Topic 1: Stoichiometric Relationships Flashcards
Melting
Solid to Liquid, Endothermic (absorbs energy)
Vaporisation ( Boiling or Evaporation)
Liquid to gas, Endothermic (absorbs energy)
Sublimation
Solid to Gas (No inbetween state), Endothermic (absorbs energy)
Condensation
Gas to liquid, Exothermic (releases energy)
Freezing
Liquid to solid, Exothermic (releases energy)
Deposition
Gas to solid ( No inbetween state), Exothermic (releases energy)
Elements:
Single substances composed of atoms of the same type.
Compounds:
Substances composed of two or mole different elements bonded together. They may have different properties to the original.
Molecules:
Two or more atoms bonded together.
The ratio of elements in a compound
Find the ratio of elements in a compound by balancing the charges. Do this using the periodic table.
E.g Mg has charge +2 and Cl has charge -1, so the compound needs one Mg to two Cl to balance the charges. MgCl2
Homogeneous mixture
The mixture is mixed such that the elements/compounds are distributed equally throughout.
Heterogeneous mixture
The mixture is not uniform in composition
To deduce the chemical equation:
- Use the data booklet to convert the reactants and products to their corresponding symbols
- Set up the equation with the reaction arrow and symbols.
- Apply correct state symbols (s), (l), (g) and (aq)
- Equate coefficients so that the same number of elements are either side.
Relative Atomic mass (Ar)
The mass of an element relative to carbon-12, where Carbon-12 is defined to have a mass of 12.
Molar mass (Mr)
This is calculated by summing the atomic masses of each element in a compound. The unit for molar mass is g mol-1
What are moles?
Moles “n” are used to count particles because particles are so unbelievably small that a huge number of them can fit in a beaker. One mole of a something is simply 6.02 x10^23 times of it.
Avogadro’s number:
The number of particles which equal one mole - 6.02 x 10^23
To convert from moles to particles
multiply the number of moles by avogadro’s number, When going the other way divide instead of multiplying
To convert from moles to grams
multiply the number of moles by the molar mass(Mr), When going the other way divide instead of multiplying
Mole ratio
The mole ratio is the ratio between the moles of different compounds in a chemical formula.
You can find the mole ratio by looking at the coefficients in front of the compounds in the formula.
Example:
2H2+O2→2H2O
The mole ratio between the oxygen and water is 1:2, as for every one mol of oxygen reacted, 2 moles of water is produced.
It is a 1:1 mole ratio between Hydrogen and water, as they have the same coefficients.
Empirical formula
This is the simplest whole-number ratio of elements in a compound. It is important that the ratio is in its simplest form. If the ratio can be simplified it must. E.g H4O2 is not an empirical formula because it can be simplified to H2O
Molecular formula
This tells the number of moles of each element. It is the same as the empirical formula multiplied by some whole number.
Finding the empirical formula given percentage composition
- Assume that the compound has a mass of 100g.
- Calculate the mass of each element by multiplying the percentage by 100g
- Divide each mass by the atomic mass to find the moles of each element.
- The simplest integer ratio between the moles of the element is also the ratio in the empirical formula
Find the Molecular formula given mass and the empirical formula
- With the empirical formula find the molar mass of the compound, with the simplified ratio.
- Divide the mass given by the molar mass of the empirical formula, to get a whole number.
- Multiply the empirical formula by this whole number to get the molecular formula.
Limiting reactant:
This is the reactant that determines the theoretical yield.
Excess reactants
All reactants which aren’t the limiting reactant are in excess, so they don’t affect the yield.
Determine the limiting reactant
If given the chemical equation and the masses do this:
- Convert the masses to moles
- Divide the moles by the mole ratio coefficients
- The reactant with the smaller number is the limiting reactant
Experimental and theoretical yield
The yield gained in an experiment is often lower in an experiment due to errors with the method. The percentage yield can tell us how much of the limiting reactant actually reacted.
Theoretical yield:
The yield expected theoretically, assuming all the limiting reactant is converted into products.
Experimental yield:
The yield in an experiment
Percentage yield:
Experimental yield divided by theoretical yield x 100
What is an ideal gas
This is a gas that exhibits the five postulates of the kinetic molecular theory, as well as obeying the gas laws.
Properties of an ideal gas:
*The particles in a gas exhibit random motion
*There are no intermolecular force between particles
*All collisions are perfectly elastic
*Particles have negligible volume
*The kinetic energy is directly proportional to the temperature
Combined gas law:
P1xV2 over T1 = P2xV2 over T2
Ideal gas equation:
PV = nRT ( By rearranging the ideal gas equation you can find the relationship between any two components)
Avogadro’s Law
States that equal volumes of gases at the same temperature and pressure contain equal numbers of particles. This is true because a gas is mostly empty space so the the nature of the particles themselves does not affect the number of particles which are in a gas. This law is important because it means any same volume of gas also has the same number of molecules and hence the same number of moles.
Molar volume:
22.7 dm3 mol-1 - This is the amount of volume one mole of idea gas takes up under STP
Moles of gas:
Volume over Molar volume= V/22.7
Concentration:
Moles of solute over the volume of solution (measured in mol dm-3)
Acid-Base Titration
A Titration is a method of finding the concentration of an unknown solution using a solution with a known concentration.
Titrant:
The solution with the known concentration in buret, can be an acid or a base
Analyte:
The solution with the unknown concentration in conical flask, can be an acid or a base.
