Energetics Flashcards
(25 cards)
What is enthalpy?
It is a measure of the heat content of a substance
What is an enthalpy change (∆H)
Change in heat content at constant pressure
What are standard conditions?
100 kPa and a stated temperature (usually 298K)
What is the energy change in an exothermic reaction?
Energy change is negative
What is the heat change in an exothermic reaction?
Heat is given out to the surroundings
Whats the energy change in an endothermic reaction?
Energy change is positive
What is the heat change in an exothermic reaction?
Heat is absorbed from the surroundings.
Define standard enthalpy change of formation
Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions.
Define standard enthalpy change of combustion
Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions.
What is the enthalpy of formation of an element?
0
Why is the enthalpy of formation of an element 0?
by definition
Define standard enthalpy change of neutralisation
Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions.
Which equation is used to find the amount of heat energy given out in calorimetry ?
q=mc∆T
In calorimetry, how do you find the heat enthalpy change of a reaction?
∆H = q/moles
What are the biggest problems with calorimetry practicals
Heat loss either as draughts or as lack of insulation
What is Hess’s Law?
The enthalpy change for a reaction is independent of the route taken
How would you find ∆H for a reaction if you were given enthalpy of formation data?
∆H = [SUM of ∆Hf products] – [SUM ∆Hf reactants]
How would you find ∆H for a reaction if you were given enthalpy of combustion data?
∆H = [SUM of ∆Hc reactants] – [SUM ∆Hc products]
What is the intermediate between reactants and products in the thermochemical cycle if you are given enthalpy of formation data?
the elements that make the reactants and products
What is the intermediate between reactants and products in the thermochemical cycle if you are given enthalpy of combustion data?
The oxides of the elements that make up the reactants and products
What is a bond enthalpy?
Bond enthalpy is the enthalpy change to break one mole of covalent bonds in the gas phase.
How are average bond enthalpies calculated?
The value for the bond enthalpy is an average taken from a range of molecules
What can be a problem when using average bond enthalpies?
They are not as accurate as they might be because the values used are averages and not the specific ones for that compound.
What phase must compounds be in to use bond energy to calculate ∆H?
They must all be gases
