Rates and orders of reaction

The Rate Expression and Orders of Reaction

Simplistically one would imagine that doubling the concentration of a reactant would double the rate of a reaction. This seems logical; there will be double the number of collisions. However, although this is often the case it is not always true. Indeed, sometimes doubling the concentration of a reactant could have no affect on the rate and sometimes it will increase the rate by more than double, for example the rate could be quadrupled.

How can this be so? The answer lies in the fact that reactions usually take place by a series of steps and one of the steps (the slowest one (rate determining step RDS)) will control the rate. Only reactants involved in this step will affect the rate.  For reactants that are not involved in the RDS their concentration will have no affect on the rate. The order with respect to this reactant will be zero and this reactant will not appear in the rate expression. If a reactant is involved once in the RDS then doubling its concentration will cause the rate to double. It is said to be first order with respect to this reactant. The rate expression would be rate ≈ [conc] or Rate = k [conc]. It could be the case that a reactant is involved twice in the RDS, in which case, doubling its concentration will cause the rate to go quadruple i.e. go up by 4. The rate expression would be: Rate ≈[conc]²  or  Rate = k[conc]². K = the rate constant and the powers are the orders for each of the reactants. The overall order is simply the addition of all of the powers in the rate expression.

For example for the rate expression:  Rate = k  [BrO₃^-] [Br^-] [H⁺]²

What is the order with respect to each reactant?

What is the overall order?

What units will k have in this rate expression.  Note that the units of k will vary depending on the expression.

Remember that the rate expression can only be determined experimentally by following the rate and changing one reactant at a time. The stoichiometry of a reaction tells you nothing about the rate expression or possible mechanism.

Deducing  a Mechanism from the Rate Expression

This can be quite complex. However, simplistically if a reactant has zero order then it is not in the RDS but must be in a faster step. If a reactant is first order then it is directly or indirectly involved once in the RDS. If a reactant is second order then it’s either directly or indirectly involved twice in the RDS. Indirectly means that it could be involved with producing an intermediate, which is then involved in the RDS.

It can be fun proposing mechanisms that fit with the rate expression. There may well be several possibilities.


Mechanisms and orders

Rate expression

Guide to writing a lab report

Guide to writing a lab report

Rates of Reaction

You've seen how concentration and surface area affect the rate of a reaction. let's now look at the effect of catalysts and how heterogeneous and homogeneous catalysts can work;

The following is a good chem guide link:

Chem guide rates

Chem guide Catalysts

Maxwell Boltsmann distribution curve
temperature and the Maxwell Boltsmann curve

Measuring the rate of a reaction

Some useful notes on rates

IB Syllabus link to rate expression

VIRTUAL EXPERIMENT

Virtual exp results

Another virtual experiment

Chem Wiki

Energetics definitions

This is from the interactive syllabus. Remember for standard values you must state under standard conditions i.e. 25 0C 1 atm and any solutions 1 mol/dm3.

Energy Definitions

It should be noted that many of the energy definitions can be considered in the reverse direction with a corresponding change of sign for the energy. Bond formation = exothermic ΔH negative, bond cleavage = endothermic ΔH positive

Enthalpy of vaporisation

The energy required to vaporise one mole of a liquid

Enthalpy of atomisation

The energy required to produce one mole of gaseous atoms from an element in its standard state

Bond dissociation enthalpy

The energy change when one mole of a specific bond is broken or created

Bond enthalpy

The average energy change when one mole of a specific type of bond is broken or created.

Enthalpy of Combustion

The energy released when one mole of a compound is burned in excess oxygen

Enthalpy of formation

The energy change when one mole of a compound is formed from its constituent elements in their standard states

Enthalpy of solution

The energy change when one mole of a substance is dissolved in an infinite amount of water

Hydration enthalpy

The energy change when a particle is taken from infinite separation in the gaseous state to its position in an aqueous lattice

1st Ionisation energy

The energy required to produce one mole of gaseous ions from one mole of gaseous atoms by removal of one mole of electrons

Lattice enthalpy

The energy change when one mole of an ionic substance is broken into its constituent atoms at infinite separation.

Energetics Useful links

The following are some useful chemguide links:

Definitions

Using bond enthalpies

using Hess's law

Using specific heat capacities

Homework- using Bond energies

Please finish the sheet on bond energies. Also read the next part in your book so that you are ready for the lessons after academic travel. Hess's law and Born Haber cycles.

Have a good week.

Allotropes of carbon

The following video is interesting about the possible future of graphene:

Bonding test

We have just about finished the topic on bonding so please start revising for a test next week. Your notes, experiments and textbook should be used. In addition the IB website is useful too. Here are the following links:

IBbonding standard level

IB bonding higher level

Bonding Useful Links

Shapes

Shapes and angles

Sigma and pi bonds

Hybridisation video clip

Delocalisation of electrons

Bonding in methane

Bonding in ethene

Bonding in ethyne

Bonding in benzene

Welcome back

Hi, I hope that you all have had a great Christmas and New Year. Cran Montana is sunny with lots of snow at the moment. :)

Please look at the following links, in preparation for the next topic, bonding.

Bonding IB site

Bonding Chem Guide

Happy Christmas

Wishing you all a very Happy Christmas

See if you can work out all the Chemistry involved with this simple demonstration of copper in silver nitrate solution. 

Reactivity Series of Metals

Note that Al displays much less reactivity that it actually has, due to an oxide coating over its surface which is unreactive. That's why it can be used for mountain bikes and window frames. The reactivity series of metals is as follows:

Melting Point Trends in Period Three

The following link shows a graph of the trend and gives some explanation. Notice that it gives a different value for phosphorous than the worksheet that I gave. Where's the mistake? Check with the data booklet and also the interactive periodic table site.

Melting and boiling points across period three



Periodic Table

Periodicity Video Clips

some video links

Homework. Period Three Trends

Period Three Trends

Don’t panic we will do bonding next and the trends can be simplified:

Sodium, magnesium and Aluminium are metals. Therefore, they have typical metallic properties i.e. shiny, malleable, and conduct electricity. The delocalized electrons can move and therefore, maintain the metallic bond even if the shape is changed, in addition they can move and so an electric current can flow. Si, P S Cl and Ar are non- metals. Ar is a noble gas and so exits as atoms but the others have covalent bonds, either as a giant covalent structure i.e. Si or as simple covalent molecules. The melting point will depend very much on the type of covalent bonding. Giant covalent substances will have very high melting points, as the strong covalent bonds need to be broken. However, simple molecular substances will have relatively low melting points due to the weak forces of attraction between the molecules (Van der Waal’s). 

The metallic oxides are ionic. These are basic and produce hydroxide solutions. Al₂O₃ is an exception as it has both kinds of bonding, ionic and covalent and so can react with acids and bases. It is said to be amphoteric. However, in water in does not dissolve and so won't affect the pH.

The covalent oxides are acidic; they react with water to form an acid. 

The ionic oxides are neutral or slightly acidic. Any slight acidity is just due to protons being pulled off water molecules, releasing H⁺ ions. The smaller and more highly charged the ion, the more this happens.

The covalent chlorides react with water to form two acids (usually), one of them being HCl.  ACl₃ is strange in that it is actually a covalent compound and is made up of a metal and a non-metal. It produces HCl when it reacts with water. 

Your homework is to: 

• Produce a neat table of results from your experimental observations
• Write an equation for all of the reactions that occurred. (include the oxides and chlorides that we didn’t have)
• Make conclusions from your results about the trends of the elements, oxides and chlorides of period three that you saw
• Explain the conclusions that you made and relate them to the type of bonding

Use your notes form the lesson, the notes / links on the blog and your text book

Period Three Trends

The following is a link to the IB website and it has good summary tables for the reactions of the oxides and chlorides. Remember to think about the type of bonding when you are explaining the trends of the period three elements, oxides and chlorides.

Period Three Trends

Chem Guide Period Three

Questions on the Halogens

Group (VII) (The Halogens)

1.      Explain the trend in atomic size in descending the group.

Each time the group is descended by one, a new quantum level is added. Therefore, the distance from the nuclear charge to the outer electrons increases and so does the screening effect, therefore, the attraction on the outer electrons is reduced and the atoms get bigger.

2.      What trend in electron affinities would you predict?

As the atomic size (distance from outer electrons to the nuclear charge) and the screening effect increases going down the group, the attraction on an added electron from the nuclear charge will decrease. Therefore, I would expect the electron affinity to decrease going down the group.

3.      Based on number 2, what trends in reactivity would you expect? Give reasons.

The halogens have seven electrons in their outer shell and so react by obtaining one more (either by sharing in covalent bonding or gaining one in ionic bonding). One of the factors influencing their reactivity will be their ability to gain an electron. This will be greatest when they are smallest with the least screening  (greatest attraction). Therefore, the reactivity should decrease going down the group or increase going up the group.

4.      The actual values for the electron affinity are; F_(g)   -328 (KJ/mol), Cl_(g)   

-349, Br_(g) -324 and I_(g) -245

What is surprising is that Cl has a higher value (more heat released) than F. This is because that F is such a small atom that the electrons in the outer quantum level are closely packed. This causes there to be more repulsion for an incoming electron and therefore, the overall heat energy released is actually less than for Cl. However, F is still more reactive than Cl and this is because in a reaction other things are occurring, such as: Atomisation and hydration energy (if its in a solution). Flourine has the lowest atomization energy and the greatest hydration energy. This compensates for the slightly lower electron affinity value. Therefore, overall fluorine is the most reactive of the halogens.

You may find the atomisation data for fluorine and chlorine surprising as they are both gases at room temperature. Therefore, atomisation involves breaking the bond between the atoms. Look at the following link which includes bond energy data, and all should be clear to you.

Chem Guide Group 7 properties

Homework The Halogens

The Halogens

Please make sure that all your observations for the reaction with iron wool and with the solutions; KI_(aq), KBr_(aq) and KCl_(aq) are recorded. Please write equations for the reactions and explain the trend in reactivity.

E.g. Cl₂ + 2I^- →  I₂ +  2Cl^-

Or showing the two half equations:  Cl₂ + 2e^- → 2Cl^-    (reduction)

                                                                  2I^- → I₂ + 2e^-        (oxidation)

This illustrates that chlorine can oxidise iodide ions and that chlorine is more reactive than iodine.

Period three reactions with water

Period three reactions with water

The oxidising ability of the halogens

look at the following link:

The Oxidising power of the halogens