Bond Enthralpy Flashcards
Exothermic vs endothermic
Exo-releases energy from reaction, temp of surroundings increases
Endo- energy is taken in by the chemical from surroundings, temp of surroundings decreases
Breaking= endo
Making=exo
Enthralpy change definition
The change in heat energy at a constant pressure
Standard conditions for enthalpy change
100kPa
298K
1.0 mol do-3 conc
Activation energy definition
Minimum energy needed to start a reaction
Mean bond enthralpy defintion
Energy required to break one mole of a covalent bond into gas atoms, averaged over a range of different compounds
Standard enthalpy deltafHØ
Enthalpy change when one mole of substance is formed from its constituent elements under standard conditions with all reactants and products being in the standard state
Formation of ammonia
Reaction forms 2 moles so have to divide by 2 to form one mole
Enthalpy of Na (S)
0- Na is an element
Na(l) would not be 0 bc it’s not the standard state
Why may the enthalpy of combustion of a compound be hard to measure
Incomplete combustion can occur
Hess law definition
The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products
Reason why the enthalpy calculated is different from the mean enthalpy quoted in the data book
The data book value is averaged over a range of different compounds
Define the standard enthalpy of combustion
The enthalpy change when one mole of substance is completely burnt in excess O2. Under standard conditions, all reactants and products being in their standard state.
Standard combustion for water, CO2 and most other oxides
0, cannot be combusted
Equation for heat energy
Q=mc(delta T)
Q= heat energy in joules
M=mass of water in grams
C= specific heat capacity of water
(Delta T)= temperature change
Equation for enthalpy change
Delta H= Q/n
Q= heat energy transferred to the water
N= moles of reactants or products
Equipment you’ll need for combustion reactions
Thermometer
Calorimeter
Water
Spirit burner, combusting fuel
Solution A and B react to produce
Reaction mixture A + B
Volume would be (A and B) added together
Solid B and solution A
Reaction mixture A and B- always use the mass of the limiting reagent
Assumption made in calorimetry
The energy transferred to the water from the reacting chemicals is equal to the energy released by the reaction.
Improvements to minimise sources of error
Add a lid—> reduced heat loss
Insulate sides—> polystyrene cup
Reduce distance between flame and beaker
Put sleeve around flame —> protect from draughts
Steps to measure an enthalpy change using a cooling curve
Record the temperature for a suitable time (3mins) before adding reactants together
To establish an initial temp
Mix reactants then record temp every minute until trend is seen
Plot a graph of temperature against time
Extrapolate the cooling curve back to the points of addition
To establish a theoretical temperature change accounting for heat loss
