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Endothermic and Exothermic Reactions. The Experiments’ Aim. You are about to discover the difference between different chemical reactions that play an important role in daily life processes and the industry.
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The Experiments’ Aim • You are about to discover the difference between different chemical reactions that play an important role in daily life processes and the industry. • A number of different dissolution processes and chemical reactions will be conducted and the change in temperature – as a measure of the change in energy – will be used to distinguish between endothermic and exothermic processes. • It is important that you understand the scientific method before attempting the project. • You will have to do the following after completing the investigation: • Ask an investigative question • Formulate a hypothesis • Identify and control different variables • Design the investigation • Collect quantitative data accurately • Interpret the data • Verify results by conducting more than one experiment • Recognize, analyze and evaluate alternative explanations of observations and data and write an appropriate conclusion.
Introduction to Enthalpy Enthalpy Changes in Enthalpy • Change in composition and change in energy are characteristic of all chemical reactions. • All chemical reactions adhere to two laws: • The Law of conservation of Mass • The Law of Conservation of Energy • Chemical reactions can be divided into two groups: • Reactions that absorb energy from the environment in the form of heat are called endothermic reactions. • Reactions that release energy into the environment in the form of heat are called exothermic reactions. • Enthalpy is the heat content of a substance at constant pressure and is represented by the symbol H. • The change in enthalpy or heat of the reaction, ∆H, is the net energy released or absorbed during a chemical reaction that takes place at constant pressure. • Enthalpy change is measured in kJ or kJ∙mol-1.
Energy in bonds • To understand why some reactions are endothermic, while others are exothermic, we have to take a look at bond energies. • The strength of the intramolecular forces determine the size of the bond energy. • When chemicals are mixed to react, existing bonds have to be broken and energy is absorbed – endothermic process. • The reactant atoms are unstable and will rearrange • New bonds are formed, while energy is released – exothermic process.
Endo and Exo thermic changes • If more energy is absorbed to break existing bonds than what is released to form new bonds, the process is endothermic. • If less energy is needed to break existing bonds than what is released to when new bonds are formed, the process is exothermic.
Calculating bond energies and ∆H Example worked Table 1: Table of bond energies • . • Natural gas (methane) produces CO2(g), H2O(g) and energy when it burns in O2(g) • CH4(g) + 2O2(g) ⟶ CO2(g) + 2H2O(g) • Bonds that must be broken: • 4(C – H) ⇒ 4(413) • 2(O = O) ⇒ 2(498) • New bonds that must be formed: • 2(C = O) ⇒ 2(745) • 4(O – H) ⇒ 4(463) • ∆H = ∑▒〖𝑒𝑛𝑒𝑟𝑔𝑦 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 −〗 ∑▒〖𝑒𝑛𝑒𝑟𝑔𝑦 𝑟𝑒𝑙𝑒𝑎𝑠𝑒𝑑〗 • ∆H = [4(413) + 2(498)] – [2(745) + 4(463)] • ∆H = 2648 – 3342 • ∆H = -694 kJ∙mol-1 • The reaction is exothermic. Use the table of bond energies given below to determine if the combustion of natural gas is endothermic or exothermic.
Potential energy profiles Introduction Graph 1: Graph of endothermic reaction • The activation energy is the minimum energy required to start a reaction. • Chemical reactions may be regarded as the result of collisions between atoms, molecules or ions. • A chemical reaction only takes place when the total energy of the colliding particles is greater than the activation energy required for the reaction. • Most collisions do not have sufficient kinetic energy that can be converted to potential energy and do not lead to a reaction. • When particles have enough energy, existing bonds between the particles can be broken. • The particles are now in an unstable, high energy state and are on the brink of rearranging to form new compounds. • This temporary, unstable high energy state is called the activated complex. • When new bonds are formed, potential energy is released and can be used to ensure that the reaction continues. • A catalyst can be added to the reactants to speed up the reaction without taking part in the reaction. • A catalyst cannot start a reaction. • It lowers the activation energy, but has no effect on the energy of the reactants or products. • The enthalpy change (∆H) is not affected by the presence of a catalyst. • Eproducts > Ereactants • ∆H is positive
Potential energy profiles Graph 2: Graph of Exothermic reactions Graph 3: Graph of exothermic reaction with catalyst • Eproducts> Ereactants • ∆H is positive • A positive catalyst has the same effect on an exothermic reaction. Eproducts < Ereactants ∆H is negative
Energy changes in daily life General Chemistry Concepts behind the Use of Cold Packs: • . • Energy changes are either occurring naturally (combustion/burning, photosynthesis, respiration, boiling, freezing, etc.) or being employed by mankind to make our lives better through its numerous application. • One of the most common application is the use of Cold Packs in our daily lives. • When you have a headache, applying a cold pack relieve the pain. • Most of the commercially available cold packs contain chemicals which absorb heat when they react. • This is why cold packs causes a cooling effect on their person’s head and temporarily relieve the pain. • What really happens is that when the cold pack is used, the chemicals inside the pack are made to react with each other. • The reaction taking place in the system, is highly endothermic in nature – heat energy is taken into the system from the environment - in this case the environment is the person’s head. • The cold pack can simply be made up of 2 bags – a small inner bag and an outer bag. • The small inner bag can be just water and the out bag can be ionic salts such as ammonium chloride or potassium nitrate. • When the pack is squeezed, the small inner bag breaks and the ionic salt dissolves in water. • This causes a reaction which is endothermic in nature.
The project • You have to conduct a practical investigation that will allow you to: • Classify different dissolution processes and reactions as exothermic and endothermic. • Do research about one exothermic and one endothermic reaction and discuss its application in daily life or industry.
Part 1 The Breakdown Diagram 1: Salts that will be tested • Dissolution processes: • When ionic salts dissolve in water, the process is known as dissociation. • Some of the dissociation reactions are exothermic and others are endothermic. • By measuring the change in temperature during the dissolution process, you can identify the process as exothermic or endothermic. • An increase in temperature is characteristic of exothermic process. • A decrease in temperature is characteristic of endothermic process. • When polar covalent substances dissolve in water, the process is called ionisation. • You are going to dissolve H2SO4(ℓ) in water. • Remember to add the acid carefully, drop by drop to the water!
Part 2 The Breakdown Diagram 2: Expected reaction • Identify and explain applications of exothermic and endothermic reactions in everyday life and industry. • Write a report on the research that you have done about the applications. • Remember to include a bibliography. • No plagiarism will be allowed! Some of the experiments are vigorous! Make sure that you wear safety clothing and that the Educator is present at all times!
