C5

• Concentration of a solution

  Measured in moles per given volume of solution e.g. moles per dm3 (mol/dm3)
• Calculating moles of solute in a given volume of a known concentration

  Moles = concentration x volume (mol = mol/dm3 x dm3)
• Smaller volume or larger number of moles of solute

  Higher concentration
• Larger volume or smaller number of moles of solute


  Lower concentration
• Titration


  The volumes of acid and alkali solutions that react with each other can be measured
• Titration method


  1. Add acid to burette, record starting volume

  2. Add known volume of alkali to conical flask with indicator

  3. Place flask on white tile

  4. Add acid to alkali until end point

  5. Calculate amount of acid added (titre)

  6. Repeat for concordant titres
• 1 dm3 = 1000 cm3
• One mole of a substance in grams is the same as its relative atomic mass in grams
• Determining the rate of a reaction


  1. Use equations to find the rate of reaction

  2. Compare the effect of changes in surface area/particle size, concentration, temperature, use of a catalyst etc.

  3. Measure the amount of reactant used or product formed over time

  4. Quantity of reactant or product can be measured by mass or volume

  5. Units of rate of reaction may be g/s or cm3/s or mol/s

  6. Do multiple experiments changing the variable to compare
• After a titration, you know the exact volume of acid that reacts with the exact volume of alkali
• Measuring the rate of a reaction when a gas is produced

  1. Measure the volume of the gas using a gas syringe, upside down measuring cylinder or burette

  2. Record the total volume of gas collected at regular intervals and plot a graph

  3. Use the rate of reaction equation: volume of gas / time taken
• Rate of reaction graphs


  • Concentration of products increases as the reaction proceeds
  • Concentration of reactants decreases as the reaction proceeds
  • The gradient of the line/slope = the rate of reaction
  • 1/t is proportional to rate and gradients of graphs
• Effect of changes in temperature, concentration, pressure, and surface area on rate of reaction

  Increasing temperature, pressure or concentration increases the rate of reaction
• Calculating concentration of unknown alkali from known acid concentration


  1. Calculate moles of acid

  2. Use mole ratio to calculate moles of alkali

  3. Calculate concentration of alkali using concentration = moles / volume
• Explaining the effects on rates of reaction of changes in temperature, concentration and pressure

  1. Increasing temperature increases the speed of the moving particles, so they collide more frequently and energetically

  2. Increasing concentration of reacting solutions increases the frequency of collisions

  3. Increasing pressure of reacting gases increases the frequency of collisions
• Molar gas volume at room temperature and pressure (RTP)

  24 dm3
• Calculating volume of gas at RTP from moles

  1. Volume (dm3) = Moles x 24

  2. Volume (cm3) = Moles x 24,000
• Effect of changes in the size of the pieces of a reacting solid on rate of reaction

  • A greater surface area to volume ratio means a greater rate of reaction
  • As surface area to volume ratio increases, there are more surfaces for a reaction to occur - increasing frequency of collisions and therefore increasing the rate
• Catalysts


  • Substances that speed up chemical reactions without being changed or used up during the reaction
  • Enzymes are biological catalysts

  
• Concentration of a solution

  Calculated as mass of solute / volume of solution
• Identifying catalysts in reactions

  Remain unchanged throughout the reaction, so is not included in the reaction equation
• Explaining catalytic action in terms of activation energy


  Catalysts increase rate of reaction by providing an alternative pathway, which has a lower activation energy - therefore there are now more particles / reactants with an energy greater than that of the activation energy, meaning rate of reaction increases

  
• Smaller mass or larger volume


  Smaller concentration
• Reversible reactions


  Reactions where the products can react to produce the original reactants
• Enzymes act as catalysts in biological systems
• Larger mass or smaller volume

  Larger concentration
• Reversible reactions


  Reaction can be reversed by changing the reaction conditions
• Calculating theoretical amount of product from given amount of reactant

  1. Calculate moles of reactant

  2. Use mole ratio to calculate moles of product

  3. Calculate mass/volume of product using moles
• Dynamic equilibrium


  Occurs in a closed system when the rates of forward and reverse reactions are equal
• When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction
• Percentage yield


  Amount of product produced / Maximum amount of product possible x 100
• Equilibrium position


  • The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction
• If a system is at equilibrium and a change is made to any of the conditions


  The system responds to counteract the change
• Le Chatelier's Principle


  Can be used to predict the effect of changing conditions on a system at equilibrium
• Atom economy


  Measure of the amount of reactants that become useful products (or the efficiency of a reaction)
• If the concentration of one of the reactants or products is changed

  The system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again
• Calculating atom economy


  Atom economy = (Mr of desired product / sum of Mr of all reactants) x 100
• If the concentration of reactants is increased


  The position of equilibrium shifts towards the products (right) so more product is produced until equilibrium is reached again
• If the concentration of products is increased


  The position of equilibrium shifts towards the reactants (left) so more reactant is produced until equilibrium is reached again
• Choosing a reaction pathway


  Consider atom economy, yield, rate, equilibrium position and usefulness of by-products