S3 - Equation, Calculations and Electrolysis Flashcards
write word equations and balanced chemical equations (including symbols):
-(g) means gas, (s) means solid, (l) means liquid, (aq) means aqueous
-example of word equation: hydrochloric acid + sodium hydroxide -> sodium chloride + water
-example of balanced chemical equation: HCl + NaOH -> NaCl + H2O
-to balance an equation: you need to make sure there are the same number of each element on each side of the equation and if there isn’t use big numbers at the front of a compound to balance it e.g: 3H2O
calculate relative formula masses (including relative molecular masses) (Mr) from relative atomic masses (Ar)
-relative formula mass (Mr) of a compound: sum of the relative atomic masses of the atoms in the number shown in the formula
-in a balanced chemical equation:
sum of Mr of reactants in quantities shown = sum of the Mr of products in quantities shown
mole (mol)
-> the unit for the amount of a substance
-chemical amounts are measured in moles, the symbol for the unit mole is mol
-the mass of one mole of a substance in grams is numerically equal to its relative formula mass
-6.02 x 10^23
-for example, the Ar of Iron is 56, so one mole of iron weighs 56g
-the Mr of nitrogen gas (N2) is 28 (2x14), so one mole is 28g
-one mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance
carrying out calculations involving amount of substance, Ar and Mr:
-you can convert between moles and grams by using this triangle or the equation:
mole = mass/relative atomic mass
mass = moles x relative atomic mass
-e.g: how many moles are there in 42g in carbon?
moles = mass/Mr = 42/12 = 3.5 moles
calculate reacting masses using experimental data and chemical equations:
-chemical equations can be interpreted in terms of moles
e.g: Mg + 2HCl -> H2 shows that 1 mol. Mg reacts with 2 mol. HCl to produce 1 mol. MgCl2 and 1 mol. H2
-masses of reactants and products can be calculated from balanced symbol equations. If you are given the reacting mass of one reactant and asked to find the mass of one product formed:
-find moles of that one substance: moles = mass/molar mass
-use balancing number to find the moles of desired reactant or product (e.g: if you had the equation: 2NaOH + Mg -> Mg(OH)2 + 2Na, if you had moles of Mg, you would form 2x2=4 moles of Na)
-mass = moles x molar mass (of the product) to find mass
calculate percentage yield:
percentage yield = amount of product produced/maximum amount of product possible x 100
-it is not always possible to obtain the calculated amount of product for 3 reasons…
-reaction may not go to completion because it is reversible
-some of the product may be lost when it is separated from the reaction mixture
-some of the reactants may react in ways different to the expected reaction
-amount of product obtained is known as a yield
molecular formula and empirical formula:
-molecular formula: the number of atoms of each element in a compound
-empirical formula: the simplest whole number ratio of atoms of each element in a compound
calculate the empirical and molecular formulae from experimental data:
empirical formula from the formula of molecule:
-if you have a common multiple, e.g: F2O4, the empirical formula is the simplest whole number ratio, which would be FeO2
-if there is no common multiple, you already have the empirical formula
molecular formula from empirical formula and relative molecular mass
-find the relative molecular mass of the empirical formula
-divide relative molecular mass of compound by that of the empirical formula
-multiply the number of each type of atom in the empirical formula by this number
-e.g: if answer was 2 and the empirical formula was Fe2O3 then the molecular formula would be empirical formula x 2 = Fe4O6
practical: know how to determine the formula of a metal oxide by combustion
1) get a crucible and heat it until its red hot (this will make sure it is clean and there are no traces of oil or water)
2) leave the crucible to cool, then weigh it, along with its lid
3) add some clean magnesium ribbon to the crucible, reweigh the crucible, lid and magnesium ribbon, the mass of magnesium you’re using minus the initial reading for the mass of the crucible and lid
4) heat the crucible containing the magnesium, out the lid on the crucible so as to stop any bits of solid from escaping, but leave a small gap to allow oxygen to enter the crucible
5) heat the crucible strongly for around 10 minutes, or until all the magnesium ribbon has turned white
6) allow the crucible to cool and reweigh the crucible with the lid and its contents, the mass of magnesium oxide you have is this reading, minus the initial reading for the mass of the crucible and lid
practical: know how to determine the formula of a metal oxide by reduction
1) place a rubber bung (with a hole through the middle) into a test tube with a small hole in the end, and weigh them using a mass balance
2) take the bung out of the test tube and spread out a small amount of copper(II) oxide in the middle of the tube
3) re-insert the bung and weigh the test tube again
4) expel the air from the test tube gently turning on the gas, after about 5 seconds, light the gas by holding a burning splint next to the hole in the end of the test tube, you can control the size of the flame by changing the amount of gas that’s flowing through the test tube
5) use a bunsen burner to heat copper (II) oxide for about 10 mins (or until the solid changed colour from black to a brownish-pink colour)
6) turn off the bunsen burner and leave the test tube to cool
7) once the tube has cooled, turn off the gas and weigh the test tube with the bung and its contents
explain why substances with a simple molecular structures are gasses or liquid or solids with low melting and boiling points:
-the term intermolecular forces of attraction can be used to represent all forces between molecules
-substances that consist of small molecules are usually gases or liquids that have low boiling and melting points
-substances that consist of small molecules have weak intermolecular forces between the molecules, these are broken in boiling or melting, not the covalent bonds
-substances that consist of small molecules don’t conduct electricity, because small molecules do not have an overall electric charge
explain why the melting and boiling points of substances with simple molecular structures increase, in general, with increasing relative molecular mass:
-the intermolecular forces increase with the size of the molecules, so larger molecules (i.e. molecules with greater relative molecular masses) have higher melting and boiling points
