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Chemical reactions 2
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Reaction rate
Reaction rate (or speed of reaction) is the speed with which reactants are transformed into products.
r =
-Δ[R] Δt =
Δ[P]
r indicates the change in concentration of reactants or products in the time interval. 3
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4. Factors influencing reaction rate
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Factors influencing reaction rate
Reaction rate depends on the chemical characteristics of the reactants, but it can be influenced by many factors, including:
concentration of reactants;
surface area available for contact between components;
temperature;
presence of catalysts or inhibitors.
Catalysts reduce activation energy because they modify the mechanism of the reaction. In this way, a larger number of particles can react even if the temperature is not changed. 4
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5. Collision theory and activation energy
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Collision theory and activation energy
According to collision theory, a chemical reaction takes place only if successful collisions occur. In successful collisions, particles of reactants have kinetic energy equal to the activation energy and have a favorable orientation. Cl Cl Cl O N O Cl O Cl N N Cl O O O
collision
no reaction
before collision O O O N Cl Cl N N Cl Cl Cl Cl O O O
before collision
collision
reaction
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Energy exchange
The heat of reaction (Q) is the heat exchanged between the system and the environment during a chemical reaction.
exothermic reactions reactants → products + Q
endothermic reactions Q + reactants→ products
The heat of reaction does not depend on the rate of reaction.
In an exothermic reaction, activation energy is lower than the energy released when reactants are transformed in products.
In an endothermic reaction, released energy is lower than the activation energy. 6
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7. Calories and basal metabolic rate
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Calories and basal metabolic rate
It is possible to experimentally measure the heat of a reaction, which is expressed in joules or calories.
A calorie is the amount of heat necessary to raise the temperature of a gram of liquid water by 1 °C.
Basal metabolic rate (BMR) is the amount of energy consumed by an organism at rest and at a neutral environment temperature.
Daily energy requirement, normally expressed in kilocalories, corresponds to the BMR plus the amount of calories required for daily activities. 7
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Reversible reactions
A chemical reaction is reversible when the reactants can transform into products (direct reaction) and the products can transform into reactants (inverse reaction).
direct reaction
reactants
products
inverse reaction 8
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9. Chemical equilibrium rd = ri
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Chemical equilibrium
A reversible reaction, which takes place in a closed system at constant temperature, is at equilibrium if the concentration of reactants and products in the reaction mixture do not change anymore.
At equilibrium, the rate of the direct reaction is the same as the rate of the inverse reaction.
rd = ri 9
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The law of mass action
At a given temperature, in a reaction mixture at equilibrium, there is a constant ratio (Kc) between the product of the concentration of products, raised to the power of the stoichiometric coefficients, and the product of the concentration of reactants, raised to the power of stoichiometric coefficients.
[A]a[B]b
= Kc
[C]c[D]d 10
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11. The concentration is constant
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Heterogeneous equilibria
In heterogeneous reactions, involving substances in different physical states, the equilibrium constant only takes into consideration the concentration of substances in gaseous or solution states.
2Hg(l) + Cl2(g) ⇄ Hg2Cl2(g)
[Hg2Cl2(g)]
[Hg]2[Cl2]
= K1
[Cl2]
= Kc 1
The concentration is constant 11
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12. Le Châtelier's principle
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Le Châtelier's principle
When an external factor (change of concentration, pressure or temperature) makes the rate of the direct reaction different from the rate of the inverse reaction, equilibrium is altered.
According to Le Châtelier's principle, the altered system partially counteracts the effect of the change, reaching a new state of equilibrium. 12
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13. Spontaneous and non spontaneous processes
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Spontaneous and non spontaneous processes
A chemical or physical process is spontaneous when it occurs without external intervention.
In order to induce a non spontaneous process, it is necessary to do work, thereby giving energy to the system. 13
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14. exothermic reactions ΔH < 0 endothermic reactions ΔH > 0
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Heat, work and reactions
The internal energy U of a system is the sum of all forms of kinetic and potential energy of particles that are part of it.
Enthalpy H expresses the thermal content of a system – the amount of heat that it can exchange, at constant pressure, with the environment.
exothermic reactions ΔH < 0
endothermic reactions ΔH > 0 14
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Entropy /1
Disorder indicates the random disposition of atoms, ions or molecules in substances.
Entropy (S) measures the disorder of a system.
Entropy depends on the physical state, chemical nature and number of moles in the system.
When temperature increases, entropy increases. 15
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Entropy /2
Transformations with positive ΔS indicate an increase of disorder.
Transformations with negative ΔS indicate a reduction of disorder.
Solutions and gases are less ordered than liquids and solids.
increase in entropy
solid
liquid
gas 16
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Free energy
Spontaneous processes increase the entropy of the Universe (environment + system) and reduce its enthalpy.
The free energy of the system (G):
G = H – TS
ΔG indicates the maximum amount of work that a system can perform during a transformation at constant temperature and pressure.
spontaneous reactions ΔG < 0
non spontaneous reactions ΔG > 0 17
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