Chemistry Lab Notes 6 PDF

1\. THEORY KEYWORDS: − reaction rate, − factors that affect the rate (the speed) of reaction, − chemical equilibrium and Le Chatelier's Principle, − catalyst/inhibitor. Reaction rate (reaction speed; speed of reaction; rate of reaction) -- is the rate of change in concentration in reactants or produ...

1\. THEORY KEYWORDS: − reaction rate, − factors that affect the rate (the speed) of reaction, − chemical equilibrium and Le Chatelier's Principle, − catalyst/inhibitor. Reaction rate (reaction speed; speed of reaction; rate of reaction) -- is the rate of change in concentration in reactants or products over time. In other words the reaction rate is defined as how quickly or slowly the reaction takes place. For this reaction: C D Rate = \[D\] /  time \[mol/dm3 · s\] the rate of appearance of product C D Rate = --\[C\] /  time \[mol/dm3 · s\] the rate of loss of reactant \[\] -- molar concentration; mol/dm3 ; mol/l The rate law -- explanation based on reaction: A + B AB; the rate usually depends on \[A\] and \[B\]. This is an equation that states the instantaneous rate of reaction as a function of reactant concentrations. First-order reaction: rate law: Rate = k1\[A\] or Rate = k1\[B\]; k = rate constant; \[A\], \[B\] -- molar concentrations of A and B. Second --order reaction: rate law: Rate = k2\[A\]2 or Rate = k2\[B\]2 or Rate = k2\[A\]\[B\]. Zero-order reaction: rate law: Rate = k The main factors that affect the rate (the speed) of reaction: − activation energy (Ea) -- is a minimum energy required to initiate a chemical reaction \[kJ/mol\], in other words, this is the energy that we have to put in to get the reaction going, − concentration -- the higher concentration of one or more reactants the faster the rate of reaction; if we increase the concentration of one or more reactants, the speed of reaction (the rate of reaction) will also increase, − temperature -- an increase in temperature increases the speed of reaction, Arrhenius equation: k = Ae--Ea/RT Ea -- activation energy; R -- ideal gas constant; T -- temperature \[K\]; A -- a frequency factor (frequency of collisions) -- it depends on reaction: − pressure of reacting gases -- the higher the pressure the faster the speed of reaction; an increase in pressure increases the rate of reaction, − surface area of solid reactants -- an increase in surface area increases the speed of reaction, − presence of a catalyst -- a catalyst is a substance that increase the speed of reaction without being affected chemically or being used up itself during the reaction of interest. A catalyst reduces an activation energy (Ea) that is why it increases the rate of reaction. 6 Chemical equilibrium -- typical for reversible reaction -- two opposite reactions (forward and reverse) are taking place at the same time in the same place and with equal rates. Represented in the chemical balanced equation by: a A + b B c C + d D at equilibrium, the process is described by constant (Kc; equilibrium constant; Keq): Kc = \[C\]eq c x \[D\]eq d / \[A\]eq a x \[B\]eq b \[ \] refers to molarity (M = mol/l; mol/dm3 ) (Guldberg and Waage -- Law of Mass Action -- the law states that equilibrium condition is expressed by equation of equilibrium constant). It is said that chemical equilibrium is dynamic, it means that reactants and products are continually being formed and re-formed. For all other conditions (except equilibrium: eq) the process is expressed by the reaction quotient, Qc: Qc = \[C\]c · \[D\]d / \[A\]a · \[B\]b \[ \] refers to molarity (M=mol/l; mol/dm3 ) when Qc = Kc -- the reaction is at equilibrium when Qc \> Kc -- the reaction will go to the left when Qc \< Kc -- the reaction will go to the right Le Chatelier's principle: When a system at dynamic equilibrium is perturbed (disturbed, changed), the system will adjust itself in order to minimize the effects of the perturbation (the effect of changes). Summary of Le Chatelier's principle: − CONCENTRATION: adding reactant/removing product -- equilibrium shifts in the forward direction; removing reactant/adding product -- equilibrium shifts in the reverse direction; Kc does not change, − TEMPERATURE: increasing temperature -- equilibrium shifts in the forward direction for endothermic reactions, in the reverse direction for exothermic reactions; decreasing temperature - equilibrium shifts in the reverse direction for endothermic reactions, in the forward direction for exothermic reactions; Kc changes, − CATALYSTS: no shift; Kc does not change, − PRESSURE: increasing pressure -- equilibrium shifts toward the side of the reaction with fewer moles of gas; decreasing pressure -- equilibrium shifts toward the side of the reaction with more moles of gas; Kc does not change. Enthalpy (H) is the measure of energy that can be released as heat; ΔH -- reaction enthalpy (the enthalpy change). When ΔH \< 0, the reaction is exothermic (heat is released; gives off heat to its surroundings); when ΔH \> 0, the reaction is endothermic (heat is consumed; absorbs energy from its surroundings). Catalyst -- is a substance that increase the speed of reaction without being affected chemically or being used up itself during the reaction of interest. A catalyst reduces an activation energy (Ea) that is why it increases the rate of reaction. Heterogeneous catalyst -- is a substance that acts in a different phase than the reactants. Homogeneous catalyst -- is a substance that acts in the same phase as the reactants. Biocatalyst -- mainly enzymes -- protein based catalysts in metabolism and catabolism. 7 Autocatalytic reaction -- the product is itself the catalyst for this reaction. Inhibitor -- is a substance that increases an activation energy that is why it slows down a reaction (decreases the speed of reaction). Example 1 -- The reaction: 2NO2 2NO + O2 is at equilibrium at some temperature, T and the following equilibrium concentrations are measured: \[NO2\] = 0.05 mol/dm3 ; \[NO\] = 0.22 mol/dm3 ; \[O2\] = 0.14 mol/dm3. Calculate the equilibrium constant Kc. Kc = \[NO\]2 · \[O2\] / \[NO2\] 2 = (0.22) 2 · 0.14 / (0.05) 2 = 2.71 Example 2 -- Write the equilibrium constant (Kc) expression for the following reaction: H2 + I2 2HI Kc = \[HI\]2 / \[H2\] · \[I2\] Example 3 -- How will the rate of reaction (Rate) change according to the kinetic equation: Rate = k · \[A\]2 · \[B\] if the molar concentrations of A and B increase 3 times? Ratex = k · 3A2 · 3B = k · 9A2 · 3B = k27A2 · B Ratex / Rate = k 27A2 · B / k · A 2 · B = 27 -- the rate of reaction will increase 27 times.