3.3 ENTHALPY Flashcards
What is enthalpy?
Enthalpy is a measure of the heat energy in a chemical system
-enthalpy change can be measured, enthalpy cannot
What is an exothermic reaction?
An exothermic is heat from the system into the surrounding. Delta H is positive
What is an endothermic reaction?
An endothermic reaction is heat from the surroundings into the system. Delta H is negative
What is activation energy?
Activation energy is the minimum energy required for a reaction to take place
What are the standard conditions?
Standard conditions:
pressure = 101 kPa
concentration = 1 mol/dm
temperature = 298K
state = physical state of substance under standard conditions
What is the standard enthalpy change of a reaction?
The standard enthalpy change of a reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation under standard conditions and states
What is the standard enthalpy change of neutralisation?
The standard enthalpy change of neutralisation is the enthalpy change that accompanies the reaction of an acid by a base to form 1mol of water under standard conditions and states
What is the standard enthalpy change of formation?
The standard enthalpy change of formation is the enthalpy change that takes place when 1mol of compound is formed from its elements under standard conditions and states
What is the standard enthalpy change of combustion?
The standard enthalpy change of combustion is the enthalpy change that takes place when 1mol of substance reacts completely with oxygen under standard conditions and states
How is energy change calculated?
Energy change calculated:
heat energy (J) = mass (g) x specific heat capacity (J/g/degrees C) x temperature change (degrees C)
What is average bond enthalpy?
Average bond enthalpy is the energy required to break 1mol of a specific type of bond in a gaseous molecule
-energy is always required to break bonds
-bond energies are always endothermic
-bond energies always have a positive enthalpy value
-actual bond energy can vary depending on chemical environment of bond
Bond breaking and bond making.
Bond breaking and bond making:
-energy required to break bonds (bond breaking endothermic)
-energy released when bonds form (bond making is exothermic)
What does Hess’ law state?
Hess’ law states that the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place provided the initial and final conditions remain the same.
In the Hess’ cycle, which way do the arrows point for enthalpy change of formation?
For enthalpy change of formation, the arrows point up.
In the Hess’ cycle, which way do the arrows point for enthalpy change of combustion?
For enthalpy change of combustion, the arrows point down.
