8. Energetics I Flashcards
(18 cards)
Give the standard conditions
Pressure: 1atm, 100 kPa
Temperature: 298K
Concentration: 1 moldm-3
Define enthalpy change
The heat energy change measured at a constant pressure
–> Symbol = ΔH
Is ΔH positive or negative in an exothermic reaction
For exothermic reaction, enthalpy change is negative (temperature increases)
So for endothermic reaction, enthalpy change is positive (temperature decreases)
Describe what happens in an exothermic reaction
In an exothermic reaction, energy is transferred from the system (chemicals) to the surroundings.
The products have less energy than the reactants as energy is let out
Describe what happens in an endothermic reaction
In an endothermic change, energy is transferred from the surroundings to the system (chemicals).
Define standard enthalpy change of reaction, ΔH°r
The enthalpy change when the number of moles of substances shown by an equation as written react under standard conditions (298K, 100kPa), with all reactants and products in the states given.
Define standard enthalpy change of formation, ΔH°f
The enthalpy change when one mole of a substance forms from its constituent elements under standard conditions (298K, 100kPa), with all reactants and products in their standard states
eg: 2C (s) + 2.5H2 (g) –> C2H5
Define standard enthalpy change of combustion, ΔH°c
The enthalpy change when one mole of a substance burns completely in oxygen under standard conditions (298K, 100kPa), with all reactants and products in their standard states
eg: C2H5OH (l) + 3O2 (g) –> 2CO2 (g) + 3H20 (l)
Define standard enthalpy change of neutralisation, ΔH°c
The enthalpy change when one mole of water is produced in a neutralisation reaction between an acid and an alkali under standard conditions (298K, 100kPa), (not standard states because all will be aqueous)
NaOH (aq) + 0.5H2SO4 (aq) –> 0.5Na2SO4 (aq) + H2O (l)
Define Hess’s Law
The enthalpy change for a given reaction is the same, independent of the route by which this is achieved (provided the temperatures, pressures, and physical states of products and reactants are the same).
What are Hess’s cycles used for
They are used to measure the enthalpy change for a reaction that cannot be measured directly by experiments. Instead alternative reactions are carried out that can be measured experimentally.
Define bond enthalpy
The energy required to break one mole of bonds, in the gaseous state.
Define mean bond enthalpy
The energy required to break one mole of bonds, in the gaseous state, averaged across a range of compounds.
Give the equation to work out enthalpy change
Q = mcΔT
Energy transferred (J) = mass (same as volume) x specific heat capacity x temperature change
Give the equation to work out the enthalpy change per mole of a substance (which can be enthalpy change of reaction, formation, combustion)
ΔH = Q/n
Enthalpy change (KJ) / number of moles of the thing that was added
Describe an experiment to measure the enthalpy change of a reaction
–> put polystyrene cup in a beaker for insulation and support
–> Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
–> measure the initial temperatures of the solution or both solutions if 2 are used using a thermometer. Do this every minute for 2-3 minutes
–> At minute 3, transfer second reagent to cup.
–> If using a solid reagent then use ‘before and after’ weighing method
–> stir mixture ( mixing ensures that all of the solution is at the same temperature)
–> Record temperature every minute after addition for several minutes
Give the reasons why the standard enthalpy change of combustion calculated could be lower than published
1) if spirit burner is uncapped, fuel will evaporate, making it appear as though more fuel has been combusted
2) not all energy is transferred to water (calorimeter and air)
3) not all of fuel underwent complete combustion
4) experiment may not have been under standard conditions
5) energy lost to surroundings
6) assuming solutions have specific heat capacity of water
7) reaction/dissolving may be incomplete or slow
A combustion reaction is always….
Exothermic
