Energetics Flashcards
Enthalpy change (delta H)
Heat energy change in a reaction at a constant pressure
kJmol-1
Standard enthalpy change of reaction delta (subscript) r H (little circle with line across symbol)
Enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions
Standard enthalpy change of formation
Enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions
Standard enthalpy change of combustion
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions
Standard enthalpy change of neutralisation
The enthalpy change when an acid and an alkali react together, under standard conditions, to form 1 mole of water
Degrees Celsius to kelvin
Add 273
Describe an experiment that could be used to find the enthalpy change of an endothermic reaction between ……
- Add set volume acid of known concentration to a polystyrene cup in a beaker
- Put lid on beaker and measure the temperature of the solution until it’s stabilised
- Add set mass of other compound to the beaker and stir
- Measure the temperature of the reaction mixture at regular time intervals until the temperature has reached a minimum (maximum for exothermic)
- Graph of temperature against time
- Extrapolate the line from minimum temperature (where the reaction is returning to it’s initial temperature) back towards the time when the reaction started
- Read off where lines cross the temperature
- Compare this with the initial reading of temperature to find change
Temperature change negative as this is measuring the temperature change of the surroundings
Positive for exothermic as temperature of surroundings increases
Measuring enthalpy change of combustion
Measure known vol water in calorimeter
Record starting temp of water and starting mass of spirit burner with fuel
Light spirit burner and heat water by a set increase in temp
Reweigh spirit burner
Calc heat change then enthalpy change
Hess’s law
The total enthalpy change of a reaction is always the same, no matter which route is taken
Bond enthalpy
Amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase
Mean bond enthalpy
The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds
Enthalpy change of reaction=
Sum of bond enthalpies of reactants- sum of bond enthalpies of products
Sources of error from measuring enthalpy change of combustion
Heat loss to surroundings
Heat loss from water
Incomplete combustion
Heat capacity of calorimeter not accounted for
Measuring enthalpy change of neutralisation (same method for any reaction adding solids and liquids)
Eg HCl and NaOH
Add set vol and known conc of NaOH to calorimeter using pipette
Start stirrer and timer and record temp every 30s for a set amount of time (eg 3mins 30s) then add a set vol and known conc of HCl
Continue to record temp every 30s up to 10 mins at least
Plot temp vs time and extrapolate both lines to where the HCl was added to find change in temp. This method accounts for heat loss
Q=mcT where m is the total volume of the solution (vol of NaOH+ vol of HCl)
H=Q/n where n is moles of water produced
Limitations of using mean bond enthalpies
Uses mean rather than actual values for the molecules involved
Check states or substances
