SL R1.1 Measuring Enthalpy Changes PDF

SL R1.1 Measuring enthalpy changes Xuesi Liu Learning outcomes  Reactivity 1.1.1—Chemical reactions involve a transfer of energy between the system and the surroundings, while total energy is conserved.  Understand the difference between heat and temperature.  React...

SL R1.1 Measuring enthalpy changes Xuesi Liu Learning outcomes  Reactivity 1.1.1—Chemical reactions involve a transfer of energy between the system and the surroundings, while total energy is conserved.  Understand the difference between heat and temperature.  Reactivity 1.1.2—Reactions are described as endothermic or exothermic, depending on the direction of energy transfer between the system and the surroundings.  Understand the temperature change (decrease or increase) that accompanies endothermic and exothermic reactions, respectively.  Reactivity 1.1.3—The relative stability of reactants and products determines whether reactions are endothermic or exothermic.  Sketch and interpret energy profiles for endothermic and exothermic reactions.  Reactivity 1.1.4—The standard enthalpy change for a chemical reaction, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states. It can be determined from the change in temperature of a pure substance.  Apply the equations Q= mcΔT and ΔH= −Q/n in the calculation of the enthalpy change of a reaction. Overview  Energy transfer in chemical reactions  Exothermic and endothermic reactions  Energy profiles  Standard enthalpy change Energy transfer in chemical reactions  All chemical reactions involve energy changes  Energy is transferred from the reaction system into the surroundings or the other way round  Most commonly, the energy is transferred in the form of heat, but it may also be in the form of electricity, sound or light Open, closed and isolated system Heat and temperature  Temperature, T, is a state function, which means any change in value is independent of the pathway between the initial and final measurements  Heat, q, is a form of energy that is transferred from a warmer body to a cooler body to a cooler body, as a result of the temperature gradient  Heat is sometimes referred to as thermal energy, which can be transferred by the processes of conduction, convection and radiation  Heat has the ability to do work—when heat is transferred to an object, its kinetic energy increases, which causes an increase in temperature and potentially a phase change Thermochemistry  Thermochemistry is the study of heat changes that occur during chemical reactions  Heat changes are often described in term of enthalpy change, , which is defined as the heat transferred from a closed system to the surroundings during a chemical reaction  The most common unit of enthalpy change is kJ Exothermic and endothermic reactions  Exothermic reaction—a chemical reaction in which heat is transferred from the system to the surroundings  Endothermic reaction—a chemical reaction in which heat is absorbed into the system from the surroundings  In a chemical reaction, chemical bonds in the reactants are broken, and new chemical bonds are made to form products  Bond breaking is an endothermic process—the energy needed is termed as bond dissociation energy can it can be quantified for each type of bond  Bond making is an exothermic process Exothermic and endothermic reactions  Enthalpy change of reaction, , can be calculated as  If , the reaction is endothermic  If , the reaction is exothermic  To determine whether a reaction is endothermic or exothermic, we can use a calorimeter  A calorimeter is any apparatus used to measure the amount of heat being exchanged between the system and the surroundings Energy profiles  Ea is the activation energy, which is the minimum energy required for the reaction to take place (more on this in Reactivity 2.2) Standard enthalpy change  The Plimsoll symbol ⦵ denotes standard temperature and pressure (273.15 K and 100 kPa)  Standard ambient temperature and pressure (SATP) refer to more practical reaction conditions of 298.15 K and 100 kPa  The standard enthalpy change for a reaction, , refers to the heat transferred at constant pressure under standard conditions and states  It can be determined from the change in temperature of a pure substance  The unit is kJ mol-1 Standard enthalpy change  To calculate for a reaction, we need to determine the amount of heat released or absorbed in the course of that reaction  This can be done by measuring the change in temperature of a pure substance to or from which the heat is transferred  E.g. to find out the of a fuel—heat is released during combustion, which is then absorbed by water. We can measure the temperature change to quantify the heat absorbed by water, thus the heat released when burning the fuel Standard enthalpy change  How to calculate the heat absorbed by water?  Q is the amount of heat absorbed by a pure substance  c is the specific heat capacity of that substance  Which is defined as the amount of heat needed to raise the temperature of 1 kg of that substance by 1 K  Specific heat capacity is an intensive property that does not vary in magnitude with the size of the system being described  m is the mass of the pure substance  is the change in temperature of that substance in K Standard enthalpy change Standard enthalpy change Standard enthalpy change  Once we have Q, how do we calculate  n is the number of moles of the limiting reactant, which is the reacting substance with the least stoichiometric amount present, which therefore limits how much product can be formed  In contrast, the other reacting substances are said to be in excess Standard enthalpy change Standard enthalpy change Standard enthalpy change  Calorimetry experiments typically give a smaller change in temperature than is predicted from theoretical values—why?  This is the result of heat loss from the system, which is difficult to measure