experimental techniques Flashcards
Time
Time can be measured using a stopwatch or stopclock which are usually accurate to one or two decimal places.
The units of time normally used are seconds or minutes although other units may be used for extremely slow reactions (e.g. rusting).
1 minute = 60 seconds.
Temperature
Temperature is measured using a thermometer which can normally give readings to the nearest degree. Digital thermometers are available which are more accurate.
The units of temperature are degrees Celsius (ºC).
Mass
Mass
Mass is measured using a digital balance which normally gives readings to two decimal places. These must be tared (set to zero) before use.
The standard unit of mass is kilograms (kg) but in chemistry grams (g) are used most often.
1 kilogram = 1000 grams.
Volume-liquids
The volume of a liquid can be determined using several types of apparatus, depending on the level of accuracy needed.
For approximate volumes where accuracy isn´t an important factor, measuring cylinders are used. These are graduated (have a scale so can be used to measure) and are available in 25 cm3, 50 cm3, 100 cm3 and 250 cm3.
Pipettes are the most accurate way of measuring a fixed volume of liquid, usually 10 cm3 or 25 cm3.
Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm3 and 50 cm3 (e.g. in a titration).
Volume-gases
The volume of a gas sometimes needs to be measured and is done by collecting it in a graduated measuring apparatus.
A gas syringe is usually the apparatus used.
A graduated cylinder inverted in water may also be used, provided the gas isn’t water soluble.
If the gas happens to be heavier than air and is coloured, the cylinder can be used upright.
Demonstrate knowledge and understanding of paper chromatography
This technique is used to separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink).
A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along with the samples.
The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don´t wash into the solvent container.
The solvent travels up the paper by capillary action, taking some of the coloured substances with it.
Different substances have different solubilities so will travel at different rates, causing the substances to spread apart. Those substances with higher solubility will travel further than the others.
This will show the different components of the ink / dye.
Interpret Simple Chromatograms
If two or more substances are the same, they will produce identical chromatograms.
If the substance is a mixture, it will separate on the paper to show all the different components as separate spots.
An impure substance will show up with more than one spot, a pure substance should only show up with one spot.
Paper chromatography is the name given to the …while a chromatogram is the name given to the …, namely the piece of chromatography paper with the separated components visible after the run has finished.
The verb run is used in this technique as the samples essentially …the chromatography paper.
- overall separation technique
- visual output of a chromatography run
- “run” up
Assessing Purity
Pure substances melt and boil at specific and sharp temperatures.
Melting and boiling points data can be used to distinguish pure substances from mixtures.
An unknown pure substance can be identified by comparing its m.p and b.p and comparing to data tables.
Mixtures melt over a range of temperatures as they contain two or more substances.
Importance of Purity
A pure substance consists of only one substance and contains nothing else.
To have a pure substance for food and drugs is very important as impurities could be dangerous even in small amounts.
Melting and boiling point analysis is routinely used to assess the purity of food and drugs.
Interpret simple chromatograms, including the use of Rf values
These values are used to identify the components of mixtures.
The Rf value of a particular compound is always the same.
Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.
Calculation
Retention factor = distance moved by compound ÷ distance moved by solvent.
The Rf value is a ratio and therefore has no units.
Outline how chromatography techniques can be applied to colourless substances by
For chromatography to be useful the chemist needs to be able to see the components move up the paper, which is not the case for invisible samples such as proteins.
In such cases, locating agents can be used to react with the sample and produce a coloured product which is then visible.
The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye.
Methods of Purification
The choice of the method of separation depends on the nature of the substances being separated. All methods rely on there being a difference of some sort, usually in a physical property such as b.p., between the substances being separated.
Mixtures of solids
Differences in density, magnetic properties, sublimation and solubility can be used.
For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities.
Mixtures of liquids
You can separate immiscible liquids with a separating funnel or by decanting (pouring carefully).
Examples include when an organic product is formed in aqueous conditions.
Filtration
Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water). Centrifugation can also be used for this mixture.
Filter paper is placed in a filter funnel above another beaker.
Mixture of insoluble solid and liquid is poured into the filter funnel.
Filter paper will only allow small liquid particles to pass through as the filtrate.
Solid particles are too large to pass through the filter paper so will stay behind as a residue.
Crystallisation
Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold (e.g. copper sulphate from a solution of copper (II) sulphate in water).
The solution is heated, allowing the solvent to evaporate to leave a saturated solution behind.
Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution. If the solution is saturated, crystals will form on the glass rod.
The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow.
Crystals are collected by filtering the solution.
They are then washed with cold, distilled water to remove impurities and allowed to dry.
Simple Distillation
Used to separate a liquid and soluble solid from a solution (e.g. water from a solution of salt water) or a pure liquid from a mixture of liquids.
The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask.
The vapour passes through the condenser, where it cools and condenses, turning into pure liquid H2O that is collected in a beaker.
After all the water is evaporated from the solution, only the solid solute will be left behind.
Fractional distillation
Used to separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two).
The solution is heated to the temperature of the substance with the lowest boiling point.
This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker.
All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture.
For water and ethanol: ethanol has a boiling point of 78 ºC and water of 100 ºC. The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker.
When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated.
State the relative charges and approximate relative masses of protons, neutrons and electrons
Electrons (symbol e–)
These subatomic particles move very fast around the nucleus.
They move in orbital paths called shells.
The mass of the electron is negligible, hence the mass of an atom is contained within the nucleus where the neutron and proton reside.
describe how elements are arranged
Elements are arranged on the Periodic table in order of increasing atomic number where each element has one proton more than the element preceding it.
Hydrogen has 1 proton, helium has 2 protons, lithium has 3 etc.
The table is arranged in vertical columns called Groups numbered I – VIII and in rows called Periods.
Elements in the same group have the same amount of electrons in their outer shell, which gives them similar chemical properties.
Define isotopes as atoms of the same element which have the same proton number but a different nucleon number
Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number.
So C-14 is the isotope of carbon which contains 6 protons, 6 electrons and 14 – 6 = 8 neutrons.
Electron shell diagrams
Electrons orbit the nucleus in shells (or energy levels) and each shell has a different amount of energy associated with it.
The further away from the nucleus then the more energy a shell has.
Electrons occupy the shell closest to the nucleus which can hold only 2 electrons and which go in separately.
When a shell becomes full electrons then fill the next shell.
The second shell can hold 8 electrons and the third shell can hold 18 electrons and the electrons organise themselves in pairs in these shells.
The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons.
The noble gases
The atoms of the Group 8/0 elements all have 8 electrons in their outer shells, with the exception of helium which has 2. But since helium has only 2 electrons in total and thus the first shell is full (which is the only shell), it is thus the outer shell so helium also has a full valency shell.
All of the noble gases are unreactive as they have full outer shells and are thus very stable.
All elements wish to fill their outer shells with electrons as this is a much more stable and desirable configuration.
Electronic configuration of the first twenty elements:
Metals and nonmetals
The Periodic Table contains over 100 different elements.
They can be divided into two broad types: metals and nonmetals.
Most of the elements are metals and a small number of elements display properties of both types. These elements are called metalloids or semimetals.
Describing Alloys
Alloys are mixtures of metals, where the metals are mixed together but are not chemically combined.
They can be made from metals mixed with nonmetals such as carbon.
Alloys often have properties that can be very different to the metals they contain, for example they can have more strength, hardness or resistance to corrosion or extreme temperatures.
Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms.
This makes it more difficult for the layers to slide over each other, so they are usually much harder than the pure metal.
Brass is a common example of an alloy which contains 70% copper and 30% zinc.
Ionisation of metals and non-metals
Metals: all metals lose electrons to another atom and become positively charged ions.
Non-metals: all non-metals gain electrons from another atom to become negatively charged ions.
Electrostatic attraction
The positive and negative charges are held together by the strong electrostatic forces of attraction between opposite charges.
This is what holds ionic compounds together.
Describe the formation of ionic bonds between elements from Groups I and VII
Explanation
Sodium is a group 1 metal so will lose one outer electron to another atom to gain a full outer shell of electrons.
A positive ion with the charge +1 is formed.
Chlorine is a group 7 non-metal so will need to gain an electron to have a full outer shell of electrons.
One electron will be transferred from the outer shell of the Sodium atom to the outer shell of the Chlorine atom.
Chlorine atom will gain an electron to form a negative ion with charge -1.
Formula of Ionic Compound: NaCl
Ionic Bonds between Metallic and Non-Metallic Elements
Explanation
Magnesium is a group 2 metal so will lose two outer electrons to another atom to have a full outer shell of electrons.
Example: Magnesium Oxide, MgO
A positive ion with the charge +2 is formed.
Oxygen is a group 6 non-metal so will need to gain two electrons to have a full outer shell of electrons.
Two electrons will be transferred from the outer shell of the Magnesium atom to the outer shell of the Oxygen atom.
Oxygen atom will gain two electrons to form a negative ion with charge -2.
Formula of ionic compound: MgO
The Lattice Structure of Ionic Compounds
Lattice structure refers to the arrangement of the atoms of a substance in 3D space.
In these structures the atoms are arranged in an ordered and repeating fashion.
The lattices formed by ionic compounds consist of a regular arrangement of alternating positive and negative ions.
