Practical Endorsement: Module 1: Practical Skills in Chemistry: Chemistry OCR A A Level

Pipette

A pipette is designed to deliver only one volume accurately; the value is stated on the bulb of the pipette.

Pipette

A pipette is designed to deliver only one volume accurately; the value is stated on the bulb of the pipette.

Volumetric analysis

Volumetric analysis is a practical technique whereby one uses reacting volumes to analyse and calculate a variety of unknown values.

Volumetric analysis

Volumetric analysis is a practical technique whereby one uses reacting volumes to analyse and calculate a variety of unknown values.

Describe the chemical test for halide ions.

Add dilute nitric acid to react with carbonate ions so no Ag_2CO_3 forms (white solid).

Add silver nitrate. Precipitate forms:

- White: AgCl (soluble in dilute ammonia)

- Cream: AgBr (soluble in concentrated ammonia)

- Yellow: AgI (insoluble in ammonia)

Describe the chemical test for halide ions.

Add dilute nitric acid to react with carbonate ions so no Ag_2CO_3 forms (white solid).

Add silver nitrate. Precipitate forms:

- White: AgCl (soluble in dilute ammonia)

- Cream: AgBr (soluble in concentrated ammonia)

- Yellow: AgI (insoluble in ammonia)

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 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 are the solubilities of the first 3 group 2 metals (Mg, Sr, Ba) in chromate solution?

Mg - soluble

Sr - soluble

Ba - insoluble

How do you purify a liquid product?

● You can use a separating funnel to isolate the organic layer from the aqueous layer.

● You can then purify the liquid by distillation, which separates the substances by boiling point.

What is the weighing by difference method?

● It is a method to weigh materials accurately.

● Mass of substance = Mass of weighing dish and substance - Mass of dish after substance has been transferred.

How do you purify a solid product?

By recrystallisation.

● Add minimum amount of warm solvent to the impure sample until it has dissolved.

● Allow to cool, crystals should form.

● When no more form you can filter under reduced pressure to obtain a dry crystalline solid.

How can we keep a substance at a constant temperature?

By using a water bath. This allows us to control temperature and keep it constant.

How can rate of reaction be measured?

● Initial rates method- i.e. the iodine clock reaction.

● A continuous monitoring method- i.e. measuring the volume of gas released in a reaction over time.

When doing a titration, what are concordant results?

Titres that are within 0.1 cm^3 of each other.

How do you use laboratory equipment to heat under reflux?

Quickfit apparatus is used to heat a substance under reflux.

1. The substance is boiled in a pear-shaped or round-bottomed flask.

2. As it evaporates, it is cooled by the water in the liebig condenser and so condenses back into a liquid and drips back down into the flask to be heated again.

How do you use laboratory equipment to filter under reduced pressure?

Using a Buchner funnel and Buchner flask, connected by

rubber tubing to the vacuum source.

● The funnel contains a layer of filter paper.

● Pour the substance onto the filter paper and the liquid will be sucked through via vacuum filtration into the flask.

● The solid will remain on the paper.

What is heating to constant mass?

The repeated heating and weighing of a substance until the mass no longer changes.

How do you measure the volume of gas given off from a reaction?

● Using a gas syringe.

● The plunger is pushed out of the syringe as more and more gas is collected.

● You can read the volume of gas from the scale along the side of the syringe.

Why are acid-base indicators used?

To detect when a reaction reaches completion/becomes neutral, usually by the presence of a colour change.

How do you determine the melting point of a substance and why can this information be useful?

● Place a small sample of the solid in a capillary tube.

● Melt using the melting apparatus available, measuring the temperature with a thermometer.

● A pure substance will usually melt at a single temperature (or a very small range) but an impure substance will melt over a range of temperatures (usually lower than that of the pure substance).

● Record the starting and ending points of the melting, when the first crystal can be seen to melt and when the last crystal becomes liquid respectively.

● You can then compare the melting point to known values to identify the substance.

How can pH be measured?

Using:

● pH charts

● A pH meter,

● A pH probe with a data logger

Why may an experimental value for enthalpy change be different to the actual value?

1. Heat loss to apparatus/surroundings.

2. Incomplete combustion.

3. Non-standard conditions.

4. Evaporation of alcohol/water.

How do you calculate an R_f value?

R_f value = Distance travelled by component ÷ Distance travelled by solvent

How do you set up an electrochemical cell?

● Two half cells are set up and connected to each other via a salt bridge (which allows the flow of ions).

● Each electrode is then connected to a

voltmeter which will measure the cell potential.

How do you make a standard solution?

● Measure, using a balance, the mass of solid required.

● Transfer this to a volumetric flask and rinse the remaining weighing bottle content into the flask so no solid is lost.

● Add a volume of distilled water to dissolve the solid. Swirl to mix.

● Then add more distilled water up to the line on the neck of the volumetric flask. Invert multiple times to mix.

What is a standard solution?

A standard solution is a solution of known concentration.

What are the uses of thin layer or paper chromatography?

To separate a mixture into its constituent components for analysis. This allows identification by the calculation and comparison of R_f values.

How do you use laboratory equipment to carry out a titration?

● A pipette is used to accurately measure out the volume of a reactant before transferring it to a conical flask.

● A burette is a measured, controlled and easy way to add small volumes of one reactant to another reactant.

How do you use laboratory equipment to carry out distillation?

● The substance you want to distill is heated in a pear-shaped or round-bottomed flask, causing it to evaporate.

● When it reaches the condenser it will cool and condense back into a liquid which will then drip out into the collecting flask.

Why is heating under reflux used?

● Allows heating for a long period of time.

● Prevents the flask from boiling dry.

● Prevents volatile reactants/products escaping.

● Ensures even heating.