Energetics Flashcards
(30 cards)
What is enthalpy change
The heat energy change measured under conditions of constant pressure
What does standard enthalpy change mean
Just refers to standard conditions
Define standard enthalpy of combustion
The energy released when 1 mole of a substance undergoes complete combustion under standard conditions
Define standard enthalpy of formation
The enthalpy change when 1 mole of a compound is formed in its standard state from its constituent elements in their standard states under standard conditions
How to find q
The heat change, q, in a reaction is given by the equation q = mc∆T
What is m in the equation q = mc∆T
where m is the mass of the substance that has a temperature change ∆T and a specific heat capacity c
define the term mean bond enthalpy
the energy required to break a particular covalent bond in one mole of molecule in a gaseous state
Other terms for bond enthalpy
Bond energy and bond dissociation energy
What are the standard conditions for enthalpy chnages
Standard conditions are:
• 100 kPa pressure
• 298 K (room temperature or 25*C)
• Solutions at 1mol dm-3
• all substances should have their normal state at 298K
What does incomplete combustion lead to and what does this mean
- Will lead to soot (carbon), carbon monoxide and water
- It will be less exothermic than complete combustion
If an enthalpy change occurs then..
energy is transferred between system and surroundings
In an exothermic change energy is transferred..
from the system (chemicals) to the surroundings
-> the products have less energy than the reactants
In an endothermic change, energy is transferred..
from the surroundings to the system (chemicals).
-> they require an input of heat energy e.g. thermal decomposition of calcium carbonate.
-> the products have more energy than the reactants.
In an endothermic reaction the ∆H is..
Positive
In an exothermic reaction the ∆H is..
Negative
Errors in the calorimetric method
• energy transfer from surroundings (usually loss)
• approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water.
• neglecting the specific heat capacity of the calorimeter- we ignore any energy absorbed by the apparatus.
• reaction or dissolving may be incomplete or slow.
• density of solution is taken to be the same as water.
Errors in measuring enthalpies of combustion using calorimetery
- Energy losses from calorimeter
- Incomplete combustion of fuel
- Incomplete transfer of energy
- Evaporation of fuel after weighing
- Heat capacity of calorimeter not included
- Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment
density of water
is 1g cm-3
eg. 25 cm3 will weigh 25 g
Trend in Enthalpies of Combustion in a Homologous Series
there is a constant rise in the size of the enthalpies of combustion as the number of carbon atoms increases
Why are calculated values of enthalpy of combustions more accurate if calculated from enthalpy of formation data than if calculated from average bond enthalpies
because average bond enthalpy values are averaged values of the bond enthalpies from various compounds
When does the mean bond energy definition apply
only applies when the
substances start and end in the gaseous state
Are the values of mean bond energy negative or positive and why
positive because energy is required to break a bond
DeltaH =
= Σ bond energies broken - Σ bond energies made
(If all substances are gases)
Breaking bonds is..
Endothermic
