energetics

Question 1

exothermic reactions

Answer 1

• heat energy is released to surroundings

- products will have less energy than the reactants

Question 2

endothermic reactions

Answer 2

• heat energy is absorbed from the surroundings

- products will have more energy than the reactants

Question 3

enthalpy change

Answer 3

• the amount of heat given out/taken in during a reaction carried out at constant pressure
• symbol: ∆H
• units: kJmol-1

Question 4

enthalpy of combustion

Answer 4

• the heat released during the complete combustion of one mole of that substance
• always a -ve value

Question 5

bond energies

Answer 5

• 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]

Question 6

bond breaking

Answer 6

• to break a covalent bond between 2 atoms, energy must be supplied
• bond breaking is an endothermic process (+ve value)

Question 7

bond forming

Answer 7

• when a covalent bond between 2 atoms is made, energy is released
• bond forming is an exothermic process (-ve value)

Question 8

limitations of bond energies

Answer 8

• 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

Question 9

energy formula

Answer 9

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

Question 10

calorimetry in aqueous solutions

Answer 10

• 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

Question 11

assumptions made in energy calculations

Answer 11

• 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

Question 12

sources of error in calorimetry calculations

Answer 12

• 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.