Topic 8- Energetics I Flashcards
Give 2 reasons why reactions may be classified as examples of combustion.
Oxygen is a reactant.
Reactions are exothermic.
Why is this reaction not a standard enthalpy change?
CH4 + 2O2 –> CO2 + 2H2O
This isn’t a standard enthalpy change of formation as there is more than one product.
What symbol is enthalpy given? Therefore what is the symbol for enthalpy change?
H
ΔH
A negative enthalpy change ………… heat energy
Gives out
A positive enthalpy change ………….. heat energy
Absorbs
What are the “standard conditions” for bond enthalpy?
25 degrees/ 298K
100kPa
What is the standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance in its standard state burns completely in O2 under standard conditions.
What is the standard enthalpy change of formation of a compound
The enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions.
State Hess’ Law.
The total enthalpy change of a reaction is the same no matter the route taken.
Define enthalpy change
the heat energy change in a reaction at constant pressure (in kJ mol-1)
The less enthalpy a substance has the more ……. it is
The less enthalpy a substance has the more stable it is.
Combustion is ………..
Exothermic
thermal decomposition is ……..
Endothermic
What is the standard enthalpy change of reaction?
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.
What is the standard enthalpy change of neutralisation?
The enthalpy change when an acid and an alkali react together, under standard conditions to form 1 mole of water.
To find enthalpy changes of combustion you need a….
Calorimeter
Give the 2 equations for calculating enthalpy changes
Q=mcΔT
Q= heat lost of gained (j) m=mass of water c=specific heat capactity (4.18J g-1 K-1) ΔT= temperature change of water/solution (K)
Q/mols = kJ mol -1
Bond breaking is an ……….. process
Bond breaking is an endothermic process
Bond making is an …………. process
Bond forming is an exothermic process,
What is bond enthalpy?
The amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase.
The less enthalpy a substance has the more …… it is
The less enthalpy a substance has the more STABLE it is
Bond making is
exothermic
Bond breaking is
endothermic
298K =…. “c
25”c
What do enthalpy level diagrams show vs reaction profiles?
Enthalpy level=overall change of reaction
Reaction profile= Enthalpy changes during a reaction
What does calorimetry mean?
measuring heat changes
what does a colorimeter do?
finds the enthalpy change of combustion by measuring temperature change of some water
