into contact with ammonium nitrate.

The two solids should be kept far apart at all times. It is recommended that ammonium nitrate is used only by post students, or by teachers as part of a demonstration. If students are to experience endothermic dissolving, they can use KCl. Consider using a digital thermometer with a clear display for the demonstration.

Procedure Carry out the following reactions. Find out whether the reaction: Gives out energy exothermic , or Takes in energy endothermic A test for water: the reaction of water and anhydrous copper II sulfate Put a spatula measure of white, anhydrous copper II sulfate powder into a test tube. Use a dropping pipette to add a few drops of water to the powder. Watch what happens and feel the bottom of the tube.

The sherbet mixture: the reaction of sodium hydrogencarbonate and citric acid In a dry test tube, mix one spatula measure of citric acid with one spatula measure of sodium hydrogencarbonate.

Add about 2 cm 3 water to the mixture. A competition reaction: the reaction of copper II sulfate solution and zinc Put about 5 cm 3 copper II sulfate solution in a test tube.

Using a spatula, add a small measure of powdered zinc. Stir with a glass rod. Teacher demonstration: dissolving ammonium nitrate Take about 10 cm 3 of water in a test tube. Ask a student to note its temperature or display the temperature with a digital thermometer. Add a large spatula measure of ammonium nitrate.