01 - kinetics

• what happens to atoms in a chemical reaction?

  the bonds between atoms in the reactant molecules break to allow the atoms to separate and rearrange to form new bonds to make the products.
• what must happen for a chemical reaction to occur?

  the reactant particles must be able to collide with sufficient kinetic energy for any covalent bonds between the atoms to be broken. This is the activation energy (Ea).
• what is the definition of activation energy?

  ‘the minimum amount of energy needed for a reaction to occur’
• what does the activation energy needed for each chemical reaction depend on?

  varies depending on the strengths and number of bonds which need to be broken.
• what could happen to a reaction with a low activation energy?

  the reaction can occur spontaneously.
• what happens to a reaction with a higher activation energy?

  the reaction needs some change to be made to the conditions so that the reaction can start.
• what’s an example of a reaction with high activation energy occurring?

  combustion of a fuel does not happen spontaneously at room temp because it needs heat energy in the form of a lit match or flame to start the reaction.
• do most collisions that occur between reactant particles occur with low activation energy of high? so?

  most collisions DONT occur with energy either equal to or greater than the activation energy. So, no reaction will occur as the bonds between the atoms of the reactants aren’t broken and so the reactant molecules stay unchanged.
• as long as successful reactions occur, what happens as any chemical reaction proceeds?

  concentration of reactants decreases and concentration of products increases.
• what is equation for rate of reaction?

  rate of reaction = change in conc of reactants or products/ time.
• what are the molecules like at the beginning of a reaction?

  • lots of reactant molecules around so chance of successful collisions occurring is higher.
• what are the molecules like as the reaction proceeds?

  • the concentration of reactants decreases as more are used up to make products.
  • concentration of products increases.
  • as reactant molecules used up, number of successful collisions occurring decreases and change in concentration of reactants/products over time decreases.
• what does the number of successful reactions occurring affect?

  rate of reaction.
• the higher the number of successful collisions occurring over a given time period…

  …the higher the frequency of successful collisions and so the higher the rate of reaction.
• the lower the rate of successful reactions occurring over a given time period…


  …the lower the frequency of successful collisions and the lower the rate of reaction.
• what are the units for concentration?

  moldm-3
• what is the definition of concentration?

  a measurement of the number of moles of the substance present in 1dm3 of the solution.
• when one or both of the reactants is a solution, how can we change its concentration?

  changing the volume of the solution or the number of moles of substance in the solution.
• what does a high concentration mean?

  there are a high number of moles present in the solution and the higher the number of particles present in the solution the higher the likelihood of successful collisions.
• what does an increase in concentration mean?

  an increase in the number of successful collisions occurring over a given time period, so, the frequency of successful collisions increases so an increase in rate of reaction.
• what does a low concentration mean?

  there are fewer moles in the solution, the lower the number of particles present the lower the likelihood of successful collisions.
• what does a decrease in concentration mean?

  results in a decrease in the number of successful collisions occurring over a given time period. The frequency of successful collisions decreases so a decrease in rate of reaction.
• what is pressure?


  a measurement of the number of gas particles present in a specified volume of gas.
• if one or both of the reactants is a gas, how can we change the pressure?

  changing either the volume of the gas or the number of gas particles.
• what does a high pressure mean?

  there are a high number of gas particles in the volume of gas. The higher the number of gas particles then the higher the likelihood of successful collisions.
• what does an increase in pressure mean?

  results in an increase in the number of successful collisions in a given time period. So, the frequency of successful collisions increases so there is an increase in rate of reaction.
• what does a low pressure mean?

  there are fewer gas particles present. So, the lower the likelihood of successful collisions.
• what does a decrease in pressure mean?

  results in a decrease in the number of successful collisions in a given time period. So, the frequency of successful collisions decreases so a decrease in rate of reaction.
• what is the motion like in a sample of gas?

  the particles have a constant and random motion. Some particles move very fast and other move very slow.
• what occurs as particles move around in a gas?

  they collide with each other, during the collisions energy transfers between the particles and some particles will gain energy and some will lose energy.
• what does a Maxwell-boltzmann distribution curve represent?

  the distribution of the energies of the particles in a sample of gas.
• draw a Maxwell-boltzmann distribution curve

  here

  
• for a Maxwell-boltzmann distribution curve, what does the x and y axis show?

  x axis = energy

  y axis = number of particles with that amount of energy
• on a Maxwell-boltzmann distribution curve, what does the position of the highest peak on the curve represent?

  the most probable energy (the amount of energy possessed by the majority of particles in the sample).
• what is the mean energy?

  the average energy of the particles in the sample, (always lower than the most probable energy).
• on a Maxwell-boltzmann distribution curve, what is the area below the curve show?

  the total number of particles in the gas sample.
• do any gas particles have 0 energy?

  NO, all particles will have some energy.
• how should you draw your curve on a Maxwell-boltzmann distribution curve?


  curve should never touch the x axis on the right (there are always some particles at the highest energies).
• does changes to concentration of a solution or pressure of a gas change energy of particles?

  no, the number of particles present changes but not their energy.
• how would the Maxwell-Boltzmann distribution curve change when changes occur to concentration of a solution or pressure of a gas?

  would have the same shape and distribution of curve, the positions of most probable energy, mean energy and activation energy don't change. The curve IS drawn at a higher position to show the increased number of particles at each energy.