Calorimetry - Calorimeters Introduction Flashcards
What is a Calorimeter used for?
The measurement of Enthalpy/Heat Content
What are the 2 types of Calorimeters
- Solution (simple) Calorimeter
- Bomb Calorimeter
What does a Calorimeter measure in the experiment?
The change in temperature that occurs during a reaction
This value can be used to find enthalpy
Each Calorimeter has its own calibration factor. What is a Calibration factor?
Calibration factor is the energy required to change the temperature within calorimeter by 1 °C. It is determined experimentally.
What does a Solution (simple) calorimeter like?
- A vessel with known amount of water.
- Container is insulated (prevents heat loss to surroundings)
- Has a stirrer (keeps temprature constant throughout vessel)
- Thermometer (record temperature changes)
Refer to diagram online
Reactants are placed iwthin solution for solution calorimeter
What is the purpose of a Bomb Calorimeter (how is it special)?
Allows for the determination of enthalpies for Combustion reactions, fuels and foods.
Reactants are not placed within solution.
What does a Bomb Calorimeter look like?
More spohisticated equipment:
* Reactants placed inside a ‘bomb’ where they are ignited
* Bomb is surrounded by water (where temperature change occurs)
* Water’s vessel is insulated
* Ignition coil placed in reactants and wires stick out of calorimeter to initiate ignition
* Thermometer and stirrer in water
Refer to diagram online
The measurement of enthalpy of substances can be found using a calorimeter. State the two methods of finding it:
- Calibration Factor Method
- Specific Heat Capacity Method
What are the two types of Calibration Methods?
- Electrical calibration
- Chemical calibration
What data needs to be known for conducting the Calibration Method for finding enthalpy?
- Calibration Factor (alternatively need to know amount of volts, amps, duration and ΔT)
- ΔT
- Mass of tested substance
Give equation for Electrical Calibration
Cf = (VxIxT) / ΔT
V = volts, I = amps, T = time/duration, ΔT = difference in temperature
Describe process of conducting an electrical calibration
- A known voltage and current is applied through a heating coil for a specific time
- Heating coil heats up water/solution
- Temperature change within this time is recorded
Electrical Calibration is used for Solution (simple) calorimeters
Why is chemical calibration used?
As bomb calorimeters usually require chemical calibration as eletrical calibration cannot be conducted on them
However, VCAA is known to use electrical calibration on bomb calorimeter
What chemical is used usually for chemical calibration and why?
Benzoic acid is usually used - it has a very accurately known enthalpy
May be asked to work out calibraton factor of bomb calorimeter using chemical calibration - all data for calculations will be provided though
What’s formula for finding enthalpy with known Calibration factor and ΔT?
Heat energy evolved = ΔT x Calibration Factor
E = ΔT x Cf
Explain the 3 steps for finding enthalpy using Calibration Factor
1) Calibrate Calorimeter
* Set V, I and t
* Record ΔT
* Use Cf = (VIt)/ΔT to find calibration factor
2) Complete reaction and record ΔT
* Use E = Cf x ΔT to get energy evolved
3) Find ΔH
* Energy evolved is for ‘x’ amount of grams
* Convert grams to moles
* Then divide by number of moles to get ΔH
What data needs to be known for conducting the specfic heat capacity method for finding enthalpy?
- Mass of water (or other solution)
- Specific heat capacity of water (or other solution)
- ΔT
- Mass of tested substance
What is the equation for finding energy evolved using specific heat capacity?
Energy (J) = Specifc Heat Capacity x Mass of water (g) x ΔT (degrees C)
E = Cp x m(H2O) x ΔT
What are the 2 steps in finding enthalpy using specific heat capacity method?
1) Find energy inputted into water by substance
* E = Cp x mass(H2O) x ΔT
2) Find ΔH
* Energy found in step 1 if for ‘x’ amount of grams
* Convert grams to moles
* Divide energy by number of moles for ΔH
Why might specific heat capacity method be used instead of calibration factor method?
- When amount of water is known
- When Calibration factor is undeterminable
When might enthalpy found using calorimeters not be given per mole, but per gram instead?
When tested substance is a mixture
For solids and liquids how do you convert kJ/mol to kJ/g?
Divide by M (molar mass)
For solids and liquids how do you convert kJ/gram to kJ/mol
Multiply by M (molar mass)
For gasses how do you convert kJ/mol to kJ/gram
Divide by Vm (molar volume of an ideal gas at SLC)
