C5

• What does the term concentration mean?

  Concentration refers to the amount or mass of a substance in a given volume of solution.
• What are the units of concentration?

  • mol/dm3
  • g/dm3
• How can concentration be calculated?

  • concentration (mol/dm3) = moles/volume (dm3)
  • concentration (g/dm3) = mass (g)/volume (dm3)
• How could you increase the concentration of a solution of hydrochloric acid?

  Either:

  • Increase the number of moles of hydrochloric acid.
  • Decrease the volume of the solution.
• What equipment is required for a standard titration?

  • Burette
  • Conical Flask
  • Pipette and pipette filler
  • White tile
  • Funnel
  • Clamp and stand
• Describe how to carry out a standard acid/alkali titration to find the concentration of the alkali.

  • Add acid to burette. Use pipette to add 25cm3 of alkali to a conical flask.
  • Add a few drops of indicator to the conical flask.
  • Record initial burette reading. Slowly add the acid to the conical flask, swirling the flask constantly.
  • When the solution in the conical flask changes colour, stop adding the acid and record the final volume in the burette.
  • Calculate the total volume of acid added from the burette.
• Why must the conical flask be swirled during a titration?

  To ensure all the particles react.
• Why is a white tile used when carrying out a titration?

  The white tile is placed under the conical flask so that the colour change of the indicator is easier to see.
• After an acid/alkali titration how can you calculate the concentration of the alkali if you know the concentration of the acid?

  • Multiply the concentration of acid by the volume from the burette to calculate the number of moles of acid.
  • Use the balanced equation to work out the molar ratio of acid:alkali then calculate the number of moles of alkali that reacted using this ratio.
  • Calculate the concentration of alkali by dividing the number of moles of alkali by the initial volume of alkali in the conical flask.
• Define the molar volume of a gas.

  The volume occupied by one mole of any gas at room temperature and pressure.
• What is the molar volume of gas at room temperature and pressure?

  24dm3
• How can the volume of gas be calculated at RTP?

  volume at RTP (dm3) = moles x 24
• Why can the theoretical mass of a product be calculated if you know the masses of all the other reactants and products?

  The law of conservation of mass means that the total mass of the reactants must equal the total mass of the products.
• How can the theoretical mass of a product be calculated using the mass of one reactant only?

  • Divide the mass of the reactant by its molecular mass to find the number of moles of this reactant.
  • Work out the molar ratio of reactant to product and use this to find the number of moles of the product.
  • Multiply the moles by molecular mass to find the theoretical mass of the product.
• How can percentage yield be calculated?

  Percentage yield = (actual mass of product/theoretical mass of product) x 100
• Why is percentage yield usually less than 100%?

  • Incomplete reaction.
  • Some of the product may have escaped (if gaseous) or vaporised.
  • Impurities in the starting material.
• What is meant by the term atom economy?

  The proportion of reactant atoms which form the desired product in a reaction.
• How can atom economy be calculated?

  Atom economy = (molecular mass of desired products/total molecular mass of reactants) x 100
• Some products can be produced by different reaction pathways. What must be considered when choosing the most appropriate pathway?


  • Percentage yield
  • Atom economy
  • Uses of by-products
  • Rate of reaction
  • Position of equilibrium
  • Availability of reactants
• How can rate of reaction be calculated?

  Rate of reaction = amount of product formed or reactant used/time (s)
• How can the rate of reaction be measured if a precipitate is produced?

  • Place a conical flask over a black cross.
  • Start the timer as soon as the reactants are combined in the conical flask.
  • Time how long it takes for the black cross to disappear.
• Why is the disappearing cross rate of reaction experiment not very accurate?

  It's very subjective, people might disagree about the exact point the black cross disappears.
• How can the rate of reaction be measured if hydrogen is produced?

  By collecting gas in a syringe or upturned measuring cylinder in a water trough:

  • Reactants are placed in a conical flask with a bung and delivery tube connected to a gas collection vessel.
  • Start timer.
  • Measure the volume of gas collected in 30 seconds.
  • Calculate the rate of reaction.
• When and how can the rate of reaction be measured using a digital mass balance?

  When a gas is produced as the gas will escape the reaction vessel causing mass to decrease.

  • Record initial mass of reactants.
  • Start timer when reactants are combined.
  • Calculate the rate of reaction.
• `How could you investigate the effect of temperature on the rate of a reaction?

  Calculate the rate of reaction using either:

  • Disappearing cross method.
  • Gas syringe method.
  • Digital mass balance method.

  Then repeat the same experiment, changing the temperature of the reactants at equal intervals to collect results. Plot the results on the same graph to compare.
• How can the rate of reaction be calculated at a specific time in a reaction?

  If the amount of product formed/reactant has been recorded at regular intervals during the reaction, plot results on a graph:

  • X axis - time (s)
  • Y axis - amount of product formed/reactant used.
  • Draw a tangent at specific time. The rate of reaction at this time is equal to the gradient of the tangent (change in y/change in x).