energetics Flashcards
1
Q
exothermic reactions
A
- heat energy is released to surroundings
- products will have less energy than the reactants
2
Q
endothermic reactions
A
- heat energy is absorbed from the surroundings
- products will have more energy than the reactants
3
Q
enthalpy change
A
- the amount of heat given out/taken in during a reaction carried out at constant pressure
- symbol: ∆H
- units: kJmol-1
4
Q
enthalpy of combustion
A
- the heat released during the complete combustion of one mole of that substance
- always a -ve value
5
Q
bond energies
A
- a measure of the average bond strength for a particular covalent bond
- measured in compounds in the gaseous state so the values only apply in calculations where all reactants and all products are in the gaseous state
- ΔH = [sum of energy breaking bonds] + [sum of energy forming bonds]
6
Q
bond breaking
A
- to break a covalent bond between 2 atoms, energy must be supplied
- bond breaking is an endothermic process (+ve value)
7
Q
bond forming
A
- when a covalent bond between 2 atoms is made, energy is released
- bond forming is an exothermic process (-ve value)
8
Q
limitations of bond energies
A
- bond energies are average values so values calculated using them are not specific to the molecules used
- thus, the actual value is likely to have a difference from the calculated value of over 10%.
- calculating the enthalpy change can only be done for reactions which take place entirely in the gaseous state
- to obtain more accurate values for the ΔH, calorimetry experiments can be used
9
Q
energy formula
A
Eh = cmΔT
- Eh = energy gained by water (in kJ)
- c = specific heat capacity of water (4.2)
- m = mass of water (in kg) 1l of water = 1kg of water
- ΔT = change in temp of water
ΔH =Eh/n (n=no. of moles), must have a +/- sign
10
Q
calorimetry in aqueous solutions
A
- a measured volume of a known conc of solution is placed in a polystyrene cup (an insulator so less heat is lost/gained to/from the surroundings)
- the temp of the solution is taken
- a measured volume of a known conc of another solution is added
- the mixture is stirred and the highest temp rise is found. -ΔT, can then be calculated and used in the formula
11
Q
assumptions made in energy calculations
A
- at room temp and pressure, the density of water is 1.00g cm−3 and the density of a solution is assumed to be the same even though it will not be quite the same
- the specific heat capacity is particular to the substance being heated. As most of an aqueous mixture is water, it can be assumed that the specific heat capacity of a solution is the same as that for pure water.
- as water is the major component, it can be assumed that only the water has absorbed any heat and that the calorimeter doesn’t absorb any energy
12
Q
sources of error in calorimetry calculations
A
- incomplete combustion: especially true when larger molecules are used as the fuel
- heat loss to surroundings is more likely in an open setting: heat can be lost before it reaches the calorimeter, and if the calorimeter not insulated
- heat is lost to the calorimeter: since it is a conductor, some heat will be absorbed by the calorimeter, (this error can be removed by calculating the energy change of the calorimeter and the water and adding these values.