Chemistry practical technique's Flashcards
What is the procedure for making a standard solution?
- First work out how many moles of solute you need using n = c X v / 1000
- Work out how many grams of solute is needed using the formula mass = moles X Mr
- Carefully weigh out the mass of solute : weigh beaker first and note this down then add correct mass
- Add a small amount of distilled water to the beaker and stir with a glass rod until all the solute has dissolved
- Tip the solution into a volumetric flask and make sure it all goes in - using a funnel if needs be
- Rinse the beaker and glass rod with distilled water and then add that to the flask to, to make sure there is no solute left over in the beaker / on the rod
- make up the rest of the volume / top up the volumetric flask to 250cm3 with more distilled water, making sure the bottom of the meniscus reaches the line
- ## stopper the bottle and turn it upside down to ensure mixing. Check the meniscus again to make sure it is full
What are the two main indicators used in titrations and what are their colour changes when adding acid to alkali ?
methyl orange - turns yellow to red
phenolphthalein - turns red to colourless
How would you carry out a procedure to measure the enthalpy change of combustion of a flammable liquid ?
- Pour 100cm3 or any other appropriate amount of water into a copper calorimeter. Put the calorimeter in a clamp stand and measure it’s initial temperature.
- Measure the mass of the spirit burner before lighting it ( before any fuel / flammable liquid is burned ), and put in position under the calorimeter.
- You can use a draught shield if you want to protect the spirit burner and therefore less heat will be lost to the surroundings.
- Light the spirit burner and allow it to heat the calorimeter until the water has increased it’s temperature by around 30 degrees celsius.
- Put out the spirit burner by putting the lid back on.
- weigh the spirit burner again and record loss in mass ( = amount of fuel burned )
- Use equation Q = mc∆T to calculate the energy transferred to the water.
- Use the change in mass and the Mr of the fuel burned to calculate the moles of fuel burnt.
- divide the energy transferred by the moles to calculate enthalpy change in KJ per mole.
Describe the test for C=C double bonds / unsaturation using bromine water :
The bromine water will quickly decolourise and go colourless when in the presence of unsaturated compounds / C=C double bonds. This is because of electrophilic addition.
How do you heat something under reflux ?
Heat the solution in a pear shaped flask fitted with a Liebig condenser, which causes the solution to continually boil then condense back down, giving it time to react.
Describe the process of distillation :
Set up a distilling flask fitted with a thermometer in the top, using a clamp stand, and position a bunsen
burner underneath. The flask should be attached to a condenser, which has cold water coming in and out. The condenser should be attached to a receiving flask at the other end, where the distillate will be collected.
- When the evaporated substance enters the condenser, it is cooled and then condensed back into a liquid, where it can then be collected.
- A thermometer shows the boiling point of the substance that is evaporating. If you know the boiling point of your pure product, you can use the thermometer to find out when to collect the distillate.
Making an organic liquid - separation - describe the process of separation and why you would need to carry it out :
- If a product is insoluble in water, you can use separation to remove any impurities that do dissolve in water.
- Once the reaction to form the product is completed, pour the mixture into a separating funnel, then add water.
- Shake the funnel then allow it to settle. The organic layer is less dense than water so therefore should eventually sit on the top. Any water soluble impurities will have dissolved into the lower aqueous layer. You can then open the stopper on the separating funnel and run off the aqueous layer, leaving you with the organic product in the funnel.
After carrying out separation to remove impurities from an organic product, what would you do to remove trace amounts of water?
Carry out the drying process.
To do this, you should add an anhydrous salt such as MgSO4 or CaCl2.
The salt acts as a drying agent and will bind to any water present, becoming hydrated.
Describe the process of filtration to isolate an organic product :
- You need to setup a Buchner funnel with a side arm going to a vacuum line. Put filter paper inside the Buchner funnel.
- Pour the mixture containing the liquid and the solid into the funnel
- The reduced pressure inside the flask caused by the vacuum line removing air, will force the liquid through the funnel, leaving the solid product on the filter paper to be collected.
How would you purify an organic solid using recrystallisation ?
- Add very hot solvent to the impure solid until it just dissolves, be careful not to add too much solvent.
- This gives you a saturated solution of the product.
- Leave the solution to cool down, and crystals of the product will form.
- Remove the crystals by filtration and wash them with cold solvent.
- Dry the crystals.
What is the initial rates method / experiment?
- You time how long it takes for a set amount of product to form at the start of a reaction then use this to calculate the initial rate of reaction.
- You repeat this several times using different concentrations of one of the reactants.
- use the equation - initial rate = amount of reactant used / time
Describe how to carry out the iodine clock experiment?
- Add a small amount of sodium thiosulfate solution and starch to an excess of hydrogen peroxide and iodide ions in acid solution, hydrogen peroxide and the iodide produces iodine.
- The sodium thiosulfate that is added to the mixture reacts instantly with iodine.
- Once all the sodium thiosulfate is used up, any more iodine that is produced will turn the solution blue black because of the starch indicator. This is your end point.
- You can use this data to calculate initial rates.
- Repeat the experiment with varying concentrations of iodide or hydrogen peroxide.
How to measure the enthalpy change of solution ?
- Use a polystyrene cup fitted with a thermometer- this will measure the temperature change.
- You need to use the number of moles and the change in temperature with the equation Q = mc∆T to calculate the enthalpy change.
