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