Paper 1&2- Energetics Flashcards
(21 cards)
Define enthalpy change
Heat energy change at constant pressure
What are standard conditions?
100kPa
298K
1.0M concentration (for reactions with aqueous solutions)
Define enthalpy of combustion
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all reactants and products in their standard state
Is combustion always exothermic or endothermic?
Exothermic
Define enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants in their standard states
What are the units for enthalpy change?
kJmol-1
What is the enthalpy of formation for an element?
0
What is the difference between temperature and heat?
Temperature is related to the average KE of the particles in a system
Heat depends on temperature, mass and type of material
What are the units for heat?
Joules
What are the units for temperature?
Celsius
What equations are needed to find enthalpy of combustion?
q=mcT
H=q/n
Is the enthalpy change of an exothermic reaction positive or negative?
Negative
How could you improve the accuracy of a calorimetry experiment?
Add a lid to reduce heat loss to the surroundings
Insulate sides to reduce heat loss and drafts
Use a bomb calorimeter
What is one error that could cause a calorimetry experiment to be inaccurate?
Incomplete combustion
When calculating enthalpy change in a reaction between solutions, what mass do you use?
Mass of both solutions added together
When calculating enthalpy change of combustion, what mass do you use?
The mass of water
Give 3 reasons why it might not be possible to measure the enthalpy change of a reaction
High activation energy
A slow reaction rate
More than one reaction taking place
What is Hess’ law?
the total enthalpy change of a reaction is independent of the route by which the reaction takes place, provided the initial and final conditions are the same
Define bond dissociation energy
The energy required to break a covalent bond with all the species in the gaseous state
Define mean bond enthalpy
The average bond dissociation enthalpy, calculate from a range of compounds for a specific type of covalent bond
Why might a calculated value for bond enthalpy differ from a data book value?
The data book value is mean bond enthalpy, which is the average bond dissociation energy, calculated from a range of compounds for a specific covalent bond
