Chemistry notes

F3252.1Rate of reaction.The rate of reactions depends on collisions between particles.The collisions must have more energy than the activation energy for a reaction to take place.Temperature can change the proportion of molecules that exceed the Ea as can the presence of a catalyst.

The rate of chemical reaction measures:-How fast a reactant is being used up.-How fast a product is being formed.

Rate of Reaction :- is the change in the concentration of a reactant or product per unit time.

Change in conc. Of reactant or product Rate = Time for the change to take place

Moldm3Units of rate = = moldm3s1} concentration/time S

Brackets [ ] are used to represent concentrations, in moldm3. E.g [CO2] means conc of CO2 in moldm3

During a reaction concentration of reactants decrease as:Fewer collisions take place between reactant particles per second therefore the rate slows down.

The rate of reaction can be determined by measuring the concentration of the reactant or product, at time intervals during the course of the reaction.A graph is produced from the results as conc against time

at any instant of time, the rate is equal to the slope of the curve. The slope is measured by drawing a tangent to the curve at this time. The gradient, or slope, of the tangent is then calculated. Initial rate of reaction is the change in concentration of a reactant, or product, per unit of time at the start of the reaction when t=0

1) What are the units of rate of reaction? = moldm3s1

2) How is rate of reaction measured from a concentration time graph?a tangent is drawn to the curve at the required time, and the gradient of the tangent is measure.3) What is meant by the initial rate of reaction?The change in concentration of a reactant or product at the start of the reaction, when t=0Measuring reaction rates from a concentration-time graph

Example Sulphur dichloride, SOCL, decomposes according to the equation:SOCL(g) SO(g) + Cl(g)

In an experiment, the conc of the reactant SOCl was measured over a period of time.A graph is formed from the results and tangents are drawn after 0 s ( the initial rate) and 3000 s

The value of the gradient of each tangent is measured to give the rate at each time , t.Change in concentration of SOCl 0.50-0.00After t=0 s initial rate = =Time for the change to take place 3300-0 =1.5 x 10 4 moldm3s1Change in concentration of SOCl 0.38 0.14After t=3000 s, rate = =Time for the change to take place 4000-0 = 6.0 x 10 5moldm 3s 1

Orders and the rate equation

OrdersIn a chemical reaction the particles that have a greater energy than the activation energy have sufficient energy to react and only these.

The greater the concentration: The larger the number of collisions per second The faster the rate of reaction.

The concentration of each reactant affects the rate of reaction, this effect is called order with respect to a reactantFor a reactant A: Rate [A] Where = order with respect to A

Order is always defined in terms of reactant concentrations.

Zero order is when the rate is unaffected by changing the concentration.First order when concentration is doubled so is the rate the change is proportional on both the rate and concentration.Second order when concentration is doubled rate quadruples, when concretion is tripled rate is increase by 9 times.

Rate constant, k = constant that links the rate of reaction with the concentrations of the reactants.

Rate equation = rate = k[B] [C]

The overall order is the addition of the powers together so in this example overall order would be 1+2 = third order.

Rate constants Zero order, rate = k[A]0 = k units of k = moldm 3s 1First order, rate = k[A] k=rate/ [A] units of k = moldm 3s 1/moldm 3 = S 1Second order, rate = k[A]2 k = rate/ [A]2 units of k-moldm 3s 1/ (moldm 3)2 = dm3mol 1s 1Third order, rate = k[A]2[B] k = rate/[A]2[B] Units of k= moldm 3S 1/ (moldm 3)3 = dm6mol 2s 1=mol 2dm6Concentration time graphs Zero order = diagonal lineFirst order = slow curveSecond order = steep curveThe half-life of a reactant is an important feature of a concentration-time graph. Half-life is the time for the concentration of a reactant to reduce by half.Rate- concentration graphsZero order = horizontalFirst order = diagonal (45 degrees)Second order = diagonal (less than 45 degrees)

Initial rates Can be determined from a concentration-time graph A tangent is drawn at t=0 and the rate is the gradient of this tangentAnother method of obtaining a value for the initial rate is to measure the time for a certain amount of product to be formed. clock reactions are ideal for this purpose. They measure the time from the start of the reaction until there is a visual change such as: Appearance of precipitate Disappearance of a solid A change in colourThe initial rate is proportional to 1/tInitial rates and rate constants Determination of orders by inspectionYou can find the order with respect to each reactant by comparing how the rate changes with changes in concentration.