Secondary Chemistry Rate of Reaction

7/29/2019 Secondary Chemistry Rate of Reaction

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Rate of Reaction and Ionic Equations

The rate of a reaction can be measured by the rate at which a reactant is used up, or the

rate at which a product is formed.

The temperature, concentration, pressure of reacting gases, surface area of reacting

solids, and the use of catalysts, are all factors which affect the rate of a reaction.

Chemical reactions can only happen if reactant particles collide with enough energy.

The more frequently particles collide, and the greater the proportion of collisions with

enough energy, the greater the rate of reaction.

Measuring rates

Different reactions can happen at different rates. Reactions that happen slowly have a low rateof reaction. Reactions that happen quickly have a high rate of reaction. For example, thechemical weathering of rocks is a very slow reaction: it has a low rate of reaction.Explosions are very fast reactions: they have a high rate of reaction.

Reactants and products

There are two ways to measure the rate of a reaction:1. Measure the rate at which a reactantis used up2. Measure the rate at which aproductis formed

The method chosen depends on the reaction being studied. Sometimes it is easier to measurethe change in the amount of a reactant that has been used up; sometimes it is easier tomeasure the change in the amount of product that has been produced.

Things to measure

The measurement itself depends on the nature of the reactant or product:The mass of a substance - solid, liquid or gas - is measured with a

balanceThe volume of a gas is usually measured with a gas syringe, or

sometimes an upside down measuring cylinder or buretteIt is usual to record the mass or total volume at regular intervals and plot a graph. The

readings go on the vertical axis, and the time goes on the horizontal axis.

For example, if 24 cm3

of hydrogen gas is produced in two minutes, the mean rate of reaction= 24 2 = 12 cm

3hydrogen / min.

Factors affecting the rate

You will be expected to remember the factors that affect the rate of reactions, and to plot orinterpret graphs from rate experiments.

How to increase the rate of a reaction

The rate of a reaction increases if:

1. The temperature is increased2. The concentration of a dissolved reactantis increased3. The pressure of a reacting gas is increased4. Solid reactants are broken into smaller pieces5. A catalystis used


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Rate of reaction and changing conditions

The graph above summarises the differences in the rate of reaction at different temperatures,concentrations and size of pieces. The steeper the line, the greater the rate of reaction.

Reactions are usually fastest at the beginning, when the concentration of reactants is greatest.When the line becomes horizontal, the reaction has stopped.

Collisions and reactions

You will be expected to explain, in terms of particles and their collisions, why changing theconditions of a reaction changes its rate.

Collisions

For a chemical reaction to occur, the reactantparticles must collide. Collisions with too littleenergy do not produce a reaction.

Changing concentration or pressure

If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas isincreased:

1. There are more reactant particles in the same volume2. There is a greater chance of the particles colliding3. The rate of reaction increases

Changing particle size

If a solid reactant is broken into small pieces or ground into a powder:Its surface area is increasedMore particles are exposed to the other reactantThere is a greater chance of the particles collidingThe rate of reaction increases

Changing the temperature

If the temperature is increased:

The reactant particles move more quicklyMore particles have the activation energy or greaterThe particles collide more often, and more of the collisions result in a

reactionThe rate of reaction increases

Using a catalyst

Catalysts increase the rate of reaction without being used up. They do this by lowering theactivation energy needed. With a catalyst, more collisions result in a reaction, so the rate ofreaction increases. Different reactions need different catalysts.Catalysts are important in industry because they reduce costs.


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Electron-half-equations

What is an electron-half-equation?

When magnesium reduces hot copper(II) oxide to copper, the ionic equation for the reactionis:

You can split the ionic equation into two parts, and look at it from the point of view of themagnesium and of the copper(II) ions separately. This shows clearly that the magnesium haslost two electrons, and the copper(II) ions have gained them.

These two equations are described as "electron-half-equations" or "half-equations" or "ionic-

half-equations" or "half-reactions" - lots of variations all meaning exactly the same thing!Any redox reaction is made up of two half-reactions: in one of them electrons are being lost

(an oxidation process) and in the other one those electrons are being gained (a reductionprocess).

Working out electron-half-equations and using them to build ionic equationsIn the example above, we've got at the electron-half-equations by starting from the ionicequation and extracting the individual half-reactions from it. That's doing everything entirelythe wrong way round!In reality, you almost always start from the electron-half-equations and use them to build the

ionic equation.Example: The reaction between chlor ine and iron(I I ) ionsChlorine gas oxidises iron(II) ions to iron(III) ions. In the process, the chlorine is reduced tochloride ions.

You would have to know this, or be told it by an examiner. In building equations, there isquite a lot that you can work out as you go along, but you have to have somewhere to startfrom!You start by writing down what you know for each of the half-reactions. In the chlorine case,you know that chlorine (as molecules) turns into chloride ions:

The first thing to do is to balance the atoms that you have got as far as you possibly can:

ALWAYS check that you have the existing atoms balanced before you do anything else. Ifyou forget to do this, everything else that you do afterwards is a complete waste of time!

Now you have to add things to the half-equation in order to make it balance completely.

All you are allowed to add are:

3.electrons


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4.water5.hydrogen ions (unless the reaction is being done under alkaline conditions - in which case,

you can add hydroxide ions instead)

In the chlorine case, all that is wrong with the existing equation that we've produced so far isthat the charges don't balance. The left-hand side of the equation has no charge, but the right-hand side carries 2 negative charges.

That's easily put right by adding two electrons to the left-hand side. The final version of thehalf-reaction is:

Now you repeat this for the iron(II) ions. You know (or are told) that they are oxidised toiron(III) ions. Write this down:

The atoms balance, but the charges don't. There are 3 positive charges on the right-hand side,but only 2 on the left.

You need to reduce the number of positive charges on the right-hand side. That's easily doneby adding an electron to that side:

Combining the half-reactions to make the ionic equation for the reaction

What we've got at the moment is this:

It is obvious that the iron reaction will have to happen twice for every chlorine molecule thatreacts. Allow for that, and then add the two half-equations together.


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But don't stop there!! Check that everything balances - atoms and charges. It is very easy tomake small mistakes, especially if you are trying to multiply and add up more complicatedequations.

You will notice that I haven't bothered to include the electrons in the added-up version. If youthink about it, there are bound to be the same number on each side of the final equation, and

so they will cancel out. If you aren't happy with this, write them down and then cross themout afterwards!

Multiple Choice

1. Why does increasing the temperature of the reactants increase the rate of a chemicalreaction?A Heat energy causes the particles of the reactants to move faster, and therefore

react with each other more frequently

B Heat energy helps to lower the amount of overall energy that is required for thereaction to occur

C

Heat energy causes some of the reactants to evaporate, increasing theconcentration of the reactants

D Heat energy increases the size of the molecules of reactants, and therefore themolecules react more frequently

2.Which of the following would decrease the amount of time a glow stick would stay bright?A Decrease the concentration of one of the reactantsB Increase the size of the glowstickC Increase the temperature of the reactantsD Decrease the temperature of the reactants

3.Why are many types of food stored in refrigerators?A Bacteria do not grow well in dark placesB The volume inside the refrigerator decreases the concentration of the food

particles, allowing them to decompose more slowly

C The lower temperatures cause the chemical reactions that spoil food to occurmore slowly than usual

D Refrigerators often contain enzymes that slow the rate of food spoiling reactions


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4.Which of the following would cause the rate of a chemical reaction to slow?A Increase the temperature of the reactantsB Increase the concentration of the reactantsC Increase the surface area of the reactantsD Increase the volume that the reactants take up

5.The burning splint test is a procedure in a chemistry lab to test for the presence ofoxygen. Here, steel wool is used instead of burning splint:

What concept does the burning splint test illustrate?A Increasing volume increases

reaction rate

B Increasing concentrationincreases reaction rate

C Increasing surface areaincreases reaction rate

D Increasing temperatureincreases reaction rate

6.Which of the following methods for increasing the rate of chemical reaction increases theeffectiveness of the interactions between reactants, instead of just causing thereactant molecules to bump into each other more frequently?

A Adding a catalyst to the reactantsB Increasing the concentration of the reactantsC Increasing the surface area of the reactantsD Increasing the temperature of the reactants

7.A chemical reaction between two substances is occurring in an aqueous solution. What canbe done to increase the rate of reaction?A Increase the concentration of the reactants by adding more to the solutionB

Add a catalytic converter

C Rapidly cool the container that the reactants are inD Pour the reactants into a large, flat container to increase the surface area


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8.The process of rusting is actually the oxidation of iron, producing iron oxides. Why does acoating of oil slow the oxidation of iron?

A The oil cools the chain so that the rusting reaction occurs much slowerB The oil is an anti-catalyst, which actually causes the opposite reaction to

occur (anti-rusting)

C The oil stops oxygen from reaching the iron, and therefore decreases theconcentration of oxygen in the rusting reaction

D The oils surface decreases the surface area of the chain, slowing the rustingreaction

9. The rate of a reaction depends upon: the concentration of the reactants. A the temperature of the reaction. B whether or not a catalyst is used. C the nature of the reactants.D All of the above are correct.

11.Which of the following statment(s) about a reaction mechanism is/are true?I. The mechanism is the series of steps needed to complete a reaction.II. The rate of a reaction is only as fast as the slowest step in the mechanism.III. You can usually deduce the reaction mechanism from its reactants and products.

A I only B II only C III only D I and II only E I, II, and III

Use your knowledge of collision theory and theenergy distribution shown at right to answerquestions 11 - 13. Assume that the graph shows therange of energies of collsion of a collection ofreactants at two temperatures.

10.Which is the highest temperature?A T1 = T2 B T1 < T2 C T1 > T2 D It is impossible to tell.

11.Which of the curves has the greatest number of collisions possessing the activationenergy?

A T1 has the greatest number of collisions possessing the activation energy B T2 has the greatest number of collisions possessing the activation energy C Both have an equal number of collisions possessing the activation energy. D It is impossible to tell.

12. If a catalyst were used, how would the distributions change?A Both curves would shift to the right. B

Both curves would shift to the left.

C The activation energy would shift to the right.

D The activation energy would shift to the left.


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E Nothing about the distributions would change.

Use your knowledge of collisiontheory and the potenial energydigram shown at right to answer

questions 14 - 16. Assume theenergy values are in Kilojoules.

13.What is the H of the reaction?A + 35 KJ B - 35 KJ C + 25 KJD - 10 KJ E None of these are correct.

14.Which letter shows the activation energy?A B C D E

15.How would the graph change if the temperature were raised?A A and D would have higher energies. B C would be greater. C The activation energy would decrease. D More than one of the above are correct. E No change in the graph would occur.


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16.


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17.


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18.

Three samples of calcium carbonate are placed in flasks for an investigation.

In flask E is 5 g of large lumps of calcium carbonate.In flask F is 5 g of medium-sized lumps of calcium carbonate.In flask G is 5 g of small lumps of calcium carbonate.

An excess of dilute hydrochloric acid of the same volume is added to each flask. The

flasks are placed on three electronic balances. The figure below shows the change inmass of the flasks plotted against time.

(a) (i) Why do the three flasks and contents lose their mass?

(ii) How do the rates of reaction change with time?

(b) (i) Sketch on the same axes the curve you would expect if 10 g of powderedcalcium carbonate is used instead of 5 g of large lumps of calcium carbonate.Label this curve H.

(ii) Use collision theory to explain why the size of the lumps affects the rate ofreaction.


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19.

(b) In terms of the particle collision theory, explain how a catalyst increases the speed of

reaction.


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20.


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21.

Sodium thiosulfate solution, Na2S2O3, reacts slowly with dilute

hydrochloric acid to form a precipitate. The rate of this reaction can be

studied by measuring the time that it takes for a small fixed amount of

precipitate needed to obscure a cross on paper.

The equation for this reaction is shown below.

Na2S2O3 + 2HCl 2NaCl + S + SO2 + H2O

(a)Identify the insoluble product of this reaction.

(b)In terms of particles, explain why, at a fixed temperature, you mightexpect the rate of this reaction to double when the concentration of

sodium thiosulfate is doubled and the concentration of hydrochloric

acid remains the same.


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22.


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23. Methanol, CH3OH, is manufactured industrially by the reaction.

CO (g) + 2H2 (g) CH3OH (g) H = - 91 kJ

In the reaction, a mixture of carbon monoxide and hydrogen is passed over a catalyst.

a) Name the substances present at equilibrium. [1]

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b) In this reaction, 1.00 mol of carbon monoxide is reacted with excesshydrogen. If the reaction is 75.0% complete, sketch the graph of volume of carbon monoxideagainst time when a mixture of carbon monoxide and hydrogen are allowed to reachequilibrium over a catalyst. Label relevant values on the graph. [2]

c) Suggest why excess hydrogen is used in this reaction. [1]

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d) The reaction is carried out at a moderate temperature of 300C. After the

initial heating to start the reaction, no further heating is required. Explain why. [2]

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time

volume of carbon

monoxide / dm3

(measured at


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Secondary Chemistry Rate of Reaction

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