Energetics: Enthalpy Change Flashcards
1
Q
What is enthalpy change?
A
- Enthalpy change, ∆H (delta H), is the heat energy transferred in a reaction at constant pressure
2
Q
What are the units of ∆H?
A
- kJmol^-1
3
Q
What is standard enthalpy change?
A
- Standard enthalpy change refers to the heat energy changes under standard conditions of temperature and pressure
- Standard conditions are 100kPa and a stated temperature (usually 298K)
- ∆H298Ɵ
- Ɵ means standard conditions
4
Q
What are the different types of enthalpy change?
A
- Standard enthalpy change of formation
- Standard enthalpy change of combustion
- Standard enthalpy change of reaction
5
Q
What is standard enthalpy of formation?
A
- ∆fHƟ
- Standard enthalpy change of formation is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
6
Q
What is standard enthalpy change of combustion?
A
- ∆cHƟ
- Standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance undergoes complete combustion in excess oxygen under standard conditions with all reactants and products in their standard states
7
Q
What is standard enthalpy change of reaction?
A
- ∆rHƟ
- Standard enthalpy change of reaction is the enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states
8
Q
What are exothermic reactions?
A
- Exothermic reactions give out energy to their surroundings
- So the temperature in the reaction usually goes up
- The products of the reaction end up with less energy than the reactants
- So the enthalpy change for the reaction ∆H will be negative
9
Q
What are endothermic reactions?
A
- Endothermic reactions take in energy from their surroundings
- So the temperature in the reaction usually falls
- The products of the reaction have more energy than the reactants
- So the enthalpy change for the reaction ∆H will be positive
