6. Kinetics HL

HL The elementary steps proposed for a mechanism must meet both of the following requirements:

* The sum of the elementary steps must give the overall balanced equation for the reaction. * The mechanism must agree with the experimentally determined rate expression.

Sketch the potential energy profile of a reaction, showing the activation energy.

..

Describe how the rate of a reaction is defined as the rate of change of concentration or amount of a reactant or product in unit time.

.. the answer is in the question kinda

Explain that for a reaction to take place the particles colliding must be in the correct orientation and have kinetic energies greater than the activation energy for the reaction.

1. The reactants (ions, atoms or molecules) must physically collide and come into direct contact with each other. 2. For many reacting molecules, steric factors are involved; the molecules must collide in the correct relative positions so their reactive atoms or functional groups are aligned. This is known as collision geometry 3. The colliding particles must have sufficient energy. Each of the colliding particles must be travelling with sufficient velocity, so that when they collide there is enough kinetic energy to enable the reaction to occur. A fixed amount of kinetic energy is required to overcome an endothermic 'energy barrier', known as the activation energy, Ea.

Describe how the characteristics of the Maxwell-Boltzmann distribution are altered by changing temperature, meaning that a greater proportion of particles will have energy greater than the activation energy at a higher temperature.

1.The peak of the curve moves to the right, so the most likely value of kinetic energy for the molecules increases. 2. The curve flattens, so the total area under it, and therefore the total number of molecules, remains constant. 3. The area under the curve to the right of the activation energy, Ea, increases. This means that at higher temperatures a greater percentage of molecules have energies equal to or in excess of the activation energy, Ea. This analysis helps us understand why increasing the temperature means that a reaction goes faster. Temperature is a measure of the average kinetic energy of the particles of a substance. When the temperature is increased, the collision rate increases because the average speeds of particles in the gas, liquid or solution are increased.

HL What are the units for the 3rd order and nth order of reaction?

3rd - k = units of rate ÷ (units of concentration)3 = mol^-2 dm^6 s^-1 nth - k = units of rate ÷ (units of concentration)^n = (mol dm^-3)^(1-n) s^-1

What is the role of a catalyst?

A catalyst is a substance that increases the rate of a reaction but remains chemically unchanged at the end of the reaction. A catalyst does this by making it possible for the particles to react by a new alternative pathway or mechanism for the reaction that has a lower activation energy than the uncatalysed pathway

Explain how the effect of increased temperature on the rate of a reaction is twofold; the rate is increased because the particles collide more frequently. And, more importantly, the particles have greater kinetic energy, so a greater proportion of the collisions have energy exceeding the activation energy of the reaction.

Again svaret är i frågan

There are many different ways in which the rate of reaction can be measured for a particular reaction. All of them measure, either directly or indirectly, a change in the concentration of either a reactant or product. Suitable changes include:

Colour. Formation of a precipitate. Change in mass; for example, a gas produced, causing a loss of mass. Volume of gas produced. Time taken for a given mass of a product to appear. pH. Temperature.

Deduce the rate of reaction at a given time during the reaction by drawing a tangent to the curve representing the course of the reaction.

Det måste du kunna. Kolla RT video eller min syllabus.

Describe how the kinetics of a reaction can be followed by tracking the concentration of a particular reactant or product with time.

Det är väl graferna?? förstod inte riktigt frågan

During a reaction, 0.04 mol of a substance is produced in a 2.50 dm3 vessel in twenty seconds. What is the rate of reaction?

Determine the amount produced in 1.00 dm3: Concentration = 0.04 mol / 2.50 dm3 = 0.016 mol dm-3 Determine the amount produced per second: Rate = 0.016 mol dm-3 / 20 s = 8 × 10-4 mol dm-3 s-1

examiner tip

Do not forget to label the axis of the Maxwell-Boltzmann curve and energy profiles when you sketch these.

What is Ea?

Ea, is defined as the minimum amount of kinetic energy which colliding molecules require in order to react. or The activation energy is the minimum amount of kinetic energy that colliding particles must possess for a collision to result in a reaction. It is the energy required to overcome repulsion, to start rearranging the bonds to produce the product molecules.

HL attention difference between heterogenous and homogenous catalyst

Homogeneous catalysts are in the same physical state as the reactants. Often both the catalyst and the reactants are in solution. Heterogeneous catalysts are in a different physical state or phase from the reactants. Often the reactants are gases and the catalyst is a solid, frequently a transition metal or transition metal compound. Many industrial processes use heterogeneous catalysts. The action of catalysts may be modified by the presence of low concentrations of certain substances, which may be classified as either promoters, inhibitors or catalyst poisons.

How does surface area affect the rate of reaction?

In a solid substance only the particles on the surface can come in contact with a surrounding reactant. If the solid is in powdered form then the surface area increases dramatically and the rate increases correspondingly. An increase in concentration as well as temperature also increases the rate of the reaction.

Explain how changing various conditions of a reaction - concentration, surface area of a solid or pressure of a gas - alter the rate of a reaction by increasing the frequency of collisions between reacting particles.

Increase of concentration effect on collision - Higher concentration --> more reactants --> more collisions per unit time - So it affects the collisions but not the geometry or activation energy - It does increase the rate of reaction since there will be more collisions per unit time *higher concentration --> higher rate Increase in pressure - increase in pressure will decrease the volume and force the molecules closer together, resulting in an increase of the frequency collisions per unit time - An increase in pressure will not affect the activation energy or geometry *However it will cause the rate of reaction to increase Increase in temperature - Increase in temperature is an increase of heat energy. So the energy of the molecules become higher than the energy of the Ea - It is also an increase of kinetic energy which causes more collisions per unit time *Which causes the rate of reaction to increase as well Increase in surface area (e.g. go from lumps to powder) - When the surface area is increased the reactants can move more freely around causing more collisions to occur - It does not affect the activation energy neither does it affect the geometry *Results in an increase of rate The key idea in all these examples is that the frequency of collision is increased in all cases - the particles collide more often, and therefore there is a greater chance of reaction. Also remember that there is a much more significant effect in the case of a temperature rise in that the particles have greater kinetic energy, and therefore the activation energy is exceeded in a larger proportion of collisions.

xtra what does the arrhenius constant stand for?

It can be referred to as the frequency factor as it is a measure of the proportion of molecules that collide with enough kinetic energy to react and which also have the correct orientation to react.

HL Describe the Activation energies in terms of their sensitivity to temp change

Low activation energies give rise to fast rates of reaction and a low sensitivity to changes in temperature. Large activation energies give rise to slow reactions at low temperatures and a high sensitivity to changes in temperature.

Examiner tip

Make that you are aware of several different experimental techniques that can be used to collect sufficient data for measuring reaction rates. For example, experiments that allow for a change in mass, volume or colour are useful. 1. Volume of gas -Reactions that produce gases are most easily investigated by collecting and measuring the volume of gas produced in a gas syringe. The volume of gas collected will increase, and the concentration of the reactants will decrease. The rate of increase of volume of gas (tangent to the volume-time curve) can be used as a measure of reaction rate. 2. For some reactions, small samples of the reaction mixture can be removed and then analysed by performing an acid-base titration or redox titration with a standard solution. This will allow the amount of a particular remaining reactant to be determined. The small sample of the reaction mixture is usually added to a large volume of cold water, so the reaction is stopped, or at least slowed down. This method can be used to measure the rate of saponification (alkaline hydrolysis) of ethyl ethanoate. 3.In some reactions, it is easy to measure the time it takes for a particular stage of a reaction to be reached. For example, in the reaction between sodium thiosulfate solution and dilute acid, a very fine precipitate of sulfur is produced. S2O32-(aq) + 2H+(aq) → SO2(g) + H2O(l) + S(s) The effect of changing the temperature or the concentrations of two reactants can be investigated by carrying out the reaction in a conical flask placed on a cross drawn on a piece of paper. Looked at from above the solution, the time from the start of the reaction until there is sufficient sulfur to hide the cross is recorded. The same total volume of solutions and the same cross have to be used in each experiment.

examiner tip

When explaining reaction rates with respect to collision theory make sure that when discussing the effect of changes in concentration you relate this to the frequency of collisions rather than just 'more collisions'. Also, of course, these more frequent collisions still have to have energy greater than activation energy in order to be a successful collisions.

Describe the major points of the collision theory of reaction rates.

Simple collision theory states that before a chemical reaction can occur, the following requirements must be met: 1. The reactants (ions, atoms or molecules) must physically collide and come into direct contact with each other. 2. For many reacting molecules, steric factors are involved; the molecules must collide in the correct relative positions so their reactive atoms or functional groups are aligned. This is known as collision geometry 3. The colliding particles must have sufficient energy. Each of the colliding particles must be travelling with sufficient velocity, so that when they collide there is enough kinetic energy to enable the reaction to occur. A fixed amount of kinetic energy is required to overcome an endothermic 'energy barrier', known as the activation energy, Ea.

Relation between temp in K and energy of particles?

Temperature in kelvin is proportional to the (average) energy of the particles Regardless of size; if particles have the same temp = same energy

HL Describe the Arrhenius equation

The Arrhenius equation describes reactions involving gases, as well as those occurring in solution or on the surface of a catalyst. k=Ae^(−Ea/RT) The expression (e^(−Ea /RT) ) is known as the exponential factor and allows for the large effect of an increase in temperature in the Arrhenius equation. - A represents the Arrhenius constant, or the pre-exponential factor (which has the same units as the rate constant, k, usually dm3 mol−1 s−1) - Ea represents the activation energy (units of kJ mol-1), -T represents the absolute temperature (K) and R represents the gas constant (8.31 J K−1 mol−1). - T (x-axis) and k (y-axis) are variables - the ones plotted against each other

How can the instantaneous rate of reaction be determined?

The instantaneous rate of reaction can be determined graphically from a graph of product, or reactant concentration, or amount against time The instantaneous rate of reaction at any time is equal to the gradient of the graph at that time. The rate will either be positive or negative, depending on whether the y-axis shows the concentration of a product or a reactant. The steeper the gradient of the graph, the faster the reaction and the higher its rate. When the graph is horizontal (i.e. the gradient is zero), the rate of reaction is zero, indicating that the reaction has finished.

HL What does a catalyst do?

The presence of a catalyst provides an alternative pathway that is more energetically favourable. More specifically it speeds up the reaction. Generally, the rate is directly proportional to the concentration of the catalyst.

HL Understandings about the rate constant K

The proportionality constant in the rate expression for a reaction is known as the rate constant, k, for the reaction. Understand that the rate constant is constant for a particular reaction at a particular temperature and is independent of the concentration of the reactants. Is constant and does not change during the reaction. It only changes with temperature

Define rate of a chemical reaction

The rate of a chemical reaction is the change in concentration, or amount of a reactant or product, with time, t: Rate = change in concentration / change in time. Or rate = (concentration at time t2 - concentration at time t1) / (time t2 - time t1).

Describe how the rate of a reaction varies as the reaction proceeds and how this is illustrated graphically.

The reaction is fastest when the graph is steepest - at the beginning of the reaction. The reaction finishes when the graph becomes horizontal, when there is no further production due to reactants(more specifically limiting reagent) running out.

How are catalysts significant in 'green chemistry´?

The search for new catalysts for industrial and commercial processes has gathered pace in recent years with the modern emphasis on 'green chemistry'. Part of the significance of enzymes in biological systems is that they allow chemical reactions to take place under mild conditions - the same is true industrially. Using catalysts, reactions can be carried out under milder conditions that are therefore economically more viable and intrinsically safer.

Explain how the population of molecules in a sample of a gas, liquid or solution has a spread of kinetic energies that is described by the Maxwell-Boltzmann distribution curve.

Theoretical calculations and experimental measurements both suggest that the kinetic energies of gas molecules in an ideal gas are distributed over a range, known as a Maxwell-Boltzmann distribution. Similar distributions of kinetic energies are present in the particles in solutions and liquids. Consideration of the features of this distribution adds further weight to the reasoning behind the collision theory. The key is to remember that activation energy, Ea, is defined as the minimum amount of kinetic energy which colliding molecules require in order to react. The total area under the curve is directly proportional to the total number of molecules, and the area under any portion of the curve is directly proportional to the number of molecules with kinetic energies in that range. So, only those molecules represented by the shaded portion on the right have sufficient energy to react.

HL What step of a reaction does the rate expression describe?

Understand that many reactions are multi-step reactions, taking place by a sequence of elementary steps where the slowest step is the rate determining step This slowest step in a reaction sequence, or mechanism, is known as the rate-determining step (RDS). The number of particles participating in the rate-determining step (RDS) is known as the molecularity of that step.

HL How can the order of reaction of reactants be determined

Understand that the order of reaction with respect to any reactant, and hence the rate expression, must be determined experimentally,

What is the reaction between EA and the rate of the reaction?

Values of barrier heights vary widely between chemical reactions. They control how rapidly reactions take place and how their rates respond to changes in temperature. Fast reactions are associated with low activation energies, while slow reactions are associated with high values of this energy barrier.

HL Figure 4 shows an upward parabolic curve. This indicates that the reaction is second order; a first order reaction would have produced a straight line (see later). But the question remains, second order with respect to what? Since the concentrations of both HCl and CH3OH are decreasing at the same rate, either of these may be second order

further experiments would have to be carried out to confirm this. Carrying out the experiment in this way simply shows that the reaction is second order overall. However, if this reaction is carried out with, say, the methanol present in large excess, only a small amount of this reactants will be consumed during reaction (Figure 5). The concentration of methanol is then effectively constant, and the rate of reaction will depend only upon the concentration of hydrochloric acid. This will clarify that the order with respect to [HCl] is 1; and a similar experiment can be carried out to establish that the order with respect ot methanol is also first order.

HL Kinetics calculation

https://www.youtube.com/watch?v=k2Z7LdjBac8&list=PL816Qsrt2Os14qTEEcXCY38rHb6BObTxh&index=13

hl graphs

https://www.youtube.com/watch?v=urVl4TJRlpc&index=14&list=PL816Qsrt2Os14qTEEcXCY38rHb6BObTxh

How can the activation energy be calculated non-graphically ?

lnk2−lnk1=( −Ea/R) (1/T2−1/T1)

HL Arrhenius equation is also used in a modified form,what is it?

lnk=lnA− Ea/RT https://www.youtube.com/watch?v=1TVoMpIiKrM&list=PL816Qsrt2Os14qTEEcXCY38rHb6BObTxh&index=19

HL Outline the meaning of the terms zero order, first order and second order in terms of how the rate of reaction varies with concentration in each case, - First Order

order of reaction: First rate expression: rate = k [A]^1 or rate = k [A] k = units of rate ÷ units of concentration = s^-1 nature of dependence: rate directly proportional to [A]; if concentration of A doubles, say, then rate doubles

HL Outline the meaning of the terms zero order, first order and second order in terms of how the rate of reaction varies with concentration in each case, - Second Order

order of reaction: Second rate expression: rate = k [A]^2 k = units of rate ÷ (units of concentration)^2 = mol^-1 dm^3 s^-1 nature of dependence: rate changes as the square of [A]; if concentration of A tripled (3x), then the rate increases nine-fold (3^2 = 9x)

HL Outline the meaning of the terms zero order, first order and second order in terms of how the rate of reaction varies with concentration in each case, - Zero Order

order of reaction: Zero rate expression: rate = k [A]^0 k = units of rate = mol dm^-3 s^-1 nature of dependence: rate is independent of [A]; any change in concentration of A makes no difference to rate

Richard thornely tip

rate has to be negative since it is slowing down

HL Describe the 5 steps of heterogenous catalysis

​The first step involves the diffusion of the reacting gas molecules onto the surface of the catalyst. The second step involves the chemisorption which is a form of adsorption (Figure 3a) of the reacting gas molecules onto the surface of the metal where they are temporarily bonded to the surface by weak intermolecular forces and/or coordinate covalent bonds. The third step involves the breaking of and formation of chemical bonds to bring about the formation of the product molecules. In the fourth step, desorption, the product molecules break free from the surface of the catalyst; this is the reverse of the adsorption step (Figure 3b). In the fifth step the gaseous product molecules diffuse away from the surface of the catalyst.