C9 - Enthalpy Changes Flashcards
What is enthalpy?
‘H’ is a measure of the heat energy in a chemical system. (The chemical system being the atoms, molecules or ions making up the chemicals.)
What is enthalpy change and how is it calculated?
The difference in enthalpies:
ΔH = H(products) - H(reactants)
What are the ‘system’, ‘surroundings’ and ‘universe’ of a reaction?
System - the chemicals/reactants and products
Surroundings - the apparatus, the laboratory and everything not in the chemical system.
Universe - both the system and surroundings
What is an exothermic change?
When energy is transferred from the system to the surroundings.
What is an endothermic change?
When energy is transferred from the surroundings to the system
What occurs in an exothermic reaction?
Heat energy is released into surroundings.
Any energy loss by the chemical system is balanced by the same energy gain by the surroundings.
ΔH is negative
The temperature of the surroundings increases as they gain energy.
What is ΔH?
Change in enthalpy
What occurs in an endothermic reaction?
The chemical system takes in heat from the surroundings.
Any energy gain by the system is balanced by the energy loss by the surroundings.
ΔH is positive
The temperature of the surroundings decreases as they lose energy.
What is the standard enthalpy change of reaction?
ΔHr⊖ is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions.
What is ΔHr⊖?
Standard enthalpy change of reaction
What is standard enthalpy change of formation?
ΔHf⊖ 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 on standard states.
What is standard enthalpy change of combustion?
ΔHc⊖ is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in standard states.
What is standard enthalpy of neutralisation?
ΔHneut⊖is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions.
How is energy change calculated?
Q = mcΔT
Q = energy change m = mass (of material being heated) ΔT = change in temperature
What are the reasons for a lower experimental value of enthalpy change of combustion than expected?
(ΔcH)
Heat loss to surroundings other than the water
Incomplete combustion
Evaporation of the fuel from the wick
Non-standard conditions
What’s average bond enthalpy?
The energy required to break one mole of a specified type of bond in a gaseous molecule.
Energy is always required to break bonds and are always endothermic.
What is the enthalpy for bond making and breaking?
Bond breaking is endothermic
Bond making is exothermic
How is enthalpy change of reaction calculated?
Sum of enthalpies of reactants - Sum of enthalpies of products
What is Hess’s law?
If a reaction can take place by two routes and the starting and finishing conditions are the same, the total enthalpy change is the same for each route.
What are the rules for calculating the enthalpy change of reaction using enthalpy of formation and combustion?
Using enthalpy of formation:
ΔrH = sum of ΔfH products - sum of ΔfH reactants
Using enthalpy of combustion: (CRAP)
ΔrH = sum of ΔcH reactants - sum of ΔcH products
What is meant by the term “average bond enthalpy”?
Average enthalpy change when one mole of gaseous, covalent bonds is broken.
Is bond breaking endothermic or exothermic?
Endothermic
Is bond making endothermic or exothermic?
Exothermic
