Energetics Flashcards
(23 cards)
What are standard conditions?
100kPa
specified temp - usually 298K
What is enthalpy change?
Enthalpy change is the overall energy exchanged with the surroundings when a change happens at constant pressure, and same temperature
How to draw an enthalpy level diagram
Exothermic reaction - energy released to surroundings:
- reactants higher than products
- arrow facing down
- negative enthalpy change
Enothermic reaction - energy taken in from surroundings:
- reactants lower than products
- arrow facing up
- positive enthalpy change
Definition of standard enthalpy change of reaction (ΔH°r)
Enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions
Definition of standard enthalpy change of formation (ΔfH°)
Enthalpy change when one mole of a substance is formed from its constituent elements, in their standard states
Definition of standard enthalpy change of combustion (ΔcH°)
Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states
Definition of standard enthalpy change of neutralisation (ΔneutH°)
Enthalpy change when one mole of water is formed in a reaction between an acid and alkali under standard conditions
How to calculate energy transferred (enthalpy change)
Energy transferred (Q) = mass x spec. heat cap. x temp change
what are the sources of error and assumptions made where:
substances are mixed in an insulated container and the temp change is measured
errors:
* energy transfer to the surroundings
assumptions:
* all solutions have the heat capacity of water
what are the sources of error and assumptions made where:
enthalpy of combustion is measured, such as using a series of alcohols in a spirit burner
errors:
* energy transfer to the surroundings
assumptions:
* Combustion is complete
what are the sources of error and assumptions made where:
enthalpy change cannot be measured directly
errors:
* Same calorimetry errors (heat loss, incomplete reactions)
assumptions:
* No side reactions occur
How to calculate enthalpy change
Enthalpy change = Energy transferred (KJ) / number of moles
energy transferred (Q) = mass x spec. heat cap. x temp change
What to remember at the end of enthalpy change calculations
include the sign:
* + for endothermic
* - for exothermic
What is hess’s law?
The enthalpy change in a chemical reaction is independent of the route it takes.
Which way do the arrows go in a Hess Cycle demonstrating an enthalpy change of combustion?
Both the arrows from the reactant and product go down towards the intermediate.
Which way do the arrows go in a Hess Cycle demonstrating an enthalpy change of formation?
One arrow goes up from the intermediate to the reactant. The other arrow also goes up, but it goes towards the products.
Which way do the arrows go in a Hess Cycle demonstrating the enthalpy change of reaction?
One arrow goes down from the product to the intermediate. The other arrow goes up from the intermediate to the product.
Which way do the arrows go in a Hess Cycle demonstrating the enthalpy change of neutralisation?
One arrow goes down from the product to the intermediate. The other arrow goes up from the intermediate to the product.
Why can the enthalpy change of a thermal decomposition reaction not be measured directly?
Thermal energy must be supplied for the reaction to occur, therefore the measured temp change would not be due to just decomposition
definition of bond enthalpy
The energy required to break one mole of a specified type of bond in a gaseous molecule
definition of mean bond enthalpy
The average energy required to break one mole of a particular type of bond, averaged over many different compounds, with all species in the gaseous state
How to calculate an enthalpy change of reaction using mean bond enthalpies
- write out the equation using displayed formula
- list bonds broken and formed and totals
- calculate enthalpy change:
ΔHr = total energy to break bonds - total energy to make bonds
limitations of using bond enthalpies to calculate enthalpy change of reaction
- Bond enthalpies are averages
- All species must be in the gaseous state
