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Module 3 - Chapter 9 - Enthalpy changes II - Coggle Diagram
Module 3 - Chapter 9 - Enthalpy changes II
Measuring energy changes
Kelvin scale
Starts at absolute sero
1K rise = 1 C rise
0K = -273 C
Calculating energy change
Heat energy = q
Measured in Joules
Mass of surroundings (m)
Identify materials that are changing temperature
Measured in grams
SHC of surroundings (c)
Energy required to raise the temperature of 1g of a substance by 1K
Good conductos of heat have low SHC
SHC of water =
Temperature change
Thermometer readings
Final temperature - Initial temperature
Determining enthalpy change of combustion
Process
Small spirit burners can easily burn liquid fuels
1) Measure 150cm^3 of water, and pour into a beaker
2) Measure initial temp of water to 0.5 C
3) Add methanol to the spirit burner, weigh it
4) Place burner under the beaker, light it and stir the water the thermometer while the methaol burns
5) After three minutes, extinguish the flame and immediately record the maximum temperature reached by the water
6) Re-weigh the spirit burner
Calculation
1) Calculate the energy change of the water
2) Calculate the mole of methanol burnt
Caclulate the enthalpy change for complete compustion of 1 mol of CH3OH
Inaccuracies
Less exothermic
Incomplete combustion of methanol - seen as black soot
Evapouration of methanol from the wick
Heat loss to surrounding other than water
Non standard conditions (More exothermic)
Draught screen an input of oxygen gas could minimise errors
Determining enthalpy change of reaction
Enthalpy change of two solutions or solution/ solid can be determined in a polystyrene cup as they offer insulation
Solution itself is immediate surroundings
Energy transfer is between chemical particles/ water molecules
Thermometer in solution will record any temperature changes
Calculation
Temperature change
solution nass and SHC are similar to water
Calculate the energy change
Calculate the amount in mole
Calculate the enthalpy change of reaction
Cooling curve
Allows you to correct for heat loss
Method
Start a stop watch and take the temperature of the starting solution every 30 seconds until the temperature stays constant
Add the reactant to the solution and stir the mixture, record the temperatre every 30 seconds until the temperature has falled for several minutes
Plot a graph of temperature agaisnt time
Extrapolate the cooling curve to the time when the zinc was added and draw a vertical lin from the time that the solutions were mixed
Determining an enthalpy change of neutralisation
Similar to previous enthalpy change of reaction, except this reaction is between two solutions
Method
Calculate the total volume (add two solution volumes together)
Calculate the energy change
Calculate the moles of reactants
Calculate the entalpy change of neutralisation from the reaction, and then find the enthalpy change required to produce 1 mol of water
