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Chemistry I.G.C.S.E. Notes – Chapter 7 – Chemical Reactions

Rate of Reaction

·         The amount of reaction which occurs in unit time

·         Fast reactions have high rates, and low rates go with slow reactions

·         Rate = amount of change (moles concentration etc.)

                       1 second

·         Rate of reaction needs to be known because in industry reaction has to be stopped when finished and not prolonged.

Aà B

[A]                                                                  [B]

                                            t/s                                                                     t/s

*shows a decreasing rate of reactiom                              *shows increasing rate of reaction

Measuring the rate of a reaction:

·         Direct method: measuring the rate of change of mass at different intervals

·         Indirect method: measuring the external product given off e.g. gas evolved

Factors that influence rate of reaction:

Particle Size

Calcium Carbonate (s) + Hydrochloric Acid (aq) à Carbon Dioxide + Calcium Chloride + water

·         Carbonate solid used in this reaction can be varied in size and accordingly the rate of the reaction determined.

·         Smaller sizes (grinded) of the particles of calcium carbonate, faster the reaction takes place. (Due to difference in surface area)

Concentration

Sodium Thiosulphate + Hydrochloric Acid --> Sulphur + NaCl + SO₂(g) + H₂O

·         The very small particles of solid sulphur which is yielded remain in suspension

·         A cross is drawn on a paper and the time it takes until there is enough Sulphur to block the view of a cross drawn on a piece of paper under the flask.

·         Expt. Is repeated with different concentrations of sodium thiosulphate

·         The cross disappears soonest when the solution is most concentrated

·         Faster reaction, shorter time for it to finish

·         Speed of reaction µ Concentration

·         Another way is to alter the concentration of the acid and keep the concentration of sodium thiosulphate constant.

·         Ions are closer together in concentrated solution

·         Closer together, more often ions collide, more chance of reacting

Catalysts

·         Substances which increase the speed of a chemical reaction without being used up in the reaction are called a catalyst.

Hydrogen Peroxide à Oxygen + Water

   2H₂O₂ (aq) à O₂ (g) + 2H₂O (l)

·         In the formation of oxygen, because this is very slow, the reaction can be speeded up by adding manganese (IV) oxide. At the end of the reaction, the manganese (IV) oxide can be filtered and used again.

·         Catalysts help in industry because they reduce the need to create high temperatures and pressures and thereby reduce costs.

·         This is the same apparatus used to collect/measure CO₂ gas evolved from the reaction of Marble with Hydrochloric Acid. However, in the flask itself, HCl is “lowered” into the marble so that the start of the reaction can be controlled.

Enzymes

·         Catalysts of reactions that take place inside cells of living organisms

·         Enzymes are proteins that allow reactions to take place at very low temps

·         Enzymes are specific for type of reaction

·         Enzymes have 3-dimensional structure, which if lost; enzyme is denatured and loses catalytic activity

·         In industry, enzymes are used in batch processes. Enzyme and substrate mixed in container and left until good yield of product formed.

Temperature

·         In the reaction of sodium thiosulphate and acid, warming the solutions make sulphus form faster

·         Normally, reactions go about twice as fast at 30°C. (It doubles for every 10°C of temperature)

·         At a higher temperature, the ions have more kinetic energy and they move through the solution more rapidly, colliding more often and vigorously, increasing chance they will react.

Light

·         Some reactions take place faster when they absorb light

·         Formation of silver from silver salts takes place when a photographic film is exposed to light. In sunlight, green plants are able to carry on the process of photosynthesis.

Other things to note:

·         In direct measurement, the moles present in reactants and products can be known by reacting a mixture of known volume and molarity with another solution of known volume.

·         The molarity of the second solution can be determined

·         The mixture or reaction is then stopped through quenching. Quenching slows reaction down to rate of almost zero.

·         Quenching is done by rapid cooling through removal of reactants or catalyst. OR diluting the mixture

Measuring intensity with colorimeter

·         CuSO_­4 (aq) + Zn (s) à ZnSO₄ + Cu (s)

           Blue                        colourless

·         The intensity of the blue determines how concentrated the solution is

·         The intensity can be measured by a colorimeter

·         Intensity-time graph can show the decreasing concentration

·         This is relayed against the intensity of a pre-calibrated intensity against concentration curve and the concentration is obtained.

Changes in electrical conductivity:

BrO₃^- (aq) + 6I^- (aq) + 6H⁺(aq) à 3I₂ + 3 H₂O (l) + Br ^-(aq)

·         Electrical conductivity depends on the ions in solution

·         Electrical conductivity will decrease with time

·         Can also be used to measure rate of reaction

Collision Theory

A + B à C

H₂ (g) + Cl₂ (g) à 2HCl (g)

·         For any reaction involving two or more reactants the reacting particles must collide

·         The more often the particles collide, the faster the reaction goes

·         Chemical reaction also involves chemical bond breaking and bond making

·         These require energy…particles must possess necessary energy for this

·         Otherwise particles collide without reacting

·         Minimum energy colliding particles must have in order to collide successfully is Activation Energy (E_a)

·         E_a is the barrier that two reactants must cross to be converted into products

Activation Energy:

·         For high E_a, chances are that collision will be low because not all particles will possess the minimum energy to overcome it.

Catalysts:

·         Catalysts give you a greater percentage of particles that has energy greater than or equal to the minimum energy requirement. It does this by lowering the energy of activation barrier to a lower level.

Other factors that affect rate of reaction:

·         Temperature

o        Affects energy + freq. of collisions

o        Rate doubles for every 10°C (time taken is half)

o        Particles have more kinetic energy

·         Concentration

o        Rate increases due to numerous molecules that frequently collide

·         Pressure

o        (acts the same way as concentration for a gas)

·         Catalyst

o        No effect on freq. of collision

o        Lower energy requirements for successful reaction

·         Light

o        Particular radiation in sunlight provides energy for reaction

Photography

o        AgBr is salt used in black and white photography.

o        Photographic film is celluloid covered with layer of gelatin containing AgBr.

o        When light is exposed on this photographic film, Ag⁺ are converted by light into Silver Atoms which are black. Therefore, all areas exposed to light turn black. Those exposed to more light turn darker. The result is a black and white image (or grey-scale) of the scenery.

o        The film is placed in a developer solution which converts more silver ions in the areas exposed to light, to silver atoms.

o        A Fixer solution removes unchanged silver bromide

o        Result is a negative

o        Negative placed on top of light-sensitive printing paper and light is exposed.

Photosynthesis

o        plants manufacture sugar by this process

o        energy of sunlight converted into energy of chemical bonds in sugar molecules.

Carbon Dioxide + Water –-- catalysed by chlorophyll in leaves of green plantsà Glucose + O₂

6CO₂ (g) + 6H₂O (l) à C₆H₁₂O_6(aq) + 6O_{2 (g)}

Reversible Reactions

o        one that can be made to proceed in both directions under given conditions

o        Sign =  Aà B, BàA

o        One which can be made to go in either direction by changing the conditions under which it is carried out.

o        When a substance decomposes to form products which can recombine, we say that the substance dissociates. When this is brought about by heat, it is called thermal dissociation.

o         

Examples of reversible reactions:

• CH₃COOH + CH₃CH₂CH₂OH ß conc. H₂SO₄ reflux à CH₃C-O-CH₂CH₂CH₃ + H₂O
• N₂ + H₂ ß 450°C, 200 atm, Fe à 2NH₃ (g)
• Fe₃O₄ (s) + H₂ (g) ß à 3Fe (s) + 4 H₂O (g)
• CaCO₃ (s) ß à CuO (s) + CO₂ (g)

aB

o        At the beginning of the reaction, the rate at which A turns to B is high

o        As the concentration of A decreases, the rate slows down

o        The concentration of B increases and gradually the rate of B to A also increases. At the point where both the rates are equal (ie. concentrations are equal) is known as CHEMICAL EQUILIBRIUM

o        Ie. when the rate of the forward reaction equals the rate of the reverse reaction

o        Once equilibrium is achieved, the amounts of the three substances remain constant

Factors that can disturb equilibrium

o        In the evolution of CO₂ gas, if the carbon dioxide escapes, more calcium carbonate dissociates and the reaction can no longer react in the reverse direction.

Equilibrium Position

o        The equilibrium position can lay on the left, centre or to the right of an equation.

Equilibrium position lies on left because there is more A than B at equilibrium

Equilibrium position lies on the right

Equilibrium concentration is constant and equal (rare) position is centre.

o        Equilibrium must be attained under certain given set of conditions. In the Haber process, the best conditions are 450°C, 200 atm. And Fe

o        In industry, the main motive is to make profit, the cheapest method is to be found

Le Chatalier’s Principle

o        If a chemical system is in equilibrium and one of the factors involved in the equilibrium is altered (letting CO₂ gas escape), the equilibrium position will shift so as to tend to allow or cancel or counteract the effect of the change.

; ∆H = -92kJ

Factors:

o        Temperature

o        Pressure

o        Concentration

*Catalysts cannot change equilibrium position; it only reduces time takes to reach equilibrium

Pressure

o        Increase in volume = reduction in pressure and vice versa

o        An increase in pressure moves the equilibrium position to the side with less gaseous atoms

N₂ (g) + 3H₂ (g) ß 450°C, 200atm, Fe  à 2NH_­3 (g)

       [4 gas atoms]                                   [2 gas atoms]      * Equilibrium shifts to right

• When pressure increased, system can absorb increase in pressure by reducing volume ie. reacting to form ammonia

CaCO₃ (s) ß à CaO (s) + CO₂ (g)

• If pressure reduced, yield will increase. Equilibrium shift to side with more gaseous atoms

Temperature
N₂ (g) + 3H₂ (g) ß 450°C, 200atm, Fe  à 2NH_­3 (g)

• Reaction is exothermic, heat given out from left to right and reaction is endothermic the other way.
• If temperature rose, system adjusts so as to absorb heat equilibrium moves in favor (in direction) of endothermic reaction. In this case, the dissociation of ammonia will occur.
• Running process at high temperature therefore reduces % conversion of elements into ammonia.

Overall, a high pressure (200atm) and moderate temp (450°C) are employed with catalyst iron to speed up rate of reaction.

N₂O₄ (g)  2N₂O (g)  2NO (g) + O₂ (g)

Pale yellow        reddish brown               colourless

o        Dinitrogen tetraoxide boils off at 21°C, it dissociates into N₂O till 140°C.

o        Endothermic, therefore equilibrium shifts to right with increase in temperature. The intensity of the Nitrogren Dioxide is deepest at 140°.

o        140°C – 620°C = reddish brown intensity decreases

o        At 620°C = colourless mixture

o        Cooling from 620°C to 140°C = intensity increases

o        As pressure    shift to left.

Concentration

CH₃COOH + CH₃CH₂OH ß conc. H₂SO₄ à CH₃COOCH₂CH₃ + H₂O

·         If concentration increased, system will respond by using it up. Concentration decrease causes system to produce more and replace that used up.

Redox

·         Oxidation: when a substance combines with oxygen

Copper + Oxygen à Copper (II) Oxide

2Cu (s) + O₂ (g) à CuO (s)

·         Oxidizing agent is the substance that brings about oxidation (Gives oxygen). In this case it is oxygen.

·         Copper has been oxidized

·         Reduction: when oxygen is removed from an element and added to another.

Lead (II) Oxide + Hydrogen à Lead + Water

PbO (s) + H₂ (g) à Pb (s) + H₂O (l)

·         Reducing agent is substance that brings about reduction. In this case it is hydrogen

·         Lead has been reduced (Takes away oxygen).

·         HYDROGEN HAS ALSO GAINED OXYGEN (oxidation) and therefore, lead (II) oxide is an oxidizing agent.

·         Oxidation never occurs without reduction therefore these reactions are oxidation-reduction or REDOX reactions.

Note from example of Copper, copper loses two electrons and oxygen gains two electrons.

·         Substance oxidized loses electrons

·         Substance reduced gains electrons

·         Oxidizing agent accepts electrons

·         Reducing agent gives electrons

·         Oxidation is the gain of oxygen or loss of hydrogen or the loss of electrons

·         Reduction is loss of oxygen or gain of hydrogen or gain of electrons

Test for Reducing Agents

·         Acidified potassium manganate (VII) is powerful oxidizing agent

·         Reducing agent will change purple colour of MNO₄^‑ (aq) to pale pink of Mn²⁺ (aq).

Test for Oxidising Agents

·         Oxidizing agents will oxidize the iodide ion I^- to iodine I₂

·         The presence of iodine can be detected because it forms dark blue compound with starch

Potassium iodide or starch used to test