Rate of reactions ()

• Define rate of reaction

  The change in concentration of a reactant/product per unit time

  
• [A]

  Concentration of A

  Units -> moldm-3
• K =


  rate constant
• Rate of reaction is proportional to

  the concentration of a particular reactant raised to a power

  
• Zero order


  Zero order is when the concentration of a reactant has no effect on the rate

  
• First order


  First order is when rate is directly proportional to concentration

  
• Second order


  Second order is when the rate is directly proportional to the square of the concentration

  
• Rate equation

  The rate equation gives the mathematical relationship between the concentrations of the reactants and the reaction ratel.

  
• Overall order =


  m + n

  The sum of the order in respect to the reactants

  
• What are the 2 main methods in determining the reaction order in respect to the concentration of a reactant?

  Initial rates method -> This involves measuring how the initial rate changes when you alter the initial concentration of the reactant in question.

  Continuous monitoring -> This involves tracking the change in concentration of the reactant over time and plotting a rate-concentration graph.
• Initial rates method


  It is found by drawing a tangent to t = 0 and finding the gradient
• The iodine clock reaction

  Relies on the formation of iodine. 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 dark blue-black colour

  -> Hydrogen peroxide

  
• Concentration-time graphs


  The gradient of a concentration-time graph is the rate of the reaction.

  
• Zero order concentration-time graph

  A zero order produces a straight line with a negative gradient.

  The reaction rate does not change at all during the course of the reaction.

  -> THE VALUE OF THE GRADIENT IS EQUAL TO THE RATE CONSTANT K

  
• First order concentration-time graph

  A first order reaction produces a downward curve with a decreasing gradient. Sd the gradient decreases with time, the reaction gradually slows down.

  -> The time for the concentration of the reactant to halve is constant

  
• Second order concentration-time graph


  A downward curve, steeper at the start

  
• Half-life


  The time taken for half of a reactant to be used up.

  
• Calculating the rate constant from the half life

  In2 = 0.6931471806

  t1/2 = how long it takes for reactant to decompose (a half life)

  OR

  Using a gradient (rate=gradient, then use rate equation).

  
• Rate-concentration graphs


  Rate concentration graphs offer a route into the direct link between rate and concentration in the rate equation

  
• Zero order rate-concentration graph

  Rate equation: rate = k

  The y-intercept gives the rate constant k

  
• First order rate-concentration graph

  Rate equation: rate=k[X]

  Rate constant can be determined by measuring the gradient of the straight line

  
• Second order rate-concentration graph

  Rate equation: rate=k[X]2

  
• What is the rate-determining step?

  The slowest step in a multi-step reaction. The rate-determining step dictates the overall rate of reaction

  
• What is the effect of temperature change on reaction rate and rate constant?

  An increase in temperature leads to an exponential increase in the rate constant (k). At higher temperatures, reactant molecules move faster, resulting in more frequent collisions.

  Additionally, higher temperatures provide greater kinetic energy to reactant molecules, allowing a higher proportion to exceed the activation energy barrier.
• What is the effect of activation energy on reaction rate and rate constant?

  The higher the Ea, the lower the rate constant (k) becomes.

  A high activation energy means that fewer reactant molecules have sufficient kinetic energy to overcome the energy barrier and react successfully.

  -> So a reaction with a high Ea will have a slower rate
• Arrhenius equation


  In(k) = rate constant (graphically = plot y)

  -Ea/R = gradient (calculate gradient = -Ea, divide by 8.31)

  1/T = plot x

  In(A) = y-intercept (e- of value = pre-exponential factor)