Chapter 14: Experimental Chemistry

Data Gathering

Quantitative chemical experiments involve numbers * the concentration of an acid in .345M
Qualitative chemical experiments do not involve numbers * Silver nitrate forms a white precipitate when added to a solution
Observations and details of an experiment should be recorded in a notebook. * contains complete experiment description, the equipment, chemicals used, and occasionally diagrams

Calculations

Calculations can be done via dimensional analysis or memorized equations/laws

Accuracy and Precision

Accuracy is the closeness between the measured value and true value
Determinate errors can affect accuracy * poor technique or incorrectly calibrated tools
Precision is the closeness of repeated measurements to each other
Indeterminate errors are a measure of precision * errors in the last digit of measurement * random errors that cannot be eliminated

Significant Figures

Significant figures (sig figs) is the precision of a measure number
Digits are significant if: * digit is not a zero * the zero is embedded * trailing zero is in a number that has a decimal point
digits are not significant if zeros are the left of all nonzero digits
Exact numbers involve no uncertainty * five plate at the dinner table * 4.184 joules in each calorie
How to obtain and write correct answers: * number with the fewest sig figs in a multiplication or division problem determines the number of sig figs in the answer * number with the fewest decimal places in addition or subtraction problems determines the number of decimal places in the answers

Uncertainty

Two types of uncertainty: absolute and relative
Absolute uncertainty is the uncertainty of the last digit of measurement. * for 45.47mL, the last digit is uncertain and the it could be ±.01mL
Relative uncertainty of a number is the absolute uncertaintly divided by the number itself * (.01mL) / (45.47mL) = 2x10^-4

Rounding

Steps of rounding: * If the digit just after the kept digits is less than 5, the remaining digits are dropped * rounding 6.23499 to three sig figs yield 6.23 because 4 is less than 5 * if the digit after the kept digit is 5 or more, the last kept digit is increased by one * 34.25589 to three sig figs yields 34.3 * 8.445000 to three sig figs yields 8.45

Graphs

Graphs illustrate the relationship between two variables
X-axis is typically independent variable * experimenter selects independent variable (IV) * concentrations of standard solutions that a chemist prepares
Y-axis is typically dependent variable * dependent variable (DV) is a measured property of the IV. * the amount of light that each standard solution absotbs
constructing graphs: * label x and y axis * number the axes * plot the data points and draw lines to connect dots

Determination of Physical Properties

Scale Reading

Parallax errors are caused by incorrect reading techniques
Meniscus of liquid is caused by the surface tension, which causes the curvature

Determination of Mass by Weighing

Weight = (gravitational constant)(mass)
Samples collected on balances should be collected in appropriate containers, not directly on the balance.

Liquid Volume Measurement

Accurate liquid measurements can be made with pipets or burets
Glassware with “TD” or “to deliver” is pouring the the volume stated * graduated cylinders
Glassware with “TC” to “to contain” will not pour the full amount in the container because a film, or excess solution, will remain * volumetric flasks
Burets are controlled by a valve called the stockpot. * should be rinsed with the solution, not distilled water, before filling

Temperature Measurement

Thermometers are used for temperature measurement
Mercury is used to measure temperature

Determination of Melting and Boiling Points

Experiments determining boiling point must be done at 1 atmosphere of pressure for accuracy
Closed-end capillary tubes can be used for melting point determination

Determination of Density

Density = Mass / Volume
density of liquid or solid = grams of material / cubic centimeters of material
density of gasses = grams of gas / liters of gas
pycnometers can be used to determine liquid density

Determination of Specific Heat

a solid, typically metal, is heated to a certain temperature then submerged in water, where the temperature of the water is measured
q = (mass)(specific heat)(ΔT)

Sample Manipulations

Heating

Bumping is a violent burst of boiling that could splatter hot liquid * typically occurs in test tubes * occurs because burner flame superheats one portion of liquid * hot water baths, steam baths, sand bath, and electrical heating can minimize bumping * powders added to very hot liquids can cause bumping

Cooling

Crushed ice bath and water is most effective to cool * Ice is particularly efficient
Heat-resistant lab glassware should be used because glass will shatter at rapid temperature change

Mixing

Volumetric flasks should be used to prepare molar concentrations of specific volumes
acid should be added to water * if water is added to sulfuric acid, it will not mix because of the density, causing high heat and splattering/boiling

Dilution

(Cinitial)(Vinitial) = (Cfinal)(Vfinal) * where c is concentration * v is volume

Gas Collection

Pneumatic troughs can collect gas produced in a reaction

Separation Techniques

Precipitation

For Ba 2+ (aq) + SO4 2- (aq) → BaSO4 (s) * Barium solution should be added slowly with rapid stirring to precipitate sulfate ions as barium sulfate * once precipitation is complete, the mixture should be heated to form the smaller crystals to larger ones for easy filtration

Filtration

Filtration is used to separate solid particles from a liquid
Folded, cone-shaped filter paper on a flask lets liquid pass through while solid is left behind

Centrifugation

Small amounts of precipitate can be collected through centrifugation * spins at high speeds to compact solid at the bottom
Supernatant is the clear liquid remaining after centrifugation

Distillation

Distillation separates two liquids with two different boiling points, one low and one high

Chromatography

The stationary phase of chromatography has different attractive forces for different parts of the mobile phase
Different attractive forces cause the mobile phase to move at different speeds
Rf = (distance of color spot)/ (distance of water)

Instrumental Techniques

pH Determination

pH meter or pH paper can determine pH
Litmus paper simply determines if the solution is acidic or basic

Spectroscopy

Spectrophotometer measures the amount of light that a colored solution absorbs
Absorbance of a sample is the quantity determined by the spectrophotometer
Reagent blank zeroes out the spectrophotometer to get an accurate reading
Beer’s Law: Absorbance (A) = abc * a is absorptivity * b is optical path * c is concentration

Experimental Reaction

Synthesis of Gasses

By decomposing certain compounds, gasses can be collected * KClO3 forms O2 when a catalyst is present
Gasses that do not react with water can be collected via pneumatic trough

Synthesis of Insoluble Salts

Precipitated in double-replacement reactions or reactions of gasses with soluble substance * Ag (aq) + Cl (aq) → AgCl (s) * Ba (aq) + SO4 (aq) → BaSO4 (s) * Ni (aq) + SO2 (g) + H2O (l) → NiSO3 (s) + 2H (aq)

Preparation of Soluble Salts

Can be separated by evaporation, with or without heat
Pure salt sample can only be obtained if the solution only contains the ions of that salt
Filtration can be used to separate unneeded ions

Qualitative Analysis of Inorganic Ions

Qualitative analysis techniques determine if a sample contains a certain ion
Sequence of reactions used to analyze a sample is called qualitative analysis scheme, or qual-scheme
When a separation is by filtration, the liquid is called the filtrate.
When separation is by centrifugation, the liquid is called the supernatant

Chemical Hazards

Highly Flammable Compounds

Organic compounds, typically with low molar masses * methane, butane, etc.

Explosive Compounds

Ethers react to produce explosive peroxides over time * Dry picric acid is explosive * nitrogen triiodide and nitroglycerin are shock sensitive

Strong Oxidizers

Perchloric acid, sulfuric acid, nitric acid, and hydrogen peroxide can cause immediate skin injury
Perchloric acid with organic material causes spontaneous combustion
White phosphorus burn spontaneously in air with hazardous fumes

Compounds Incompatible with Water

Active metals may react explosively with water * Li, Na, K, Rb, Cs, Ca, Ba, Sr

Compounds with High Heats of Solution

Substances can emit large amounts of heat in water * active metals, their solible oxides and hydroxides, and concentrated sulfuric and nitric acids * calcium oxide and phosphorus pentoxide * mixing concentrated bases with concentrated acids

Compounds with Possible Health Hazards

Benzene, chloroform, carbon tetrachloride
Chlorinated organic compounds