chemistry - topic 3

• Balancing Equations


  Know how to balance chemical equations
• Balanced chemical equations

  • CaCO3 + 2HCl → CaCl2 + H2O + CO2
  • 2Mg + O2 → 2MgO
• The 2 is put in front of the HCl to balance the numbers of H’s and Cl’s on both sides
• The 2 is put in front of the MgO to balance with the 2O’s on the left and then a 2 needs to be put in front of the Mg to balance with the 2Mg’s on the right
• When balancing equations you cannot change the formulae
• Conservation of Mass


  The law states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants
• Chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation
• Relative Formula Mass (Mr)


  The sum of the relative atomic masses (Ar) of the atoms in the numbers shown in the formula
• Be able to work out the relative formula mass (Mr) of a substance using data from the periodic table
• In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown
• In thermal decompositions of metal carbonates, carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product
• Some reactions may appear to involve a change in mass but this can usually be explained because a reactant or product is a gas and its mass has not been taken into account
• Mass changes when a reactant or product is a gas
• When a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal
• Calculating moles

  1. Moles = mass/Mr

  2. mass = Mr x moles
• Moles


  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
• Avogadro constant


  The number of atoms, molecules or ions in a mole of a given substance is 6.02 x 10^23 per mole
• One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance
• Whenever a measurement is made there is always some uncertainty about the result obtained
• The range of a set of measurements about the mean can be used as a measure of uncertainty
• Chemical equations can be interpreted in terms of moles
• The masses of reactants and products can be calculated from balanced symbol equations
• General method for reacting mass questions

  1. Step 1: Work out the number of moles of the substance for which the mass has been given

  2. Step 2: Use the ratios of moles in the balanced equation to work out the moles of the other substance

  3. Step 3: Work out the mass of the second substance
• The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant
• Avogadro's Constant can be used for atoms, molecules and ions
• Using Avogadro's Constant


  No of particles = number of moles x Avogadro's constant
• Example calculation for Tin

  1. Step 1: Calculate the number of moles

  2. Step 2: Use Avogadro’s number to calculate no of atoms
• Steps for carbon dioxide calculation

  1. Step 1: Work out moles of sodium hydrogencarbonate

  2. Step 2: Use balanced equation to give moles of CO2

  3. Step 3: Work out mass of CO2
• Steps for sodium calculation

  Mass of 1 atom = mass of 1 mole of sodium(Ar)/Avogadro’s number
• Working out balancing numbers from masses

  1. Step 1: Calculate moles of each chemical

  2. Step 2: Using conservation of mass work out mass of missing substance

  3. Step 3: Divide each moles in step 2 by the smallest number of moles to get a whole number ratio

  4. Step 4: Write balanced equation
• In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used
• Limiting Reactant


  The reactant that is completely used up is called the limiting reactant because it limits the amount of products
• The number of moles of the limiting reactant will determine the number of moles of product formed
• Not all of the excess reactant will react
• General method for Limiting Reactant Questions

  1. Step 1: Work out the number of moles of the substance for each reactant

  2. Step 2: Use the ratios of moles in the balanced equation to work out which reactant is the limiting reactant

  3. Step 3: Use the ratios of moles in the balanced equation to convert the moles of the limiting reactant to the moles of a product

  4. Step 4: Work out the mass of the product
• Steps for limiting reactant calculation

  1. Step 1: Calculate the number of moles of the substance for each reactant

  2. Step 2: Use the ratios of moles in the balanced equation to work out which reactant is the limiting reactant

  3. Step 3: Use the ratios of moles in the balanced equation to convert the moles of the limiting reactant to the moles of a product

  4. Step 4: Calculate the mass of the product
• Concentration calculations

  1. Concentration(in mol/dm3) = moles/volume (in dm3)

  2. Concentration(in g/dm3) = mass (in g) /volume (in dm3)
• Percentage Yield

  % Yield = Mass of product actually made / maximum theoretical mass of product