Chapter 9 - Enthalpy Flashcards
What is enthalpy?
A measure of heat energy in a chemical system (H)
What is enthalpy change?
ΔH = H products - H reactants
Measured in kJmol-1
What happens to energy, temperature and products in exothermic reactions?
- Energy is transferred to the surroundings from the system
- Temperature of surroundings increases
- Products have less energy than reactants
- Enthalpy changes is negative
What does the reaction profile of an exothermic reaction look like?
x - axis = reaction progress
y - axis = enthalpy
Reactants above products
Activation energy from reactants to top of the curve
Enthalpy change from reactants to products pointing downwards
What happens to energy, temperature and products in an endothermic reaction?
- Energy is transferred from surroundings to the system
- Temperature of the surroundings decreases
- Products have more energy than reactants
- Enthalpy change is positive
What does the reaction profile for an endothermic reaction look like?
x - axis = reaction progress
y - axis = enthalpy
Reactants below products
Activation energy from reactants to top of the curve
Enthalpy change from reactants to products pointing upwards
What is activation energy?
Minimum amount of energy required to start a reaction
What is activation energy required for?
Energy required to break bonds in the reactants so that the atoms can rearrange to form new products
Why do we have standard enthalpy conditions?
Enthalpy change can vary depending on conditions
Chemists use standard conditions to compare enthalpy changes for different reactions
What is standard enthalpy change and symbol?
ΔH⦵ = an enthalpy change under standard conditions
What are the standard conditions for standard enthalpy change?
Standard pressure - 100kPa
Standard temperature - 298K (25 degrees)
Standard concentration - 1moldm-3
Standard state - the physical state of a substance under standard conditions
What is the definition for standard enthalpy change of reaction?
Standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with all reactants and products in standard states
How does enthalpy change increase if moles are doubled from 1 to 2?
Enthalpy change doubles
What is the definition of enthalpy change of formation?
Standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in standard states
What is the enthalpy change of formation to form an element from itself? O2 (g) -> ?
O2 (g) -> O2 (g)
ΔHf = 0kJmol-1
