Chemistry Quiz Flashcards
Compounds
Chemical combination of different elements in fixed ratio of atoms
Stoichiometric coefficients
Number of units (particles) in a reaction equation
Homogeneous mixture
2 or more substances mixed with uniform properties and composition. (looks like pure substance)
Heterogeneous mixture
2 or more substances mixed with different properties and composition (you see 2 substances)
Diffusion
evenly distribution of particles over space as result of random movement of the particles (fluids and gasses)
Changes of state
melting-freezing; evaporating-condensing; sublimation-deposition
Avogadro’s constant
6.02x1023, is the number of particles in one mole of a substance
Relative atomic mass,
Ar is the weighed average of 1 atom of an element relative to 1/12 of an atom of C-12
Water of crystallisation
ratio of chemically bonded water molecules to salt ions
Concentration
the amount of solute per unit of volume of solution
Formula, empirical
The formula obtained by experiment, showing the simplest whole number ratio of atoms of each element in a particle of a substance
Formula, molecular
The formula showing the actual number of atoms of each element in a particle of a substance.
Uncertainty
An estimate of the range of values within which the true value of a measurement is expected to be. It is normally expressed as a range of values such as 27.0 ± 0.2
Significant figures
The number of significant figures in a measurement is the number of digits that are known for certain plus the first uncertain one.
Random errors
Random errors in thermochemistry are caused by uncertainties in the readability of the measuring instruments and the effects of changes in the surroundings, such as temperature variations.
Systematic error
These occur when there is an error in the experimental procedure, such as heat losses in an exothermic reaction which lead to smaller temperature changes.
Repeatability
Experiments are repeatable if the same person duplicates the experiment with the same results.
Reproducibility
Experiments are reproducible if several experimentalists duplicate the results.
Accuracy
The accuracy of a result is a measure of how close it is to some accepted or literature value. If an experiment is accurate then the systematic error is small.
Precision
Precise measurements have small random errors and are reproducible in repeated trials.
Percentage uncertainties
The absolute uncertainty expressed as a percentage of the measured value.
Percentage uncertainty = (absolute uncertainty/measured value) * 100%
Absolute uncertainty
To find the absolute uncertainty in a calculated value for ab or a/b:
- Find the percentage uncertainty in a and b.
- Add the percentage uncertainties of a and b to find the percentage uncertainty in the calculated value.
- Convert this percentage uncertainty to an absolute value.
Fractional uncertainty
Fractional uncertainty =
absolute uncertainty/measured value
Percentage error
The difference between the accepted and experimental value, expressed as a percentage of the accepted value.
Percentage error =
((accepted value-experimental value)/accepted value) * 100%
Accepted value
literature value
Propagation of error
Uncertainties in raw data lead to uncertainties in processed data. Propagation should be done in a consistent way.
Adding and subtracting measurements
When adding or subtracting measurements, the uncertainty in the calculated value is the sum of the absolute uncertainties. The answer should be quoted to the same number of decimal places as the least precise value.
Multiplying and dividing measurements
When multiplying or dividing measurements, the total percentage uncertainty is the sum of the individual percentage uncertainties. The answer should be quoted to the same number of significant figures as the least precise data.
