4 Energetics

Created by

Rayan Zzy

Cards (27)

Enthalpy change is the heat energy change at constant pressure and temperature
If an enthalpy change occurs then energy is transferred between the system and the surroundings
In an exothermic reaction:
Energy is transferred from the system (chemicals) to the surroundings
The products have less energy than the reactant
The enthalpy change is negative
For example: combustion of fuels
In an endothermic reaction:
Energy is transferred from the surroundings to the system (chemicals)
The products have more energy than the reactants
The enthalpy change is positive
For example: decomposition
This is an exothermic reaction
A) Activation Energy (Ea)
B) Enthalpy change
This is an endothermic reaction
A) Enthalpy change
B) Activation energy Ea
C) oops
Standard enthalpy of formation:
The enthalpy change under standard conditions when 1 mole of a compound is formed from its elements, with all reactants and products in their standard states
The enthalpy of formation of an element is 0 by definition
Standard enthalpy of combustion:
The enthalpy change under standard conditions when 1 mole of a substances is burned completely in oxygen, with all reactants and products in their standard states.
Incomplete combustion is less exothermic than complete combustion
Standard conditions are:
100 kPa Pressure
298 K
Solutions at 1 mol dm-3
All substances should have their normal state at room temperature (298K)
$Q =$ $mcΔT$
Q = energy change in joules
m = mass of solution in grams
c = specific heat capacity
ΔT = Temperature change in kelvin
Calorimetry method:
Rinse and dry the equipment with solutions which will be used
Place the polystyrene cup in a beaker for insulation
Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
Clamp the thermometer into place so the thermometer bulb is immersed in solution
Measure the temperature of the solution in (one minute) intervals
After a few minutes transfer the second reagent to the cup and stir the mixture
Record the temperature every minute for several minutes
In calorimetry, if the reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction so the readings are extrapolated with two lines of best fit
Errors in calorimetry method:
Energy lost to the surroundings
Approximation in specific heat capacity of solution (assumes that all solutions have the specific heat capacity of water)
Reaction may be incomplete or too slow
Density of solution is taken to be the same as water
$ΔH =$ $Q/n$
ΔH = enthalpy change
Q = energy change
n = number of moles
Hess's Law states that the total enthalpy change for a reaction is independent of the route taken from reactants to products
To calculate the enthalpy change of formation, the arrows are pointing upwards
To calculate the enthalpy change of combustion, the arrows are pointing downwards
The mean bond enthalpy is the energy needed to break the covalent bond into gaseous atoms, averaged over different molecules.
In an exothermic reaction, the sum of the bonds in the reactants will be less than the sum of the bonds in the products.
In an endothermic reaction, the sum of the bonds in the reactants will be more than the sum of the bonds in the products.
In general if all substances are gases:
ΔH = Σ bond energies broken - Σ bond energies made
Enthalpy change values calculated using mean bond enthalpies are less accurate than using formation or combustion data because the mean bond energies are not exact
As you go up a homologous series, there is a constant number and type of extra bonds being broken and made, so the enthalpy of combustion increases by a constant amount
Because oxygen is the only substance that has the bond
Exam-style question
A) pipette
B) polystyrene
C) Reweigh
D) initial
E) intervals
F) Plot a graph
G) Extrapolate