Enthalpy Flashcards
What affects the amount of energy released in a reaction? What effect does this having on the results we receive and how is this overcame?
The energy released varies with conditions - temp., pressure, concentration of solutions etc. That means you have to state the conditions under which measurements are made, e.g. you normally measure changes at a constant atmospheric pressure.
What is enthalpy change? What is its symbol?
heat change at a constant pressure.
The symbol for enthalpy is H, therefore the symbol for enthalpy change is ΔH.
What are the standard conditions for measuring enthalpy changes? How is ΔH written under standard conditions?
A pressure of 100kPa (approx. normal atmospheric pressure)
Temperature of 298K (around normal room temperature
It is written as ΔH
When is a reaction complete?
When heat is produced (exothermic reaction) heat is given to the surrounding
The reaction is not thought of as being over until the products have cooled back to the conditions at the beginning.
When is delta H negative?
In a exothermic reaction, delta H is negative because the products have lost energy, heating up the surroundings.
How does pressure affect the heat energy given out in a reaction that involves gases?
If a gas is given out, some energy is required to push away the pressure.
The greater the atmospheric pressure, the more energy used for this.
This means that less energy remains to be given out as heat by the reaction.]
This is why it is important to have a standard of pressure for measuring energy changes.
Why are state symbols always needed in a equation?
The physical states of the reactants and products also affect the enthalpy change of a reaction
E.g. heat must be in to change liquid to gas and is given out when a gas is changed to a liquid.
Why does bond enthalpy vary?
mean bond enthalpy found by taking an average for Xe-F in a range of compounds
suggest why the value for entahlpy of formation of liquiod antimony is not zero
liquid is not standard state
What is the standard enthalpy of combustionb
- enthalpy chance / heat change at constant pressure when 1 mol of a substance combusts completely in oxygen
- with all substances in standard states under standard condtitions
