Calorimetry Flashcards
If temperature of water has risen, what sign must be in front of ∆H in our final answer? Why is this the case?
- Negative sign.
- Because, if temperature has risen, it’s an exothermic reaction.
How to approach this Q?
25cm3 2moldm-3 nitric acid reacted with 25cm3 of 2moldm-3 of KOH in an insulated cup. Temp rose from 20.2⁰C to 33.9⁰C. Calculate ∆H for reaction, given specific heat capacity of water is 4.18Jg-1K-1.
Extra workings on Gautum doc 11.12.24
- Use Q =mc∆θ –> 50g (of both solutions together) x 4.18 x (33.9-20.2) = 2863.3J
- 2863.3J/ 1000 = 2.8633kJ.
- Make sure to put minus sign
(-2.8633kJ/ 0.05) - moles of limiting reactant–> moles of both reactants same in this Q! = - 57.27kJmol-1
When acids and alkalis reacting are in aqueous solutions, what do we assume?
- We assume they have the same density - 1g/cm3- and specific heat capacity as water.
When we are putting ∆T value into q=mc∆T equation, does ∆T need to be in K?
- No, can leave in ⁰C because it is a change in temp (will be exact same regardless of units.)
You have set up a water bath, underneath the water bath, you are burning a fuel that will increase temp of water in bath. What equation would you use to calculate THE ENERGY TRANSFERRED to the water? What do different aspects in this equation stand for, what are the units??
q = mc∆T
m –> mass of water/ whatever solution is increasing in temp (UNITS = g.)
c –> specific heat capacity (will be given in Q.)
∆T –> change in temperature (doesn’t matter what units are!!)
q = energy transferred (UNITS = J.)
100g water heated from 23⁰C to 57⁰C by 1.8g ethanol. Calculate energy transferred hence enthalpy of combustion of fuel.
q = 100g x 4.18 x 34
q= 14212J
Moles of ethanol: 1.8g/ 46 = 0.039mol
14212J/ 1000 = 14.212kJ
(- 14.212kJ/ 0.039mol = -364.4kJmol⁻¹)
Negative sign included as temp rose (ie. it’s exothermic!!)
How is apparatus set up when determining enthalpy of combustion of fuel using calorimetery?
- Lid placed on top of beaker with solution (ie. water) being heated.
- Wind shields prevent movement of the flame.
- This prevents heats loss in experiment.
How do we calculate the mass of the fuel burnt in calorimetery experiment determining enthalpy of combustion of fuel?
- Weigh the fuel before and after burning to work out mass of fuel burnt.
What two different set ups can we have in calorimetery?
- Can measure enthalpy of combustion of fuel (ie. fuel burning underneath a water bath.)
- Can measure enthalpy changes of solutions (ie. neutralisation reactions.)
How do we set up our apparatus/ what steps do we take when determining enthalpy change of neutralisation using calorimetery?
- Reaction occurs in polystyrene cup to prevent heat loss.
- Add acid first + measure initial temp. Then add alkali/ stir and measure temp change.
When we add copper sulphate to polystyrene cup of water (in required practical) , do we measure the temperature right after we have added the other substance?
- No! We leave the reaction for a minute to allow it to actually change in temperature.
What should our graph look like for this RP, showing temp change after adding copper sulphate to water?
- Have two lines of best fit - one for temp of water initially (over specific period of time) and one for change in temp of water after copper sulphate was added (over specific period of time.)
- Join these two lines of best fit by a straight vertical line to show ∆θ on graph.
() - extra info
If you want to work out enthalpy change for anhydrous copper sulphate –> hydrated copper sulphate using calorimetery what would you do?
- Work out ∆H for copper sulphate –> copper sulphate solution
- Work out ∆H for hydrated copper sulphate ( crystals) –> copper sulphate solution (use lower volume of water than in first reaction to account for fact crystals have some water in them.)
- Use Hess’ law to calculate ∆H for copper sulphate to hydrated copper sulphate.
