Unit 1 - Kinetics to Equilibrium Flashcards
(52 cards)
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Q
Potential Energy
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- The energy of position
- The energy particles have because of their positions relative to one another
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Q
Kinetic energy
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- The energy of motion
- The energy particles have because they are moving
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Q
Kinetic Molecular Theory of Matter
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1) The tiny particles in all forms of matter are in continuous and random motion
2) Solid particles vibrate in fixed positions due to strong intermolecular (or interparticle) attractions
3) Liquid and gas particles have more freedom of movement due to weaker intermolecular attractions
4) When the temperature of a substance is increased, there is an increase in the motion of the particles, an increase in the frequency and intensity of collisions between particles, and a decrease in the density of the substance
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Q
Two pieces of evidence that support the Kinetic Molecular Theory
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1) Pressure
2) Diffusion
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Q
Pressure
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- Moving bodies exert forces when they collide with other bodies
- Many simultaneous collisions of billions of gas particles on an object are what we call gas pressure
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Q
Diffusion
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- Particles tend to move from areas of higher concentration to areas of lower concentration due to collision
- Ex: perfume sprayed in a room
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Q
Collision Theory
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- All reactions are the result of collisions between particles
- For example, in order to attain butane gas, butane molecules must collide with oxygen molecules
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Q
Reaction Rate
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- The number of atoms, ions or molecules that react in a given time to form products
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Q
Collision Theory and Reaction Rate
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- Most chemical reactions involve the transfer of atoms from one molecule to another, thus contact between the reactants is very important
- For any reaction involving 2 or more reactants, the reacting particles must collide
- The more often particles collide, the faster the reaction should go
- Chemical reactions also involve the making and breaking of chemical bonds, and therefore involve energy
- The colliding particles must have enough energy for this process or no
reaction will occur. The required energy is called activation energy. (Ea) - Activation energy is like a barrier that the reactants must cross to be converted to products
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Q
Collision Theory and Concentration
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- The rate of reaction increases if there are more collisions per unit of time
- The more reactant particles (ie. the greater the concentration), the faster the reaction rate
- For gases, increasing the pressure has the same effect as increasing the concentration
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Q
Collision Theory and Surface Area
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- Collisions can occur only at the surface of a solid
- Increasing the surface area causes more surface to be exposed, increasing the rate of reaction
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Collision Theory and Nature of Reactants
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- Reactions that involve ionic compounds and simple ions are generally faster than those involving molecular compounds
- Reactions that involve breaking weaker bonds are generally faster than those involving breaking stronger bonds (e.g. Ethane VS Ethene, Ethane is faster … C=C bond in Ethene is stronger)
- Reactions that involve breaking fewer bonds are generally faster than those involving breaking a greater number of bonds
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Q
Collision Theory and Temperature
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- Increased temperature increases the frequency of collisions
- Increased temperature increases the intensity of collisions
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Activation Energy
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The minimum collision energy that is required for a successful reaction
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Q
Collision Theory and Orientation
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- The particles must collide with the correct orientation in order for reactions to occur
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Q
In order for a successful reaction to occur …
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Particles must collide with sufficient every and correct orientation
- Otherwise the particles collide without reacting
- Not all collisions result in chemical change. Ex: butane + oxygen
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Q
Chemical Reaction
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The rearrangement of particles that occur by colliding with sufficient energy and correct orientation
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Q
Evidence that reactions have taken place:
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- Color Change: the final product(s) have a different color than the colors of the starting material(s)
- Odor Change: the final products may have a different odor than the starting materials
- State Change: the final materials may include a substance in a different state then the originals, usually a gas or a solid (precipitate)
Volume Change: the volume of the products may be greater or lesser than that of the reactants
pH Change: the product(s) may have different pH’s from the reactants, as in an acid/base neutralization reaction
MASS DOES NOT CHANGE:
In chemical reactions the total mass of matter present before the change is
ALWAYS the same as the total mass present after the change, no matter how different the new substance may appear (Law of Conservation of Mass)
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Q
Activated Complex
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During a reaction, particles which are neither reactants nor products form
momentarily. An activated complex is the arrangement of atoms at the peak of the activation energy barrier.
- This group of atoms is on its way to becoming an ionic or molecular product.
- Very unstable
- May go either way - to reactants or to product
- Lifetime of only 10(exponent -13) second
- Also known as the transition state
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Q
Change in heat
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- the enthalpy of reaction
- the difference between the potential energy of the products and the potential energy of the reactants
- breaking a bond is a process that requires energy; creating a bond releases energy
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Q
Reactants and Products
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- Original and final compounds in the reaction
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Potential Energy Diagram
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- A diagram which charts the potential energy of a reaction against the progress of that reaction
- The flat region labeled reactants shows the potential energy content of
the reactants relative to the products. - The rising part of the graph represents the increase in potential energy that occurs when reactants collide. The kinetic energy of reactants is converted to potential energy as they collide because the force of the collision causes the chemical bonds to stretch and distort. The minimum energy the colliding particles must have in order to react is the
activation energy. (EA) activation energy is a sort of “barrier” that the reactants must cross in order to be converted into products. - The top of the graph is where the bonds of the colliding particles are stretched to the breaking point.
- Here, particles which are neither reactants nor products form
momentarily. These particles form an activated complex - an unstable arrangement of atoms at the peak of the activation energy barrier. The life of an activated complex is only about 10(exponent -13) seconds and during that time the complex either forms new bonds to become products, or reforms old bonds and returns to being reactants. At this stage the activated complex is in the transition state. - The falling part of the curve represents the energy released when bonds form between particles that make up the products.
- The second flat region represents the potential energy of the products.
The difference between the potential energy of the reactants and products is
called heat of reaction (ªH).[ªE] The heat of reaction is the heat absorbed or released as reactants become products
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Q
Exothermic Reactions
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- reactants have more potential energy than the products.
- Reaction releases energy to surroundings
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Endothermic Reactions
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- Reactants have less potential energy than the products.
- Reaction absorbs energy
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The effect of a catalyst on the rate of reaction
A catalyst works by lowering the activation energy of a reaction so that a larger fraction of the reactants have sufficient energy to react.
- It does this by providing an alternative mechanism for the reaction
- the catalyzed reaction consists of a two (or more) step process
- Cancels out when written in overall reaction
- A catalyst is consumed than produced
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Homogeneous Catalyst
- Same phase as reactants
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Heterogeneous Catalyst
- Different phase than reactants
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Nature of Reactants & Collision Theory: Detailed explanation
The state of matter, type of bonding, valence orbital configuration, and many
other features of a substance influence the speed of the chemical change.
The chemical and physical properties of substances are collectively called the nature of reactants.
Nature of Reactants
Examples:
a. Solid lead nitrate and potassium iodide react very slowly. Aqueous solutions of these react instantly to produce a bright yellow precipitate.
* - Ions in solution react rapidly because the ions of opposite charge are attracted to each other and can bond upon colliding
* - Changing the state of matter can influence the rate of reaction
b. A strong acid (one which ionizes completely) such as HCl metal reacts faster than a weak acid (one that doesn’t ionize completely) like acetic
acid. Strong acids have higher hydrogen ion concentrations than weak acids of the same molar concentration
* The strength of an acid (the degree to which it ionizes) can influence the rate of reaction
c. Cesium metal reacts so fast and so vigorously with water that the hydrogen gas produced explodes violently. However, lithium, which is also a group 1A metal, reacts relatively slowly
* low electronegativity metals tend to react faster
d. Chloride ions react faster with silver ions than chlorine gas reacts with solid
silver. In the reaction involving chlorine gas, the two covalently bonded chlorine atoms have to be separated before they can react with silver atoms, which in turn are held together by metallic bonds
*Type and strength of bonds influence reaction rates
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Temperature & Collision Theory: Detailed explanation
- Increasing the temperature of a chemical system increases the frequency of collisions between the reactant particles.
- more collisions means that more particles will hit with greater frequency,
intensity, and correct orientation and will therefore enable products to
form
- more colliding particles become energetic enough to slip over the
activation energy barrier to become products
Example: paper does not appear to be burning at room temperature (even
though it is slowly oxidizing). Touching it with a match allows the paper and air molecules to collide with greater frequency and high energy. Some of the energy released when the products are formed enables more paper/air to get over the activation barrier.
- removing energy from the system by decreasing the temperature will
result in a decrease in the reaction rate because fewer particles will collide with the minimum energy and correct orientation
* even a small increase in temperature (like 10 degree Celsius) can double a reaction rate due to more particles having enough kinetic energy to overcome the activation energy barrier
As a general rule:
Increase in temperature, increase in intensity and frequency of collisions, increase reaction rate
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Concentration & Collision Theory: Detailed explanation
- Chemical reactants are more likely to collide and react if there are more
of them occupying a volume of space at the same time
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- an increase in the concentration of a solution will increase the frequency
of collisions (# / sec) and thereby increase the chances for a chemical change to occur. This will result in an increase in the reaction rate reducing the concentration of a solution will decrease the reaction rate
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As a general rule:
Increase concentration, increase frequency of collisions, increase rate of reaction
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Surface area (particle size) & Collision Theory: Detailed explanation
- Total surface area of a reactant has an important effect on the reaction rate
- the smaller the particle size, the larger the surface area for a given mass of particles
- an increase in surface area increases the collision frequency and
therefore the reaction rate
- the best way to increase the surface area of solid particles is often to
dissolve them in a liquid. This separates the particles
- homogeneous mixtures of particles often react more quickly than
heterogeneous mixtures
- it’s often impossible to obtain homogeneous mixtures; heterogeneous mixtures of reactants result in heterogeneous reactions
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Catalyst & Collision Theory: Detailed explanation
- a catalyst is a substance that increases the rate of reaction without being used up itself in that reaction
-for any chemical reaction, there is a sequence of events that must occur
if reactants are to become products
-the series of steps involved in rearranging reactant particles to form
products is called the reaction mechanism
- a catalyst speeds up a reaction by providing a reaction mechanism that
requires less activation energy
- catalysts can cause the formation of activated complexes that have lower
potential energy than those of uncatalyzed reactions
- with a lower activation energy barrier, more collisions between the reactants will have the required energy to form products and, as a result, the rate of reaction will increase
- catalysts do not appear as reactants or products in the equation for the
reaction. They appear above the line
- a substance that interferes with the action of a catalyst is called an
inhibitor
General rule:
Catalysts increase reaction rates by providing alternate reaction mechanisms that have lower activation energy.
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Reaction Mechanism
- A reaction mechanism is a series of steps that make up an overall reaction.
- Each step, called an elementary reaction, involves a single event (e.g., a collision between atoms, molecules or ions)
In elementary reactions:
- reactants are converted to products in a single step
- there is only one intervening activated complex
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Reaction Intermediate
Molecules (or atoms or ions) that are formed in elementary reactions and
consumed in a subsequent elementary reaction are called reaction
intermediates. Although they are not reactants or products in the overall
reaction, they are essential for the reaction to take place. Produced than consumed.
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Define Molecularity, Bimolecular, Unimolecular, Termolecular
Molecularity:
- the number of reactant particles involved in an elementary reaction
- can be molecules, atoms or ions
Bimolecular:
- when two particles collide and react, the elementary reaction is said to be bimolecular
Unimolecular:
- when one molecule or ion reacts in an elementary reaction
Termolecular:
- when three particles collide in an elementary reaction
- very rare
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Rate Determining Step
Elementary reactions in mechanisms all have different rates. Usually one elementary reaction - the rate determining step - is much slower (highest activation energy). It determines the overall rate of reaction.
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Dynamic Equilibrium
Dynamic equilibrium occurs when opposing changes are occurring at the same time and at the same rate.
At equilibrium, the rate of the forward reaction = the rate of the reverse reaction.
This competition requires that the system be closed so that reactants and products can't escape from the reaction container.
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Conditions for Equilibrium
1. Equilibrium is achieved in a reversible process when the rates of opposing changes are equal.
2. The observable properties of the system at equilibrium are constant. At equilibrium there is no overall change in the properties that depend on the total quantity of matter in the system (e.g., color, pressure, concentration and pH will not change). There are changes at the molecular level, but you won’t see them at an observable level.
3. Equilibrium can only be reached in a closed system. Nothing gets in or out, including energy. What this really means, in energy terms, is that the equilibrium can occur only in a system at constant temperature.
4. Equilibrium can be approached from either direction.
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Homogeneous Equilibrium
A reaction with reactants and products in the same phase (e.g., all gaseous or
aqueous).
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Heterogeneous Equilibrium
A reaction with reactants and products in different phases (e.g., aqueous ions combine to form semi-soluble product).
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How is dynamic equilibrium established?
Establishment of a dynamic equilibrium involves 3 steps:
1. One reaction is initiated, or perturbed. The forward rate is initially very high
2. The reverse rate responds (products collide to form reactants)
3. Equilibrium is established. The forward and reverse reaction rates become equal
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Le Chateliers Principle
When a chemical system at equilibrium is disturbed by a change in some property of that system, the system will react to partially counteract the change until a new equilibrium is established.
It provides a method of predicting the response of a chemical system to
an imposed change.
The application of Le Chatelier’s Principle involves a 3 stage process:
- an initial equilibrium state
- a shifting non-equilibrium
- a new equilibrium state
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3 Types of easily applied change to a system
1. Changes in concentration of species in the system. Many important equilibrium systems involve ions in aqueous solution. The common ion effect holds here. The addition of an ion to a solution in which the ion is already present causes a shift of equilibrium away from the added ion, as predicted by Le Chatelier. That is the addition of more reactant or the removal of a product will increase the overall yield of product(s) by shifting the equilibrium to the right
2. Change in the temperature of a system. The position of the equilibrium changes with temperature because the rates of the forward and reverse reactions are affected. Energy in a chemical equilibrium is treated as though it were a reactant or a product.
3. Change in the pressure (and volume) of a system. Increasing the pressure on a system at equilibrium (by decreasing volume) should result in a shift to establish a new equilibrium. A change in pressure, however, will only affect an equilibrium with an unequal number of gaseous reactants and products. We must therefore consider: Boyle’s Law and Avagadro's hypothesis
If you increase the pressure on a closed system at equilibrium (e.g., by decreasing the volume of the vessel) the system should respond by decreasing the number of moles of gas in the system this results in an equilibrium shift to the side of the equation with the least number of moles of gases (If there are fewer gas particles, they will require less space)
If you decrease the pressure on a closed system (e.g., by increasing the volume of there will be an equilibrium shift toward whichever side contains the greatest number of moles of gases
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Do changes in surface area and adding catalyst shift equilibrium?
Effect of Adding a catalyst: According to Le Chateliers adding a catalyst at equilibrium should result in a shift to establish a new equilibrium however, catalysts slow the activation energies of both the forward and the reverse reaction equally because of this, the equilibrium is not disturbed, the only effect that a catalyst has on the equilibrium is that it reduces the time it takes to reach equilibrium
General Rule:
Addition of a catalyst does not shift the position of an equilibrium
Change in the Surface Area: has a similar effect to that of adding a catalyst. It reduces the time required for a system to reach an equilibrium
but no effect on the actual equilibrium concentrations
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Law of Chemical Equilibrium
At equilibrium there is a constant ratio between the concentrations of the products and reactants in any change.
Chemists use a mathematical relationship to provide a constant value for a chemical system over a range of concentrations. This constant value is a mathematical expression called the equilibrium constant, K, for the reaction system.
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Equilibrium Constant, K
The equilibrium constant, K, is a mathematical expression that represents the ratio of product concentration to reactant concentration with each concentration raised to the power of its coefficient.
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K equations
- Products over reactants
- Coefficient becomes exponent
- Only concentrations of gases and solutions (i.e., ions ... aq) are included in the expression K
- Each chemical system has its own constant. Thus, a specific value of K is only accurate when calculated at a specific temperature
- The value of K will change with changes in temperature
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Temperature & Equilibrium
For a given system at equilibrium, the value of the equilibrium constant depends only on temperature.
- changing the temperature changes the rate of forward and reverse reactions by differing amounts
- this is because the forward and reverse reactions have different activation energies
- a reacting mixture at one temperature has an equilibrium constant whose value changes if the mixture is allowed to reach equilibrium at a different temperature
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Removing/adding reactants & equilibrium
This contrasts with adding or removing of a reactant or product. The rate of the reaction will at first change, but the equilibrium will be re-established with the same equilibrium constant. (This is because if you increase the reactants, you decrease the products proportionally.)
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Interpreting K values
K > 1 - products are favored. The equilibrium lies far to the right. Reactions where K is greater than 10(exponent)10 are usually regarded as going to completion
K < 1 - reactants are favored. The equilibrium lies far to the left. Reactions in which K is smaller than 10(exponent -10) are usually regarded as not taking place at all
K = 1 - neither reactants nor products are favored. There are approximately
equal amounts of reactants and products at equilibrium
When K values closely approach 1 you should interpret them in the same way that K = 1 is interpreted. In other words, neither the reactants nor the products are favored.
If K = 0, then the concentration of products must be equal to 0 and therefore no reaction has occurred.
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K values and Exothermic & endothermic reactions
1. If a forward reaction is endothermic, an increase in temperature will result in a larger value of K
2. If the forward reaction is exothermic, an increase in temperature leads to a
decrease in K
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The meaning of a small equilibrium constant
When Kc is small compared with the initial concentration, the value of the initial concentration minus x is approximately equal to the initial concentration. Thus you can ignore x.
- if the initial concentration of the substance is 0, any equilibrium concentration of the substance, no matter how small, is significant
- in general, values of Kc are not measured with accuracy better than 5%. Therefore making the approximation is justified if the calculation error you introduce is less than 5%
-* to help decide whether or not the approximation is justified: Divide the
smallest initial concentration by the value of Kc
- If the answer is greater than 500, the approximation is justified.* If the answer is between 100 and 500, it may be justified. If the answer is less than 100 it is not justified and the equilibrium expression must be solved in full.
