Topic 5.1: Energetics – Measuring energy changes
The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.
• Understanding: Heat is a form of energy.
▪ Heat (thermal energy): form of energy that is transferred from a warmer body to a cooler body through that can do work
• Process of heat transfer: conduction, radiation and convention
• When heat is transferred to an object, there is an increase in the average kinetic energy of particles
• Understanding: Temperature is a measure of the average kinetic energy of the particles.
▪ Temperature: measure of the average kinetic energy of the particle
• Temperature is proportional to average kinetic energy of particles
• Absolute zero: temperature in which all particle motion theoretically stops
• Understanding: Total energy is conserved in chemical reactions.
▪ Law of conservation of energy: states that energy can neither be created nor destroyed; can only be converted
• Energy, therefore, is conserved in chemical reactions and merely transferred
• Understanding: Chemical reactions that involve transfer of heat between the system and the surroundings are described as
endothermic or exothermic.
▪ Surroundings: rest of the universe distinct from the system (apparatus, external devices etc. not directly involved in reaction)
▪ System: content in which the chemical reaction takes place (all reactants, products, and any solvents)
• Open system: transfer of matter and energy is possible across its boundary
• Closed system: transfer of energy is possible but not the transfer of matter
Endothermic reaction Exothermic reaction
Definition Chemical reaction when heat is transferred Chemical reaction when heat is transferred
from the surrounding to the system from the system to the surrounding
Diagram
Enthalpy change (∆H) Positive Negative
Surrounding temperature Decreases Increases
Example Liquid to gas state change Combustion, acid-base neutralization
• Understanding: The enthalpy change (∆H) for chemical reactions is indicated in kJ mol -1.
▪ Enthalpy change (∆H): heat transferred between system and surrounding in a closed system during a chemical reaction
• Enthalpy change is indicated in kJ mol-1
• ∆H° reaction = Σ(∆Hf° products) − Σ(∆Hf° reactants)
▪ Standard enthalpy change of formation (∆Hf°): energy associated with the formation of 1 mol of a substance from constituting
elements in their standard state
• Must be 1 mol of substance; even if it gives a fraction of diatomic molecules in the reaction side
• Understanding: ∆H values are usually expressed under standard conditions, given by ∆H°, including standard states.
▪ Standard conditions: temperature of 25°C/298K, pressure of 1000 kPa, with species in their standard state
• Standard condition is denoted by a ° symbol; ∆H° denotes enthalpy change in standard condition
▪ Standard state: normal, stable state of a substance measured at 100 kPa
• Applications and skills: Calculation of the heat change when the temperature of a pure substance is changed using 𝑞 = 𝑚c∆𝑇.
▪ Specific heat capacity (c): amount of heat needed to raise the temperature of 1g of substance by 1 K
• Intensive property: specific heat capacity does not vary with the size of the system and is always the same
• Specific heat capacity of water: used in enthalpy change in aqueous systems (4.18 kJ Kg-1 K-1)
▪ Process of finding enthalpy change (∆H) with temperature change
• Volume of system is identified (in aqueous, the volume of water)
• Specific heat capacity is identified (in aqueous, heat capacity of water)
• Temperature change is identified
• Enthalpy change is determined through formula of 𝑞 = 𝑚c∆𝑇
The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.
• Understanding: Heat is a form of energy.
▪ Heat (thermal energy): form of energy that is transferred from a warmer body to a cooler body through that can do work
• Process of heat transfer: conduction, radiation and convention
• When heat is transferred to an object, there is an increase in the average kinetic energy of particles
• Understanding: Temperature is a measure of the average kinetic energy of the particles.
▪ Temperature: measure of the average kinetic energy of the particle
• Temperature is proportional to average kinetic energy of particles
• Absolute zero: temperature in which all particle motion theoretically stops
• Understanding: Total energy is conserved in chemical reactions.
▪ Law of conservation of energy: states that energy can neither be created nor destroyed; can only be converted
• Energy, therefore, is conserved in chemical reactions and merely transferred
• Understanding: Chemical reactions that involve transfer of heat between the system and the surroundings are described as
endothermic or exothermic.
▪ Surroundings: rest of the universe distinct from the system (apparatus, external devices etc. not directly involved in reaction)
▪ System: content in which the chemical reaction takes place (all reactants, products, and any solvents)
• Open system: transfer of matter and energy is possible across its boundary
• Closed system: transfer of energy is possible but not the transfer of matter
Endothermic reaction Exothermic reaction
Definition Chemical reaction when heat is transferred Chemical reaction when heat is transferred
from the surrounding to the system from the system to the surrounding
Diagram
Enthalpy change (∆H) Positive Negative
Surrounding temperature Decreases Increases
Example Liquid to gas state change Combustion, acid-base neutralization
• Understanding: The enthalpy change (∆H) for chemical reactions is indicated in kJ mol -1.
▪ Enthalpy change (∆H): heat transferred between system and surrounding in a closed system during a chemical reaction
• Enthalpy change is indicated in kJ mol-1
• ∆H° reaction = Σ(∆Hf° products) − Σ(∆Hf° reactants)
▪ Standard enthalpy change of formation (∆Hf°): energy associated with the formation of 1 mol of a substance from constituting
elements in their standard state
• Must be 1 mol of substance; even if it gives a fraction of diatomic molecules in the reaction side
• Understanding: ∆H values are usually expressed under standard conditions, given by ∆H°, including standard states.
▪ Standard conditions: temperature of 25°C/298K, pressure of 1000 kPa, with species in their standard state
• Standard condition is denoted by a ° symbol; ∆H° denotes enthalpy change in standard condition
▪ Standard state: normal, stable state of a substance measured at 100 kPa
• Applications and skills: Calculation of the heat change when the temperature of a pure substance is changed using 𝑞 = 𝑚c∆𝑇.
▪ Specific heat capacity (c): amount of heat needed to raise the temperature of 1g of substance by 1 K
• Intensive property: specific heat capacity does not vary with the size of the system and is always the same
• Specific heat capacity of water: used in enthalpy change in aqueous systems (4.18 kJ Kg-1 K-1)
▪ Process of finding enthalpy change (∆H) with temperature change
• Volume of system is identified (in aqueous, the volume of water)
• Specific heat capacity is identified (in aqueous, heat capacity of water)
• Temperature change is identified
• Enthalpy change is determined through formula of 𝑞 = 𝑚c∆𝑇
