Energetics Flashcards
Exothermic vs endothermic processes
Exo- energy is released from reaction. Temp of surroundings increases
Endo- energy is taken in by the chemical from surroundings. temp ov surroundings decreases
Breaking= endo
Making=exo
Enthalpy chang definition
the change in heat energy at a contant pressure.
Standrad conditions for enthaply change
100KPa (1 atm pressure)
298 K
1.0 mol dm-3 conc
Activation energy definition
the minimum energy needed to start a reaction
Mean bond enthalpy def
Energy required to break one mole of a covalent bond into gas atoms, averaged over a range of different compounds.
Standrad enthaply deltafHø
the enthaply change when 1 mole of substance is formed from its cnstituent elements under standard conditions with all reactants and products beibg in the standard state.
Formation of ammonia
Reaction would form 2 moles, so have to divide by 2to form 1 mole.
Enthaply of Na(s)
0- Na is an element
Na(l) would not be 0 because its not the standard state of Na
Why may the enthalpy of combustion of a compund be hard to measure.
Inomplete combution may occur.
Hess Law definition
The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
Give one reason why the bond enthalpy that you calculated above is differnt from the mean bond enthalpy quoted in a data book
The data book value is averaged over a range of differnt compounds.
Q=mcdeltaT
Q=energy (J)
M= mass of water (g)
c= Specific heat capacity (4.18)
Delta T= change in temp.
Delta H= Q/n
Delta H= Enthaply change
Q=energy taken in by reaction
n= number of moles
Combustion reactions (type 1)
Equipment: Thermometer, calorimeter, water, combusting fuel
Method: the energy released by the fuel is transferred to the water and temperature rise of the water is measured. The specific heat capacity of water allows us to use temp rise of water to calculate energy transferred.
CR (type 2)
Two solutions reacting- solution B reacts with solution A to produce a reaction mixture of B and A
use the total water volume when calculating mass.
CR (type 3)
Adding a solid to a solution
Add solid B to solution A to produce reaction mixture A and B. Limiting reagent is always used for the moles in calculating Q
Assumption made in calorimetry
The energy transferred to the water from the reacting chemicals is equal to the energy released by the reaction.
Sources of error of combustion calorimetry and Improvements to minimise error
Heat lost to surroundings
Incomplete combustion
Heat energy transferred to the metal calorimeter
Some fuel evaporates
Improvements: Add a lid- reduces heat loss, insulate sides of calorimeter- reduces heat loss, reduce distance between flame and beaker- reduces heat loss, put sleeve around flame to protect it from draughts.
Sources of error with solution calorimetry and Improvements to minimise error
Heat lost to surroundings
Improbements: add a lid- reduces heat loss
Insulate calorimeter-reduces heat loss.
Steps to measure enthalpy change using a cooling curve
Record the temperature for a suitable time (3 mins) before adding reactants together to establish an accurate initial temp. mix reactants then record temp every minute until a trend is seen. Plot a graph of tep against time, extrapolate the cooling curve back to the point of addition, to establish a theorectical temp change accounting for heat loss.
