Rate and extent of Chemical changes

• ‘As the concentration of sodium thiosulfate solution doubles, the rate of reaction doubles.’ Explain the student’s conclusion in terms of particles.

  the number of particles per unit volume increases so the frequency of collisions increases so the number of particles increases
• Gold ions are used as a catalyst. How does a gold atom (Au) become a gold ion (Au3+)?

  It loses 3 electrons
• Carbon dioxide has a very low boiling point. Explain why.

  because carbon dioxide is simple molecular there are weak intermolecular forces
• Rate of reaction

  Amount (e.g. grams, cm3) of reactant used or product formed / time
• Rate of reaction (mol/s)

  Moles of reactant used or product formed / time
• Units for rate of reaction

  • g/s
  • cm3/s
  • mol/s
• Common ways of measuring rate of reaction

  • Loss in mass of reactants
  • Volume of gas produced
  • Time for a solution to become opaque
• Measuring rate by monitoring mass loss

  Place the reaction flask on a balance. In these reactions (e.g. metal carbonate + acid) a gas is given off, so record the decrease in mass in time intervals (note hydrogen is too light). Plot a graph of mass vs time.
• Measuring rate by monitoring volume of gas

  Connect a gas syringe to a reaction flask and measure the volume of a gas formed in time intervals. Plot a graph of volume vs time.
• Measuring rate by monitoring disappearance of a cross

  Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this cross. Mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross.
• Finding rate of reaction at time t from graph of amount of reactant vs time

  Pick a point corresponding to the time t, and find the tangent to the curve at this point. The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point. The steeper the tangent line, the faster the rate. Gradient of tangent can be expressed in change in y values over change in x values.
• Factors affecting rate of reaction

  • Concentration of reactants
  • Pressure of gases (volume)
  • Surface area
  • Temperature
  • Catalysts
• Collision theory

  Chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to activation energy)
• Increasing temperature

  Faster reaction
• Increasing concentration

  Faster reaction
• Increasing pressure of gases

  Faster reaction
• Increasing surface area

  Faster reaction
• Catalyst

  Changes the rate of reaction but is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
• Enzyme

  A molecule that acts as a catalyst in a biological system.
• Reversible reaction

  A reaction where the products can react backwards to produce the original reactants.
• Dynamic equilibrium

  In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.
• Le Chatelier's Principle

  If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium.