Enthalpy Change

1. Enthalpy Change Unit 2 – Energy Changes in Reactions

2. What energy is contained in a system? • Kinetic energy created by • the movement of molecules and atoms in the form of vibration, rotation and translation. • the movement of electrons around the nucleus of an atom. • Potential energy produced by • the chemical bonds between atoms in a molecule. • the interactions between molecules. • the attraction within the nucleus. • The attraction between the nucleus and electrons.

3. Enthalpy • The sum of all energy, kinetic and potential, contained in a system is called enthalpy (measured in J or kJ). • It is difficult to measure the enthalpy of a system because there are many factors to take into account. However, we may calculate the enthalpy change of a transformation, referred to as the heat of reaction. • The enthalpy change of a physical change or a chemical reaction corresponds to the total energy exchange between a system and its environment (under constant pressure).

4. Enthalpy • Energy, in the form of heat, is exchanged between the system and its environment during a physical change or a chemical reaction. • It is possible to measure the heat absorbed or released by a system during a transformation. The enthalpy change of a physical or chemical change is equal to the difference between the total enthalpy of the products and the total enthalpy of the reactants: • The enthalpy change of a system is determined using a calorimeter; the heat released or absorbed may be calculated using Q=mcT.

5. Enthalpy • The standard enthalpy change of a transformation is the enthalpy at SATP. • Energy that is exchanged during a transformation depends on the quantity of substances in the reactions and their temperature. The enthalpy change for the transformation of mole of substance is represented as standard molar enthalpy H (expressed as kJ/mol). The terms enthalpy and molar heat of reaction are often interchanged. • When the quantity of a substance is not expressed in moles, then we are not looking at molar enthalpy change. When the conditions of a reaction are not at SATP, then the enthalpy is not standard.

6. Enthalpy • When enthalpy change is negative, heat is released. • When enthalpy change is positive, heat must be absorbed for the reaction to occur.

7. Reactions • Physical and chemical changes lead to an exchange of heat with the environment. • Endothermic reactions absorb heat from the environment; they involve the breaking of interactions between particles. • Exothermic reactions release heat into the environment; they involve the formation of interactions between particles.

8. Physical Changes • Phase changes are physical changes. For a substance to change from one phase to another, the interactions between particles must change. • When a substance changes from a solid to a liquid, the distance between particles becomes greater. • When a substance changes from a liquid to a solid, the interactions between the particles becomes more frequent.

9. Physical Changes • A phase change may involve the breaking of the forces of attraction between particles. To occur, the particles need to distance themselves from one another and this requires energy. Therefore, these are endothermic reactions. • Fusion: the melting of an ice cube in your hand is an example of this; heat is being transferred from your and to the ice cube. • Sublimation • Evaporation

10. Physical Changes • A phase change may involve the formation of interactions between particles. To occur, the particles must release energy in order to become closer together. Therefore, these are exothermic reactions. • Solidification • Condensation: snow formation; intermolecular bonds form between water molecules

11. Reaction Progress Endothermic • When ice receives heat, its temperature increases steadily until it reaches 0C. • The heat provided breaks the interactions between the molecules. • The temperature remains constant until the ice becomes liquid. • The heat absorbed by the ice corresponds to the enthalpy of the fusion of water, Hfusion. At this point the water is in both the solid and the liquid phase. • Once all the ice is melted there are no more interactions to break and the temperature of the water will begin to rise again until it reaches the boiling point, 100C. • At boiling point, the temperature will remain constant until the water becomes gaseous. • The heat absorbed by the liquid water corresponds to the enthalpy of the vaporization of water, Hvaporization.

12. Reaction Progress • During a phase change, the temperature remains constant. This is due to the fact that the heat provided allows the particles to move away from each other. • Note the exothermic phase changes are the opposite and are represented by a graph that is the opposite of the one shown. • The enthalpy change of a phase change is always equal to that of the change in reverse, but with the opposite sign: Hsolidification=-Hfusion

13. Chemical Changes • During a chemical reaction, the reactant molecules break apart and their atoms form new products. • Energy is required to break bonds; therefore, bond breaking is an endothermic process. • The formation of bonds releases energy; therefore it is an exothermic process.

14. Chemical Changes • In order to determine whether a reaction is endothermic or exothermic, the energy absorbed through the breaking of the reactant bonds must be compared with the energy released through the formation of the new bonds in the products. • If the energy absorbed by the breaking of bonds is greater than the energy released by the formation of bonds, the reaction is endothermic. • If the energy absorbed by the breaking of bonds is less that the energy released by the formation of bonds, the reaction is exothermic.

15. Chemical Changes A chemical reaction may be expressed as a thermochemical reaction. A thermochemical reaction is one where the enthalpy change is introduced into the equation. The equation allows us to determine whether a reaction is endothermic or exothermic. Exothermic: Reactants Products H=-energy (kJ/mol)  Reactants  Products +energy (kJ) Note: H is negative and when written into the chemical equation, the energy appears on the side with the products. Endothermic: Reactants  ProductH=+energy (kJ/mol)  Reactants +energy (kJ)  Products  • Note: H is positive and when written into the chemical equation, the energy appears on the side with the reactants.

16. Chemical Changes • When the enthalpy change is expressed outside of the equation it is in kJ/mol. • When the enthalpy change is expressed in the equation, it must respect the stoichiometric coefficient of the substance that is formed or broken down. • In order to include the enthalpy change in a thermochemical equation, you need to know the type of chemical reaction. Example: • Synthesis reaction: the value of the molar enthalpy change must respect the stoichiometric coefficient of the substance being formed. • Combustion reaction: the value of the molar enthalpy must respect the stoichiometric coefficient of the substance that is undergoing combustion.

17. The diagram above shows the relative enthalpy of the reactants and the products. To the left, the enthalpy of the reactants is greater than the enthalpy of the products. Therefore, the enthalpy change is negative and the reaction is exothermic. To the right, the enthalpy of the products is greater than the enthalpy of the reactants. Therefore, the enthalpy change is positive and the reaction is endothermic. Chemical Changes