ACSI HL Energetics & Thermochemistry 2022 (Teacher)

IBDP Chemistry HL/ Energetic s & Thermochemistry Page 1 Anglo − Chinese School (Independent) Year 5 (2022) IBDP Chemistry HL (IBDP syllabus Topic 5) 5.1 Measuring Energy Changes - Essential Idea: The enthalpy c hanges from chemical reactions can be calculated from their effe ct on the temperature of their surroundings. 5.2 Hess’ Law − Essential Idea: In chemical tr ansformations energy can neither be created nor destroyed (the firs t law of thermodynamics). 5.3 Bond Enthalpies − Essential Idea: Energy is abs orbed when bonds are broken and is released when bonds are formed. (IBDP syllabus Topic 15) 15.1 Energy Cycles − Essential Idea: The concept of the energy change in a single step reaction being equivalent to the summation of smaller steps can be applied to changes involving ionic compounds. 15.2 Entropy and Spontaneity − Essential Idea: A reaction is spontaneous if the overall transformation leads to an increase in total entropy (system plus surroundings). The direction of spontaneous change always increases the total entropy of the universe at the expense of energy available to do useful work. This is known as the second law of thermodynamics. TOPIC 5 Energetics & Thermochemistry TEACHER COPY – WITH SUGGESTED SOLUTIONS

IBDP Chemistry HL/ Energetics & Thermochemistry Page 2 5.1 Measuring energy changes Essential idea: The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.  Nature of Science:  Fundamental principle—conservation of energy is a fundamental principle of science. (2.6)  Making careful observations—measurable energy transfers betwee n systems and surroundings. (3.1) Understandings:  Heat is a form of energy.  Temperature is a measure of the average kinetic energy of the particles.  Total energy is conserved in chemical reactions.  Chemical reactions that involve transfer of heat between the s ystem and the surroundings are described as endothermic or exothermic.  The enthalpy change (ΔH) for chemical reactions is indicated i n kJ mol−1.  ΔH values are usually expressed under standard conditions, giv en by ΔHθ, including standard states. Applications and skills:  Calculation of the heat change when the temperature of a pure substance is changed using q = mcΔT.  A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated. Guidance:  Enthalpy changes of combustion (ΔH θc) and formation (ΔHθf) should be covered.  Consider reactions in aqueous solution and combustion reaction s.  Standard state refers to the normal, most pure stable state of a substance measured at 100 kPa. Temperature is not a part of the definition of standar d state, but 298 K is commonly given as the temperature of interest.  The specific heat capacity of water is provided in the data bo oklet in section 2.  Students can assume the density and specific heat capacities o f aqueous solutions are equal to those of water, but should be aware of this limitation.  Heat losses to the environment and the heat capacity of the ca lorimeter in experiments should be considered, but the use of a bomb calorimeter is not required.

IBDP Chemistry HL/ Energetics & Thermochemistry Page 3 5.1 What is thermodynamics?  Energetics (also known as thermochemistry) is the study of energy changes in chemical reactions. Total energy in a chemical reaction is conserved in a chemical reaction. Chemical potential energy is stored in the chemical bonds of reactants a nd products, while the temperature of the reacting mixture is a function of the kinetic energies of the atoms, ions and molecules present.  Heat, q, is a form of thermal energy that is transferred from a war mer body to a cooler body, as a result of the temperature gradient. When heat is transferred to an object, the result is an increase in the average kinetic energy of its particles and the refore an increase in its temperature or a change in phase.  At absolute zero, 0 K (−273.15 oC), all motion of the particles theoretically stops. The absolute temperature (in kelvin) is proportional to the average kinetic energy of the particles of matter. As the temperature increases, the kinetic energy of the particles increases 5.1.1 Enthalpy and thermochemistry  Enthalpy, H, is a measure of the heat energy contained in a substance. It is stored in the chemical bonds and intermolecular forces as potential energy. T he actual enthalpy of a substance is not quantifiable (cannot be directly measured). However, when substances react, the difference in enthalpy between the reactants and products ( at constant pressure) results in a heat change which can be observed.  Thermochemistry is the study of heat changes that occur during chemical reactions. At constant pressure, the change in enthalpy ΔH is defined as the heat transferred by a closed system during a chemical reaction. The term “change in enthalpy” or “heat of reaction” is commonly used when describing the thermodynamics of a reaction. The unit of enthalpy change ΔH is kJ mol−1.  All chemical reactions involve the breaking and forming of bon ds. Bond breaking requires energy to be absorbed and bond forming releases energy . Thus, there is usually a net change in the total amount of energy of the reacting particles in most chemical reactions. This net change in the energy of the reacting particles is called the enthalpy change of reaction, Δ H = H products − Hreactants where H reactants = energy content of reactants H products = energy content of products The higher the enthalpy of a substance the less stable it is.

IBDP Chemistry HL/ Energetics & Thermochemistry Page 4 5.1.2 Enthalpy change of reaction, ΔH  It is the enthalpy change when molar quantities of reactants a s specified by the chemical equation reacts to form products. E.g. N2 (g) + 3H2 (g)  2NH3 (g) ΔH = –100 kJ mol −1  According to the equation, 100 kJ of heat is evolved when 1 mo le of N2 (g) reacts with 3 moles of H2 (g) to give 2 moles of NH3 (g). E.g. 1 2N2 (g) + 3 2 H2 (g) → NH3 (g) ΔH = –50 kJ mol −1 5.1.3 Thermochemical equation  A thermochemical equation shows the enthalpy change of a react ion besides the number of moles of reactants and products as well as their state symbols. E.g. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) ΔH = –57.3 kJ mol −1 5.1.4 Exothermic reaction  The reaction mixture is called the system and anything around the system is called the surrounding or environment.  Exothermic reaction is a chemical reaction in which heat is transferred from the s ystem to the surroundings that results in a net loss of energy where ΔH is negative (ΔH < 0).  This net loss of energy is usually shown by an increase in the temperature of the surrounding as energy is released as heat to the surroundings. Exothermic reactions are common in chemistry. Neutralisation reactions between acids and alkalis liberate heat and result in an increase in temperature of the mixture. Combustion reactions where fuels, petrol or hydrogen gas are burnt in air or oxygen are exothermic reactions.  For all exothermic reactions or changes, the products are energetically more stable than the reactants. A common way to represent the enthalpy change of an exothermic reaction is using the enthalpy diagram as shown:

IBDP Chemistry HL/ Energetics & Thermochemistry Page 5 5.1.5 Endothermic reactions  Endothermic reaction is a chemical reaction that absorbs heat from their surroundings results in a net gain of energy where ΔH is positive (ΔH > 0). This net gain of energy is usually shown by a decrease in the temperature of the surr