Enthalpy 3.9 Flashcards
What is Enthalpy?
It is a measure of heat energy in a chemical system. Enthalpy is sometimes thought of the energy stored within bonds.
What does the chemical system refer to?
The chemical system refers to the atoms, molecules or ions making up the chemical.
Enthalpy cannot be measured but….
Enthalpy cannot be measured but Enthalpy changes can.
What is an exothermic Enthalpy change?
The chemical system loses energy and surroundings gain energy.
What is an endothermic enthalpy change?
Chemical system gains energy from the surroundings, temperature decreases.
What is Standard Enthalpy Changes?
The enthalpy changes measured in standard conditions.
Why is it important for enthalpy to be in a standard condition?
This is because enthalpy changes under different conditions and it offers comparison between different reactions.
Standard Pressure
100kPa 1atm = 101 kPa
Standard Temperature
298K = 25C
Standard Concentration
1 mol dm-3
Standard State
State of an element in room temperature and pressure
What is Standard Enthalpy Change of Formation?
Is the Enthalpy Change that takes place when one mole of a compound is formed from its element in standard conditions, with all the reactants and products in their standard state.
Elements —> 1 mol
What is the Standard Enthalpy Change of Combustion?
Is the Enthalpy change that takes place when one mole of a substance reacts completely with oxygen in standard conditions, with all the reactants and products in their standard state.
What is the Standard Enthalpy of Neutralisation?
Is the enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) under standard conditions, with all the reactants and products in their standard state.
Acid + Base ——> 1 mol
Why is actual bond enthalpy different from average bond enthalpy?
Actual bond enthalpy is the energy required to break that bond in a specific compound. Average bond enthalpy is the mean energy required to break that type of bond in its gaseous state across a range of compounds.
BENDO and MEXO
Breaking is endothermic and making is exothermic.
Products - Reactants
Formation
Reactants - Products
Combustion
Arrows up
Formation
Arrows down
Combustion
Average bond enthalpy
The energy required to break one mole of a specific type of bond in a gaseous molecule across a range of compounds.
What are the main sources of error in combustion experiments?
- Heat loss to the surroundings
- Incomplete Combustion
- Not in standard conditions
- Evaporation of alcohol from wick (use lid to prevent)
What are ways to prevent heat loss in an combustion experiment?
- Replace beaker with polystyrene cup
- Distance between flame and beaker
- Volume of water
Hess cycle equation
Enthalpy change = reactants - products , going in the opposite direction of the U means you have to flip the sign.
What arrows should you use when different enthalpy changes are shown?
Use the arrows of the enthalpy change shown in the table/ the one provides
Average bond enthalpy equation
reactants - products
