Flashcards in To determine the heat of reaction of hydrochloric acid with sodium hydroxide Deck (29):
Why are liquids mixed in an insulated container
To prevent heat loss to surroundings
Why is it important that heat is not lost to the surroundings
We wish to ensure all the heat produced in the reaction is used to raise the temperature of the solution
Why is it important that both solutions should be at the same temperature before they are mixed?
Since the calculations involve measuring the rise in temperature of the final solution, it is important that both solutions must be at the same temperature in order to measure the rise of temperature accurately
When adding the base to the acid care should be taken to avoid splashing and explain why it is important
Splashing would give rise to loss of liquid and hence cause an inaccuracy in the calculations
What is the purpose of placing a lid on the plastic cup?
Prevents loss of heat to the surroundings
Why is the maximum temperature recorded?
In order to accurately calculate the heat liberated if it is not recorded then the value for temperature rise will be incorrect.
Why is the mixture of liquid stirred continuosly?
To ensure that the heat liberated is uniformly distributed throughout the solution i.e to make sure that the temperature measured does not depend on the position of the thermometer in the solution
How is the heat liberated calculated?
Mass of solution x specific heat capacity x temperature rise
Why is a plastic container particularly suitable for use in the experiment?
Plastic absorbs very little heat i.e specific heat capacity of plastic is negligible
When carrying out the calculations associated with this experiment, what assumptions are made about the density of the solutions and the density of water and why is it valid?
Density of solutions is the same as the density of water and it is valid as the solutions used are not highly concentrated
List the sources of experimental error in this experiment
Mixing solutions which are not at the same initial temperature
Forgetting to wash the thermometre after measuring the temperature of the acid and before measuring the base
Not measuring the maximum temperature rise
Forgetting to place a lid on the final mixture
Would you expect a similar result if nitric acid were used instead of HCL?
Yes, a similar result would be obtained since nitric acid is similar to HCl as both are strong acids i.e both dissociate in water
Why does the heat of neutralisation of a strong acid and strong base always have a constant value?
Because the same reaction is taking place in all cases i.e H+ +OH- -> H2O
The other ions are 'spectator ions' and are not involved in the reaction
Define heat of reaction
Heat change when the numbers of moles of reactants in the balanced equation react completely
State one advantage and one disadvantage of the use of a burette instead of a graduated cylinder for measuring out the base and adding it to the reaction vessel
Adv - More accurate (delivers measured volume)
Disadv- Slow addition and difficult to measure 75cm^3 (you would need two burettes)
State two ways to ensure that the rise in temperature was measured as accurately as possible
Accurate reading of thermometer (reading to .2C)
How many moles of HCl were neutralised in the reaction with NaOh (1.0M in 75cm^3)
1.0 / 1000 x 75 = .075M
Calculate the heat produced in the reaction vessel as a result of the HCl with the NaOH. Take the density and the specific heat capacity of the reaction mixture as 1.0g/cm^3 and 4.2Kjkg/K respectively. (75cm^3 of HCl and NaOh each.
Temperatures - HCl (13) NaOh (15) with highest temp (20.9)
1) MASS -> 75 + 75 = 150cm3 / 1000 = .15kg
2) TEMP RISE -> 13+15/2 = 14
20.9 - 14 = 6.9K
3) HEAT PRODUCED
.15 x 4.2 x 6.9 = 4.347kJ
Calculate the heat of reaction for neutralisation reaction between HCl and NaOh with heat produced being 4.347 and no.of moles of HCl being .075M
Heat produced divided by no.of moles
4.347 / .075 = 57.96
The solutions used in this experiment were moderately concentrated. Identify the hazard associated with the use of these solutions
What experimental problem would have been encountered if .1M NaOH and .1M HCl solutions had been used instead of 1.0M solutions
Small temperature rise and therefore harder to measure
The expected result is -57KJmol/l but the result is lower, why?
Specific heat capacity of polystyrene calorimeter is not zero
There is loss of heat to the surroundings
The densities of the acid and base are not exactly equal to the density of water
Give the procedure
Use two separate graduated cylinders, add HCl to one and place into one polystyrene cup and do the same with sodium hydroxide in a separate cup
Measure the temp of the HCL solution
Using a second thermometer measure temp of sodium hydroxide
When both solutions are at same temperature, quickly add base to the acid, stirring well and take care to avoid any loss of liquid
Place lid on polystyrene cup and with continuous swirling record the max temperature reached
State one precaution the student should take when using these solutions
Avoid contact with eyes and skin
Use protective clothing [gloves, lab coat]
Use eye protection [glasses]
If the HCL and NaOH solutions had been stored at slightly different temperatures, explain how the initial temperature could have been obtained
-get average temperature of both solutions
-wait until both are at same temperature [room temp]
List three precautions which should have been taken in order to obtain an accurate value for the highest temperature reached by the reaction mixture
-thermometer to 0.2 oC
-add without splashing
-replace cover quickly (immediately)
What was the advantage of mixing the solutions in a polystyrene foam cup rather than a glass beaker or in a metal calorimeter
Good insulator -> doesn’t lose heat easily to environment
Taking the total heat capacity of the reaction mixture to be 420JK, calculate the heat released if the temp rise was recorded to be 6.7 and hence calculate the heat of reaction
420 x 6.7 = 2814
2814 / no.moles [.05] = -56280