Enthalpy changes Flashcards
Draw a general exothermic energy profile
*Refer to new enthalpy changes notes
Draw a general endothermic energy profile
*Refer to new enthalpy changes notes
Definition of standard states
The physical state of a substance under standard conditions.
What are the standard conditions?
298 K (25 degrees)
100 kPa (1 atmosphere)
1 moldm^-3 for concentration of solutions.
Definition of enthalpy change of reaction
The enthalpy change associated with a stated equation (in their standard states under strandard conditions).
Definition of the enthalpy chage of formation
The enthalpy change when one mole of a compound is made from its elements (in their standard states under standard condition).
Definition of the enthalpy change of combustion
The enthalpy change whne one mole of a substance is completely combusted with oxygen (in their standard states under standard conditions).
Definition of the enthalpy change of neutralisation
The enthalpy change when an aqueous acid reacts with an aqueous base forming one mole of water (in their standard states under standard conditions).
2 equations…
Q = -mc(change in temp)
Enthalpy change = Q / n
*Refer to new enthalpy changes notes
Why are the values calculated experimentally less than those determined theorteically and where doe these occur?
- Heat loss
- Incomplete conbustion
- Non-standard conditions
Through the material of the container / through the open top of the conatiner.
Ways to minmise heat loss from an experiment
- Use and insulating lid.
- Add a lid.
Which part of a reaction is endothermic?
When bonds are broken as energy is absorbed from the surroundings into the reaction.
Which part of a reaction is exothermic?
When the bonds are made as energy is released from the reaction into the surroundings.
Explain an endothermic reaction in terms of making and breaking bonds
If more energy is required to break the bonds in the reactants than is released when the bonds in the products are made, then the overall energy change of the reaction is endothermic. Enthalpy change is always positive.
Explain an exothermic reaction in terms of making and breaking bonds
If more energy is released when bonds are made in the products than was required to break bonds in the reactants, then the overall energy change of the reaction is exothermic. Enthalpy change is always negative.
Definition of average bond enthalpy
The average bond enthalpy change when breaking one mole of a given type of bond in the molecules of a gaseous species.
Calculating enthalpy change using average bond enthalpies equation
Enthalpy change = total of bonds broken in the reactants (endothermic: +ve) — total of bonds made (exothermic: -ve)
For Hess’s law: if given combustion data in the question draw arrows going…
…downwards from the reactants and products, to the products of combustion.
For Hess’s law: if given formation data in the question, draw arrows going…
…upwards to the reactants and products from the elements that makes them.
Definition of Hess’s law
The enthalpy change of a reaction is independent of the route taken providing the initial and final conditions are the same.
