Chapter 18

• How do you normally calculate rate?

  
• What is used as shorthand for concentration in moldm⁻³?

  Sqaure brackets. For example, [A] is shorthand for concentration ofAin moldm⁻³.
• What is the order of reaction?

  The effect of the concentration of a reactant on a reaction rate is called its order.The rate of reaction is proportional to concentration of a reactant raised to a power- e.g.:rate ∝ [A]ⁿThe power is the order of reaction for that reactant, which can be 0, 1, or 2.
• What is zero order?

  When the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant:rate ∝ [A]⁰In a zero order reaction:-any power raised to the power zero is 1-concentration doesn't influence rate
• What is first order?

  A reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of one:rate ∝ [A]¹In a 1st order reaction:-if conc of A is doubled, the reaction rate increases by a factor of 2¹ = 2
• What is second order?

  A reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of two:

  rate ∝ [A]²

  In a 2nd order reaction:

  -if conc of A is doubled, the reaction rate increases by a factor of 2² = 4
• What is the rate equation?

  
• What is the rate constant?

  k,the proportionality constant. It is the number that mathematically converts between rate of reaction and concentration and orders.
• What is the overall order?

  The overall order gives the overall effect of the concentrations of all reactants on the rate of reaction.For the rate equationrate = k[A]ᵐ[B]ⁿoverall order = m + n
• How do you work out the rate constant units?

  
• [worked example- rate constant from experimental results]

  
• What is continuous monitoring?


  Taking continuous measurements during the course of a reaction.
• How can you continously monitor reactions that produce a gas?

  -Gas collection

  -Mass loss
• What does a colorimeter measure?

  It measures the intensity of light passing through a sample and records absorbance, which is directly linked to the concentration of the solution.
• How does a colorimeter work?

  
• What do concentration-time graphs look like for zero order reactions?

  - straight line

  - gradient = k

  
• What do concentration-time graphs look like for first order reactions?

  - curve- constant half life (time for the conc to half), akaexponential decay

  
• What do concentration-time graphs look like for second order reactions?

  - steeper curve

  - half life increases

  
• What is the half life?


  Half life, t₁/₂, is the time taken for half of a reactant to be used up.
• [example- constant half lives from a conc-time graph]

  
• How do you calculate rate constant from the value of the rate in a concentration-time graph, using a tangent?


  Substitute in the value of rate and concentration at the position where the tangent has been drawn

  
• How do you calculate rate constant from the half life?

  
• What are rate concentration graphs?


  Graphs which plot concentration against rate, using the initial rate for a number of experiments carried out at different concentrations.

  
• What do rate concentration graphs look like for first order reactions?

  - rate is directly proportional to concentration for a first order relationship

  - rate constant can be determined by measuring the gradient of the straight line of the graph

  
• What do rate concentration graphs look like for zero order reactions?

  - the y intercept gives the rate constantk- the reaction rate doesn't change with increasing concentration

  
• What do rate concentration graphs look like for second order reactions?

  - rate constant cannot be obtained directly from graph due to curve- plotting a second graph of rate against concentration squared results in a straight line through the origin, the gradient of which is equal to the rate constantk.

  
• [worked example- rate constant from a rate-concentration graph]

  
• What is initial rate?

  Rate when t=0.
• How can you calculate orders from initial rates?

  This must be deduced from experimental results.

  Compare two experiments with different concs of the reactant you're calculating the order for, and the same concs for all other reactants.

  By comparing the change in rate of the experiments and the change in concentration, you can determine the order.
• What is a clock reaction?

  A more convenient way of obtaining initial rate by taking a single measurement.

  The time t is then measured for a visual change to be observed, often a colour or precipitate.
• How can you use a clock reaction to find initial rate?

  Provided there is no significant change in rate over time, it can be assumed that average rate of reaction over this time will be the same as initial rate.

  Initial rate is then proportional to 1/t.

  The reaction is repeated several times with diff. concentrations, and values of 1/t are calculated for each separate run.
• What is an iodine clock?

  As aqueous iodine is coloured orange brown, the time from the start of the reaction and the appearance of the iodine colour can be measured. Starch is usually added since it forms a complex with iodine, which is an intense blue-black colour.
• [example- iodine clock procedure]

  
• [worked example- rate equation from a clock reaction]

  
• What are the problems with using clock reactions?

  -This method only works if you measure initial rate- once the reaction has progressed, you're no longer measuring rate for initial conc.

  -This causes problems if you're using a reaction where you wait for a colour change/solution to go cloudy, as this may mean the reaction has gone too far

  
• What is the rate determining step?

  The slowest step in the reaction mechanism of a multi-step reaction.
• What does the rate equation tell you?

  The chemicals present in the rate determining step of a mechanism, and the steps before it.
• How can you use the rate equation and overall equation to determine the steps of a reaction?

  [this will give you the answer in the 'accept' section of the markscheme, but it works!]1- Use the rate equation to find the reactants in step 1 (the slow step)2- Use those to find the first product of step 1, which should be the same as one of the overall products.3- Write in whatever's left over.4- For step 2 reactants, write in the product in step 1 that isn't in the overall equation, so it cancels out.5- Write in whatever's left over from the overall equation for the reactants and products of step 2.(this sounds complicated but trust me it makes sense when you actually do it)
• How does an increase in temperature affect rate constant?

  -Increasing temperature shifts Boltzmann distribution to the right, increasing the proportion of particles that exceed the Eₐ.

  - As temperature increases, particles move faster and collide more frequently.

  Therefore, rate increases and rate constant also increases.

  Only temperature can change the rate constant.

  
• What is the Arrhenius equation?

  An exponential relationship between the rate constant and temperature, used to calculate Eₐ.