rates and equilibrium

• rate of reaction

  • tell us how fast reactants turn into products in a reaction
• 3 ways to measure rate of reaction

  • measuring a gas volume:
  • one product needs to be gas
  • use a gas syringe to measure the gas released, or measuring cylinder and water trough
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  • magnesium ribbon and acid:
  • Mg(s) + 2HCl(aq) > MgCl2(aq) + H2(g)
  • marble chips and acid:
  • CaCo3(s) + 2HCl(aq) > CaCl2(aq) + H2O(l) + CO2(g)
• 3 ways to measure rates of reaction

  • Measuring the decreasing mass of a reaction mixture
  • eg~
  • CaCO3 + 2HCl > CaCl2(aq) + H2O(l) + CO2(g)
• 3 ways to measure rates of reaction

  • Disappearing cross reaction
  • 2HCl(aq) + Na2S2O3(aq) > 2NaCl(aq) + H2O(l) + S(s) + SO2(g)
  • flask goes cloudy because sulfur is formed which is a solid
• changing rate:

  • concentration (solutions)
  • temperature (solutions, gases)
  • surface area (solids)
  • pressure (gases)
  • catalysts (gases, solutions)
• COLLISION THEORY

  • particles react to collide
  • need to collide in correct orientation and have enough activation energy
  • changing rate means you need to change the frequency of collisions or increase the energy of the collisions
• ACTIVATION ENERGY:

  • minimum energy required for a reaction to take place
• TEMPERATURE

  • when temperature is increased, particles move faster so there is a higher frequency of collisions
  • particles also have the activation energy meaning more collisions are likely to lead to a successful reaction
• SURFACE AREA

  • in a higher surface area, more solid particles are exposed
  • meaning higher frequency of collisions and a higher rate
• CONCENTRATION

  • refers to the amount of a substance dissolved in a fixed volume, this means there'll be a higher frequency of collisions with a faster rate
• PRESSURE (for gases)

  • change pressure by changing the size of the reaction vessel
  • higher pressure = particles more close together OR they're in a smaller volume
  • this means a higher frequency of collisions > higher rate
• CATALYSTS

  • a substance that increases the rate of reaction without being changed chemically itself by the end of the reaction
  • it makes:
  • a reaction faster
  • more product in a shorter time
  • may allow for milder reaction conditions
  • reusable
• HABER PROCESS

  • reaction to make ammonia from nitrogen and hydrogen:
  • N2(g) + 3H2(g) >< 2NH3(g)
  • REVERSIBLE REACTION
• How do catalysts work + advantages

  • provide an alternate reaction pathway of lower activation energy
  • reusable but may need to be 'cleaned'
  • lower pollution
  • saves money by using less energy since milder reaction conditions
• REVERSIBLE REACTIONS

  • energy released by the exothermic reaction is equal to the energy absorbed by the endothermic reaction
• EXOTHERMIC: let out or release energy
• ENDOTHERMIC: take in or absorb energy
• Equilibrium

  • occurs in a reversible reaction when the rate of forward and backwards reactions are equal
  • only occurs in closed systems
• STAGES

  1. start of the reaction, the forward reaction is fast
  2. as it continues, the backwards reaction begins to increase
  3. the backward reaction starts increasing in rate, the forward slowing down due to reactants being used up
  4. eventually both rates are the same, dynamic equilibrium is reached
• Effect of pressure on EQUILIBRIUM

  • increase in total pressure shifts equilibrium to the side with FEWER gaseous moles
  • decrease in total pressure shifts equilibrium to the side with MORE gaseous moles
  • if both sides have the same amount of gaseous moles, equilibrium will not shift
• Effect of temperature on EQUILIBRIUM

  • an increase in temperature shifts equilibrium in the ENDOTHERMIC direction
  • a decrease in temperature shifts equilibrium in the EXOTHERMIC direction
  • when the energy change is given and it is negative, the forward reaction is exothermic (visa versa)
• Effect of concentration on EQUILIBRIUM

  • an increase in the concentration of a substance shifts equilibrium to use up the substance and decrease the concentration once again
  • a decrease in concentration of a substance shifts equilibrium to make more of the substance and increase its concentration again
  • eg, 2SO2 + O2 >< 2SO3
  • if more O2 is added then equilibrium shifts right to use it up
  • if SO2 is removed then equilibrium shifts left to replace it
• Effect of CATALYSTS on EQUILIBRIUM

  • they do NOT affect position of equilibrium, they only increase rate at which it is reached
  • this is because catalysts increase the rate of both the forward and backwards reactions