topic 6. kinetics - mattlidenkinetics refers to movement and is the study of how fast a reaction...
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● Kinetics refers to movement and is the study of how fast a reaction goes.
Topic 6. Kinetics
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Examples of fast reactions:● Airbag inflation
● Egg in a hot pan
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Examples of slow reactions:
● Esterification
● Rusting
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6.1 Rates of reaction
Rate = 1/time = 1/s = s-1
H2 (g) + I2 (g) → 2HI (g)
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● Because the graphs are curves (and not straight lines) the rate of reaction is not constant.
● The reaction rate at any point in time (for example 50s) is given by the tangent to the curve and is the gradient of the graph at that time.
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1. Change in volume of gas produced
Measuring rates of reaction:
CaCO3 (s) + 2HCl (aq) → CaCl
2 + H
2O (l) + CO
2 (g)
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2. Change in mass
CaCO3 (s) + 2HCl (aq) → CaCl
2 + H
2O (l) + CO
2 (g)
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3. Change in transmission of light
H2 (g) + I2 (g) → 2HI (g)
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4. Change in concentration (titration)
● At regular intervals a sample of the reaction mixture is taken and titrated against Hcl. The HCl reacts with the remaining NaOH.
● The smaller the volume of HCl solution required for neutralization, the further the reaction has progressed.
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5. Change in concentration (conductivity)
● The small and fast moving hydroxide ions are consumed and replaced by slower moving bromide ions.
● The electrical conductivity decreases as the reaction proceeds.
C2H5Br (l) + OH- (aq) → C2H5OH (aq) + Br- (aq)
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6. “Clock reactions”
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Determine the reaction rate after 60 s and 120 s2 H
2O
2 (aq) → 2 H
2O (l) + O
2 (g)
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Rate of reaction, (mol dm-3 s-1)
● How fast reactants are being converted to products during a chemical reaction.
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6.2 Collision theory
● Particles in a substance have kinetic energy and move randomly.
● If a substance is heated, the kinetic energy of the particles increases and the temperature of the substance raises.
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Collision theory:● For a reaction to take place between two particles, the
two particles must:
– Collide with each other
– Collide in the correct orientation (the reactive parts of the two particles come into contact with each other)
– Collide with sufficient kinetic energy
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Activation energy, Ea
● The minimum amount of energy required to bring about a reaction.
= The minimum amount of kinetic energy that must be given to reactants before they will react.
= The minimum amount of energy necessary to break bonds.
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Movement of particles in different states of matter:
● Solid
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● Liquid/aqueous
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● Gas
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Maxwell-Boltzmann distribution
● The area under the graph represents the total number of particles and for a fixed mass of gas this area must be constant.
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Geometry of collision
● The particles must collide in the correct orientation in order to react.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/collis11.swf
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How reactions happen
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/activa2.swf
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https://www.youtube.com/watch?v=WeBE-_LRb2s
Factors affecting rate of reaction
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Concentration
● Increasing the concentration of the reactants increases the number of collisions between the particles and therefore increases the rate of the reaction.
Zn + 2 HCl → ZnCl2 + H
2
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Pressure
● If one or more reactants are gases an increase in pressure forces the particles together (=increases the “concentration”) and increases the collision rate.
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Temperature
● When particles in gases, liquids or solutions are heated:
- they move faster and thus collide more- a larger portion of the colliding species will have enough energy to overcome the energy barrier
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Particle size
● When one of the reactants is a solid, the reaction takes place on the surface of the solid.
● If the solid is broken down into smaller pieces, this surface area is increased.
CaCO3 (s) + 2HCl (aq) → CaCl
2 + H
2O (l) + CO
2 (g)
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Light
● If the reactants are photosensitive, the rate of the reaction is greatly increased by exposure to sunlight or ultraviolet light.
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Catalyst● A catalyst is a substance that is present in small amounts
and increases the rate of a chemical reaction but which is not consumed during the reaction.
● A catalyst provides a new pathway with a lower activation energy.
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● Biological catalysts are known as enzymes and consist of proteins.
● A substance that decreases the rate of a chemical reation is called inhibitor.
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Demo: Elephants toothpaste
● Pour ~50 mL of 30% hydrogen peroxide solution into the graduated cylinder.
● Squirt in a little dishwashing detergent and swirl it around.
● You can place 5-10 drops of food coloring along the wall of the cylinder to make the foam resemble striped toothpaste.
● Add ~10 mL of potassium iodide solution. Do not lean over the cylinder when you do this, as the reaction is very vigorous and you may get splashed or possibly burned by steam.
● ( An exothermoc reaction, the oxygen that is produced is captured as bubbles by the dishwashing detergent).
2 H2O
2(aq) → 2 H
2O(l) + O
2(g)