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practicals Flashcards

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1
Q

how do you determine the enthalpy of combustion of a fuel such as methanol or ethanol

A
  1. accurately measure 100cm3 of water into a calorimeter/ beaker
  2. weigh a spirit burner containing the liquid to be burnt
  3. measure the initial temperature of the eater using a thermometer
  4. use the spirit burner to heat the water
  5. stop heating when there is a reasonable temperature rise. Stire and measure the final temperature of the eater using the thermometer
  6. reweigh the spirit burner
  7. calculate the temperature change and the heat change in joules
  8. calculate the mass of fuel in the burner by subtraction and calculate the number of moles of fuel using moles = mass/Mr
  9. calculate the energy change per mole of fuel used
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2
Q

why is the value of enthalpy of combustion determined experimentally is frequently less exothermic than the value found in data books

A

Reasons for this:
- heat losses to the surroundings from the spirit burner, wick and calorimeter ( the heat flame is affected by draughts)

  • loss of fuel from the wick or burner or by evaporation
  • loss of water by evaporation
  • incomplete combustion of the fuel, leaving soot on the bottom of the calorimeter
  • heat is used to raise the temperature of the calorimeter
  • the reaction is unlikely to occur under standard conditions, especially temperature

-

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3
Q

how can we improve the experiment

A
  • using a draught shield to reduce heat loss to the surroundings
  • Use a lid on the calorimeter to reduce heat to the surroundings
  • minimising the distance between the flame and the calorimeter
  • insulating the calorimeter and the spirit burner to reduce heat loss
  • Use a top on the spirit burner, with wick protruding, to minimise evaporation
  • if possible, burning in a supply of pure oxygen to prevent incomplete combustion
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4
Q

how do we determine the enthalpy of change of neutralisation

A
  1. place a polystyrene cup in a glass beaker for support
  2. rinse a measuring cylinder with 1.0mol dm-3 HCL and then 25cm3 of this acid and transfer into the polystyrene cup
  3. stir acid with a thermometer and record the temperature
  4. rinse a second measuring cylinder with 1.0 moldm-3 of NaOH and then measure out 25cm3 of NaOH
  5. Add the NaOH to the acid, stir and record the highest temperature reached
  6. calculate the temperature change used, the amount in mol of water formed and the enthalpy of neutralisation

this method involves measuring the temperature before mixing and the maximum temperature after mixing the reactants and finding the difference

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5
Q

what is an alternate method to record to work out the enthalpy change of neutralisation

A

an alternate method to record the temperature against time

plot a graph

extrapolating the cooling curve after the reaction takes place in order to calculate the temperature change at the point of mixing the reactants

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6
Q

what is the extrapolation method best for

A

the extrapolation method is best when the reaction is highly exothermic because more heat energy is lost at the point of traction compared with that of a less exothermic reaction

Hence simply measuring the temperature difference may significantly underestimate the temperature change and the value of delta H

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7
Q

the enthalpy change of neutralisation for a reaction between a strong acid and strong alkali should be identical regardless of the strong acid and alkali used

Suggests why

A

only hydrogen and hydroxide ions are involved in the reaction - H+ + OH - - H2O

So the nature of the strong acid and alkali does not matter

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8
Q

how do we measure the rate of a reaction by a continuous method

A

In this method, the reaction is monitored throughout its course and the amount of reactant or product is established at different time intervals throughout the reaction

A variety pg methods can be used to follow the progress of the reaction against time

most measure a change in the amount or concentration of a reatnt during the reaction

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9
Q

how do we determine which continuous monitoring method to use

A

to decide which method you need to examine the equation of the reaction and identify a reactant or a product that can be measured:

Examples include:

  • a gaseous product, which can be monitored by measuring the volume of gas produced by loss in mass of the reaction system
  • coloured reactant or product which can be monitored using calorimetry
  • a titratable reactant or product which can be monitored by sampling, quenching and titrating
  • a direct measurable reactant or product i.e. H+ ions or OH- ions by measuring pH using a pH meter
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10
Q

how do we measure gas volume for gases that are not soluble in water

A

for gases that are not very soluble in water such as oxygen or hydrogen, the gas may be collected under water in an inverted measuring cylinder or burette

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11
Q

how do you measure gas volume if the gas is soluble in water

A

if agas is soluble in water (carbon dioxide) then a gas syringe may be attached to a sealed reaction vessel to measure the volume of gas produced

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12
Q

what are some errors when it comes to collecting gas

A
  • to start the reaction magnesium is dropped into the flask and the stopper is quickly replaced: there are errors in this method:
  • sine gas may escape due to the time lag between adding the magnesium and replacing the bung, resulting in a decrease in the measured volume of gas
    To reduce this error the magnesium could be suspended by a string above the acid and the stopper loosened just enough to release the threads, dropping the magnesium into the acid

Alternatively, the magnesium could be placed in a small tube in the conical flask, acid added, bung replaced and the flask swirled to mix the reactants

when the stopper is replaced, the volume of the bung displaces the same volume of air into the measuring cylinder, increasing the volume. Again the alternate methods could reduce this error

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13
Q

what happens when a solid is heated to produce a gas

A

if a solid is heated to generate a gas, a test tube or boiling tube should be used rather than a conical flask

the volume of the gas is dependent of pressure, therefore, is you increase the temperature it causes a gas to expand significantly and this could cause an error in this type of experiment

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14
Q

why is the syringe having some air that is pushed into it not important to the results

A

the gas syringe will always contain some air pushed into the syringe by the gas that is generated.

This will not affect the result because the total volume of gas is collected in the syringe will be identical to the volume of gas produced

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15
Q

measuring the change in mass

A

an experiment where gas is produced can also be monitored by measuring the mass over a period of time

This method is not suitable for hydrogen which has a very low formula mass, so the mass lost would be small and very difficult to measure

a graph of mass against time is often plotted

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16
Q

why is it difficult to determine the rate when the reaction is exothermic

A

If a reaction is very exothermic, it is difficult to accurately determine the rate because the higher temperatures can cause the reaction to proceed faster

17
Q

measuring the change in a reactant or product by titrating

A

sometimes samples of the reacting mixture are taken at various times during the course of a reaction and the reaction is quenched - this means the reaction is stopped

each sample may then be titrated to find the concentration of the reactant or product and a graph of concentration against time is drawn

18
Q

what are the different ways in which a reaction can be quenched

A
  • rapid cooling
  • adding a chemical to remove a reactant that is not being monitored
  • adding a large known volume of water to the sample
19
Q

measuring a coloured reactant or product

A

a calibration curve should be set up with known concentrations of reactants or products so that colourimeter readings are related to concentration

In the continuous rate method, one experiment is carried out and the colourimeter readings are converted to concentration using the calibration curve

A graph of concentration against time is drawn and the shape of this curve can give the order with respect to the coloured reactant

20
Q

how do we find the order of a reaction

A
  • by a continuous monitoring method

alternatively, if more reagents are present the reaction can be set up so that the rate of the reaction depends solely on one reagent and all the other reagents are present in such a large excess that their concentration is nearly constant and so they have little effect on the reaction rate

The order can be determined from the shape of the concentration-time graph

21
Q

measuring the rate of reaction by an initial rates method

A

in this method, the rate is determined immediately after the start of the reaction

At this point, all of the concentrations are known

A series of experiments are carried out in which the initial concentration of one of the reactants is =variedsystmatyicallyt while the concentration of the other reactants is kept constant

the time t is measured to a fixed point in the reaction ]

In the method, the initial concentration is necessary to determine the initial rate of the reaction

Often results are plotted on a concentration-time graph
Initial rates can be determined from the concentration-time graph

A tangent is drawn at t - 0 and the rate is the gradient of this tangent

22
Q

why how do we use clock reactions

A

altering the concentrations of one reagent to work out the initial rate is time-consuming therefore we use a clock reaction

This gives an approximation to obtain an initial rate used

take hydrogen peroxide with iodide ions

A small known amount of sodium thiosulfate ions is added to the reaction mixture which also contains starch indicator

At first, the thiosulfate ions react with any iodine, I2, as soon as it formed, turning it back to iodide ions, so there is no colour change

At the instant when all the thiosulfate ions have been used up, free iodine is produced and this immediately gives a deep blue-black colour with the starch

The reaction time t for the initial part of the reaction to take place is measured and 1/t is calculated as a measure of the initial rate of the reaction

the experiment can be measured with different initial concentrations of the reactants and the time taken to reach the blue-black colour recorded

23
Q

how do we identify ammonium ions (NH4+)

A

Warm the sample with sodium hydroxide solution in a test tube and test the gas produced with moist red litmus/ universal indicator paper

OR test the gas produced with a glass rod dip[pped in concentrated HCL

24
Q

What are the results for testing for ammonium ions

A

Pungent-smelling gas changes moist red litmus paper/ universal indicator paper blue

The gas is alkaline (ammonia)

white fumes of ammonium chloride are produced indicating ammonia is present

this indicates that the ion in the salt was ammonium

25
Q

how do we test for magnesium ions

A

make a solution of the compound and add a few drops pos sodium hydroxide solution and then excess sodium hydroxide solution

a white precipitate should be formed if magnesium ions are present

26
Q

how do we test for calcium, barium and strontium ions

A

dip a nichrome wire in concentrated hydrochloric acid and then flame into the sample

Place in a blue bunsen burner flame and record the colour change of the flame

27
Q

what is the result for calcium ions

A

brick red flame

28
Q

what is the result for barium ions

A

green flame

29
Q

what is the result for strontium ions

A

red flame

30
Q

how do we test for chloride bromide and iodide ions

A

make a solution of the compound Na(x)

add some acidified silver nitrate solution followed by aqueous ammonia

31
Q

what is the result for chloride ions

A

add some white precipitate which dissolves in dilute aqueous ammonia to give acidified silver nitrate solution followed by aqueous ammonia

32
Q

what is the result for bromide ions

A

cream ppt which is insoluble in dilute aqueous ammonia but dissolves in concentrated aqueous ammonia to give a colourless solution

33
Q

what is the result for bromide ions

A

yellow ppt which is insoluble in both dilute and aqueous ammonia solution

34
Q

how do we test for sulfate ions

A

make a solution of the compound and add acidified barium chloride solution a white ptt will form

35
Q

how do we test for hydroxide ions

A

warm with some solid ammonium salt and test the gas produced with red litmus paper/ glass rod dipped in concentrated HCL

a pungent gas (ammonia) released, which changes litmus to blue and produces white fumes with concentrated HCL

This is essentially the reverse for ammonium ion

36
Q

how do we test for carbonate ions

A

add some dilute nitric acid and test the gas with lime water

effeverescense and the gas changes the colourless limewater milky

chemistry a level (24 decks)

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