Chemistry 1 - The Development of Practical Skills in Chemistry Flashcards
Describe how to set up the apparatus to measure the volume of gas produced in a reaction.
A gas syringe or an upside-down water-filled measuring cylinder in a trough of water may be used to collect the gas.
What are the formulae relating the volume of a gas and the number of moles present?
At room temperature and pressure (RTP), use the equation n=V/24 Where: n=Number of moles V=Volume (dm³) If not at RTP, use the ideal gas equation PV=nRT Where: P=Pressure (Pa) V=Volume (m³) n=Number of moles R=Ideal gas constant (8.314) T=Temperature (K)
What is used to accurately measure mass?
A digital balance that records mass to 2 decimal places.
Why is it important to rapidly put the bung into the conical flask after starting a reaction that releases a gas?
The reaction can start as soon as the reactants mix, and gas will start being released. So to reduce the amount of gas that escapes, the bung should be put in place as quickly as possible.
Give three possible sources of error when investigating the volume of a gas produced in a reaction.
Some gas could have escaped before the bung us put in the conical flask.
The reaction may be incomplete.
Some gas may dissolve in water meaning a smaller volume would be collected than was released.
What is the purpose of safety goggles?
To protect the eyes from chemicals and small impacts to prevent damage.
What safety procedures should be followed when working with potentially irritant chemicals?
Safety glasses must be worn.
Skin contact to chemicals should be avoided.
Food mustn’t be eaten near the workspace, and not until hands have been thoroughly washed.
Avoid bringing chemicals close to face.
What apparatus is used in a titration to accurately measure volumes?
A burette is used to measure the volume of solution added.
A pipette filler is used to measure a specific volume of solution into the conical flask.
Describe the ‘weighting-by-difference’ method.
Add roughly the desired mass of a substance to a weighing bottle and weigh
Transfer the substance to a beaker/conical flask
Reweigh the weighing bottle
The difference between the masses is the mass of solid
Describe how to carry out an acid-base titration.
Use a pipette to add 25cm³ of acid to a conical flask. Add a few drops of indicator.
Pour alkali into the burette. Record the initial burette volume.
Complete a trial titre. The conical flask should be swirled constantly above a white tile. Stop adding the alkali as soon as the end point is reached. Record the final burette volume.
Repeat the titration until two concordant results are obtained. Add alkali drop by drop near the end point.
(25cm³ of alkali may initially be put into the conical flask if the burette is filled with acid.
When transferring a solution from one beaker to another, how can you ensure as much of the dissolved substance has been transferred as possible?
Use distilled water to wash any leftover solution from the old apparatus into the new beaker.
How do you accurately fill a volumetric flask to the graduated mark?
Fill so that the bottom of the meniscus rests on the graduated mark.
What is the meniscus?
The curve on the surface of a liquid.
What is a standard solution?
A solution of a known concentration,
Describe how to make a standard solution of NaHCO₃.
Weigh out a specified amount of NaHCO₃.
Add a small volume of distilled water to the beaker containing the NaHCO₃ to dissolve it.
Use a funnel to add the solution to a 250cm³ volumetric flask.
Rinse the beaker with distilled water and add the washings to the flask.
Make the flask up to the graduation mark with distilled water.
Stopper and shake the flask.
Suggest 2 indicators that could be used in an acid-base titration.
Phenolphthalein and methyl orange
What are concordant results in a titration?
Titres that are within 0.1cm³ of each other.
What degree of precision should burette readings be recorded to?
To the nearest 0.05 cm³.
Which results are used when calculating a mean titre?
Two concordant results (within 0.1cm³ of each other).
When completing a titration, what piece of apparatus is used to make the colour change easier to observe?
A white tile, placed underneath the conical flask.
What three things are required to take an accurate burette reading?
Read from the bottom of the meniscus.
Allow any liquid on the walls of the burette to settle before taking a reading.
Ensure that there are no air bubbles.
What is the end point of a titration?
The first point at which the indicator changes colour permanently.
Why is the conical flask swirled continuously during a titration?
To ensure that all the reactants are combined so that the reaction is complete.
Why is it better to have a titre volume of 25cm³ than 10cm³?
The larger the titre volume, the smaller the percentage error.
What equation links the number of moles and concentration?
n=CV Where: n=Number of moles C=Concentration V=Volume
If you know the volume of acid required to neutralise an alkali, how could you calculate the concentration of the acid given the alkali concentration and volume?
Calculate the number of moles of the alkali using the known volume and concentration.
Use the chemical equation to work out the ratio of acid and alkali that react and hence work out how many moles of acid have reacted.
Divide the moles of acid by the volume used in neutralisation.
What is calorimetry?
A process used to find the amount of energy released or taken in during a chemical reaction.
What is an exothermic reaction?
A reaction which releases energy to its surroundings.
What is an endothermic reaction?
A reaction which takes in energy from its surroundings.
What is a neutralisation reaction?
A reaction between an acid and a base, forming a salt and water.
What is meant by the term standard enthalpy change of combustion?
The enthalpy change that accompanies the formation of one mole of water from a neutralisation reaction under standard conditions.
Give the word and chemical equations for the neutralisation reaction between sodium hydroxide and hydrochloric acid.
sodium hydroxide +
hydrochloric acid → sodium chloride + water
NaOH+HCl→NaCl+H₂O
The enthalpy change of neutralisation is being investigated. Why might the neutralisation reaction be carried out in a Styrofoam cup?
A Styrofoam cup is a better thermal insulator than a glass cup, so less heat energy will be lost to the surroundings, meaning that the calculated temperature is more accurate.
What equation is used to calculate the energy absorbed by a solution in calorimetry?
Q=mcΔT Where: Q=Heat energy (J) m=Mass of the solution (g) c=Specific heat capacity of the solution (often the value for water is used) ΔT=Temperature change (°C or K)
How is enthalpy change of neutralisation calculated from the thermal energy absorbed by the solution?
Q/n
Where:
Q=Heat energy (J)
n=Number of moles =Volume×Concentration of specified substance/species used
How can you find the maximum temperature reached during a neutralisation reaction?
Plot a graph of temperature (y-axis) against time (x-axis) and extrapolating to the time that the reactants were combined.
Suggest three possible sources of error when investigating the enthalpy change of neutralisation. How could these errors be reduced?
Heat transfer to and from the surroundings, mitigated through insulating the reaction.
Incomplete combustion if using fuel, a bomb calorimeter could be used to combat these two errors.
Inaccuracy in temperature measurement, mitigated by using an electronic thermometer.
Describe and experiment to investigate the enthalpy change of neutralisation of a reaction between hydrochloric acid and sodium hydroxide.
Add 25cm³ HCl(aq) to a polystyrene cup. Place the cup in a glass beaker. Measure 25cm³ NaOH(aq).
Start the timer. Record the initial temperature of the HCl at minute intervals for 3 minutes.
At minute 4, add the sodium hydroxide, do not record the temperature.
Continue recording the temperature at minute intervals for 5 to 10 minutes. Stir the solution when measuring the temperature.
Describe the chemical test for halide ions.
Add dilute nitric acid to react with carbonate ions, so no Ag₂CO₃ forms (a white solid)
Add silver nitrate; a precipitate forms:
-White: AgCl (soluble in dilute and concentrated ammonia)
-Cream: AgBr (soluble in concentrated ammonia)
-Yellow: AgI (Insoluble in ammonia)
How can SO₄²⁻ ions be identified?
Add aqueous barium nitrate. A white precipitate (BaSO₄) will form if sulfate ions are present, This precipitate is insoluble in excess dilute strong acids.
Describe the chemical test used to detect CO₃²⁻ and HCO₃⁻ ions.
Add aqueous acid.
If either ion is present bubbles of CO₂ will be released.
Test the gas by bubbling it through limewater via a delivery tube. If it is CO₂, the limewater will turn cloudy.
What are the solubilities of the first 3 group 2 metals (Mg, Sr, Ba) in sulfate solution?
Mg - soluble
Sr - insoluble
Ba - insoluble
What are the solubilities of the first 3 group 2 metals (Mg, Sr, Ba) in chromate solution?
Mg - soluble
Sr - soluble
Ba - insoluble
What type of reaction occurs when a haloalkane is formed from an alcohol?
Nucleophilic substitution.
Write the word and chemical equations for the formation of 2-chloro-2-methylpropane from 2-methylpropan-2-ol.
2-methylpropan-2-ol + hydrochloric acid → 2-chloro-2-methylpropane + water
CH₃C(CH₃)₂OH+HCl → CH₃C(CH₃)₂Cl+H₂O
What are the 3 main stages in the synthesis of a haloalkane from an alcohol?
Preparation
Separation
Distillation
Why do 2 layers form in the separation funnel when synthesising a haloalkane from an alcohol?
The aqueous and organic layers have different densities and they are immiscible.
What is the purpose of anti-bumping granules?
To prevent the formation of large gas bubbles that cause violent boiling.
During distillation, anti-bumping granules prevents the mixture boiling over into the condenser, meaning impurities won’t contaminate the product.
When producing a haloalkane from an alcohol, why is sodium hydrogen carbonate added to the organic layer after it has been separated from the aqueous layer?
To react with any leftover hydrochloric acid.
Why is it important to open the stopper of the separating funnel regularly?
To release the build up of pressure and reduce the risk of the apparatus breaking.
When producing a haloalkane from an alcohol, why is anhydrous sodium sulfate added to the organic mixture after separation?
To remove water (small amounts only)- it acts as a drying agent.
Why is distillation used when synthesising a haloalkane from an alcohol?
To separate and purify the organic product.
Describe the key features of the apparatus set-up for distillation.
Tilt the condenser down so any liquids flow into the beaker.
The water must enter at the bottom of the condenser and leave at the top for efficient cooling.
The collection flask should not be sealed to the condenser- it should not be airtight as the hot air in the system could cause the apparatus to crack.
Use a thermometer to identify when the desired product has evaporated from the reaction mixture.
Why might a water bath or electric heater be used instead of a Bunsen burner to heat a reaction mixture?
If the reactants are flammable as using a Bunsen burner would be a safety risk.
What safety precaution should be taken when carrying out the formation of a haloalkane from an alcohol practical?
2-methylpropan-2-ol is highly flammable and harmful if inhaled- to mitigate this risk, keep it away from naked flames and keep the lab well ventilated.
Concentrated sulfuric acid causes severe skin burns and eye damage. Use a fume cupboard and wear safety goggles and gloves.
Take care when heating apparatus. Move hot apparatus with tongs to avoid burns.
What two reactants are used to synthesize methyl 3-nitrobenzene?
Methyl benzoate
Nitric acid
What is the purpose of concentrated sulfuric acid in the synthesis of methyl 3-nitrobenzoate?
it acts as a catalyst, speeding up the rate of reaction.
In the preparation stage of the synthesis of methyl 3-nitrobenzoate, what should the temperature be kept below? Why?
Below 10°C, this is to maximise yield.
Describe the process of filtration under reduced pressure.
Place a piece of filter paper in a Büchner flask. Connect the side arm of the flask to a water pump.
Connect the water pump to a tap and turn the tap on to create the vacuum. Pour the solution into the funnel.
Once the solution has drained through, rinse the solid on the filter paper with a small volume of solvent.
Describe the process of recrystallisation.
Dissolve the solid in the minimum volume of hot solvent.
Filter to remove insoluble impurities.
Cool the filtrate.
Filter to remove soluble impurities (the product remains on the filter paper).
Why is ethanol used for the recrystallisation of methyl 3-nitrobenzoate?
Methyl 3-Nitrobenzoate is soluble in ethanol but not in water, it is also a cheap and relatively safe organic solvent.
During recrystallisation, why is the product dissolved in a minimum amount of solvent?
If there is a large is a large volume of solvent when the mixture is cooled (and solubility is decreased), some of the product may remain in solution.
How does recrystallisation improve the purity of a product?
The desired product is dissolved in an appropriate solvent that the impurities are insoluble in. Subsequent filtration leaves the insoluble impurities on the filter paper while the solution flows through.
Why is the solution cooled in ice immediately after recrystallisation?
To decrease the solubility of the product so that it crystallises. Lower temperatures increase the size of crystals formed.
Why must the crystals be completely dry before measuring their mass to calculate the percentage yield?
Excess solvent on the crystals could falsely increase the yield.
How can percentage yield be calculated?
(%Yield)=((Max theoretical yield)/(Actual yield))×100
