Enthalpy: Chemistry
How can one determine the temperature at which a process becomes spontaneous if the enthalpy and entropy values are both either positive or negative?
(delta H)/(delta S) = Temperature
What are some examples of state functions?
-Composition -Energy -Level -Temperature -Pressure -Volume
What are some examples of reactions that are not considered a state function?
-Distance -Elapsed time -Cost
What two conditions must be met in order for a reaction to occur when two gas-phase molecules collide?
-Enough sufficient energy -Proper orientation
What are the two types of special catalysts?
-Enzyme -Inhibitor
What are the two types of catalysts?
-Homogenous catalyst -Heterogenous catalyst
How does one determine the Rate Law and "k" from the initial reactant concentrations?
-Measure the initial rates of a reaction at different initial concentrations of A and B -To obtain x, the order in A, compare Rate2 to Rate1 (if [B] is constant)
What factors affect how reactive a reactant is?
-Nature of the reactant -Concentration -Surface area -Temperature -Presence of a catalyst (type of catalyst)
In what cases is a spontaneous reaction likely to occur?
-Negative enthalpy value (favorable) -Positive entropy value --> increase in disorder (favorable)
In what cases is a spontaneous reaction never going to occur?
-Positive enthalpy value (unfavorable) -Negative entropy value --> decrease in disorder (unfavorable)
What happens to the enthalpy change during endothermic reactions?
-Reactants gains potential energy (heat is taken in) -Reaction converts absorbed heat into potential energy -delta H is positive
What happens to the enthalpy change during exothermic reactions?
-Reactants lose potential energy --> the lost potential energy is converted into heat which is released -delta H is negative --> potential energy decreases when reactants covert into products
How does the surface area of a reactant affect the reactivity of a reactant?
-Smaller particles have more surface area for a given amount of a reactant -As the surface area increases, the area of interaction increases, the number of collisions increases, and the reaction rate increases
In what cases does it depend on the temperature at which a spontaneous action will take place?
1.) -Negative enthalpy value (unfavorable) -Negative entropy value (favorable) 2.)-Positive enthalpy value (unfavorable) -Positive entropy value (favorable)
How can one determine the order (x and y) in the Rate Law by comparing what happens to the Rate when one reactant changes?
1st order, the rate change is the same as the concentration change 2nd order, the rate change is the square of the concentration 0 order, the rate does not change in concentration
Enzyme
A biological catalyst that decreases the activation energy and improves orientation
Enthalpy Change
A change in potential energy from reactants to products -equivalent to the amount of heat released or absorbed by a system -represented by delta H
How does the temperature of a reactant affect the reactivity of a reactant?
A higher temperature means there is an increased kinetic energy value. -more collisions with kinetic energy sufficient to overcome the activation energy -higher percentage of collisions are effective
Hess's Law
A law that helps find the enthalpy change of a reaction that can not be performed in a lab. "If a reaction can be represented by the sum of a series of reactions then its enthalpy of the reaction is the sum of the enthalpy of the reaction's values of the component reactions"
Spontaneous process
A process that occurs under a given set of conditions without outside intervention.
State function
A reaction that only depends on initial and final conditions and not the path
Inhibitor
A type of catalyst that reduces the reaction rate by increasing the activation energy or reducing orientation
Is heat being released or absorbed during an endothermic reaction?
Absorbed
How does adding heat increase entropy?
Adding heat increases kinetic, vibrational, and rotational energy of molecules
The Rate Law
An expression of the relationship of the rate of a reaction to the concentration of the reactants
Why does the temperature remain consistent when any substance is changing from a liquid to a solid?
As bonds form, energy is being released. The temperature should be cooling, but the release of energy keeps the temperature constant. -Exothermic process
Why is the transition state temporarily unstable?
At this state the reaction is reversible. Therefore, it can return to reform reactants or continue to form products. -Partial bonds
Why are endothermic reactions considered the "not favored direction?"
Because the products of endothermic reactions are of a higher energy level than the reactants
How do catalysts affect the reactivity of a reactant?
Catalysts lowers the activation energy needed to produce a reaction. Therefore, the rate of the reaction increases (higher percentage of collisions with the kinetic energy being greater than the activation energy at the same temperature)
Is the potential energy of the molecules increasing or decreasing during an exothermic reaction?
Decreasing
What is the symbol that represents spontaneity?
Delta G
What is the formula associated with Hess' Law?
Delta H of the reaction = (the sum of the (coefficient * the standard heat of formation of the product)) - (the sum of the (coefficient * the standard heat formation of the reactant))
What is the symbol for entropy?
Delta S
Is an exothermic reaction uphill or downhill?
Downhill
What is the favored change in enthalpy and why ?
Exothermic because products are more stable (strong bonds) and the temperature of surroundings goes up
What type of reactions usually require activation energy?
Exothermic reactions
Collision Theory
Gas-phase chemical reactions occur when molecules collide with sufficient kinetic energy and proper orientation.
Is the potential energy of the molecules in an endothermic reaction increasing or decreasing?
Increasing
What are reaction rates measured in?
Measured in change in concentration with change in time
Activation Energy
Minimum energy reacting particles must have in order for a reaction to occur
What is the favored change in entropy?
More disorder
When given a series of reactions, how might one use Hess' Law to determine the final reaction?
Multiply a given react ion by some factor or to reverse it to obtain the final reaction
How does multiplying and or reversing a reaction affect the enthalpy of that said reaction?
Multiplying a reaction: -multiplies the enthalpy for the reaction by the same factor Reversing a reaction: -multiplies the enthalpy for the reaction by -1
Is the delta H value positive or negative during an exothermic reaction?
Negative
What does the Rate Law depend on?
Number of Each Species in the RDS -matches the stoichiometry of the RDS Ex: A+B -> C Rate Law = k[A][B] Ex part 2: A+A -> C Rate Law = k[A]^2
Activated Complex or Transition State
On the reaction path between the initial and final arrangements of atoms or molecules, there exists an intermediate configuration at which the potential energy has a maximum value. The configuration corresponding to this maximum is known as the activated complex, and its state is referred to as the transition state.
Is the delta H value of endothermic reactions positive or negative?
Positive
What is the rate law for "A -> products?"
Rate = k[A]
What is the rate law for "A+A+B -> products?"
Rate = k[A]^2[B]
RDS
Rate Determining Step: -slowest elementary reaction -requires the most energy -determines the rate law function
Reaction Rate
Rate at which products are produced and reactants are consumed
What is the mathematical equation that demonstrates The Rate Law?
Rate=k[A]^x[B]^y K= the rate constant X and Y= the reaction order for reactants A and B
Intermediates
Reactants that are produced in one step and consumed in a subsequent step: -does not appear in overall reaction
As a reaction proceeds, what happens to the values of the reactants versus products?
Reactants' values decreases as the products' values increases
Endothermic reactions
Reactions including the absorption of heat from the surroundings -surroundings get colder
Exothermic reactions
Reactions that include the releasing of heat -the surroundings get hotter
Is heat being released or absorbed during an exothermic reaction?
Released
Reaction Mechanism
Sequence of elementary steps that represent progress toward and add up to an overall reaction
Why are reaction mechanisms used?
Some reactions with three, four, or more reactants will unlikely react. Thus, a series of elementary reactions that lead up to the overall reaction may be needed.
How does the nature of a reactant affect the reactivity of a reactant?
Some substances are naturally more reactive than others. Thus, these substances react faster -reactivity increases
Energy
The ability to do work or release heat
Gibb's Free Energy
The energy available from a chemical reaction or a process to do work -Release of energy in a spontaneous process increases the entropy of the universe
Which term in the Free Energy equation is more significant at lower temperatures?
The enthalpy value is more significant at lower temperatures since it is independent of the temperature value.
Homogenous catalyst
The form of catalysis where the catalyst is present in the same phase as the reactants
Heterogenous catalyst
The form of catalysis where the phase of the catalyst differs from that of the reactants -usually a solid catalyst in aqueous or gas
Standard Heat of Formation
The heat released or absorbed by the reaction that produces one mole of a substance from its elements, with all components at their standard states
What happens to any substance over the range when the temperature reaches a plateau?
The kinetic energy of any substance is zero: -decrease in potential energy -formation of intermolecular forces (attractions)
Why does the temperature remain constant while melting any substance and does not continue to rise as you heat it?
The phase change from solid to liquid is endothermic. Thus, the temperature remains constant because the energy that would increase the temperature goes into breaking the bonds (intermolecular forces)
In cooling curves of molecules, what is the plateau?
The plateau is where the substance changes from a liquid to a solid (freezing point).
Why does the activated complex have potential energy?
The potential energy of the reactants is being converted into kinetic energy.
How does the concentration of a reactant affect the reactivity of a reactant?
The presence of more particles leads to more collisions: -more total particles results in more total collisions
What does "k" represent in the Rate Law equation?
The rate constant: -proportionally between the reaction rate and the concentration of the reactants -only depends on the reactions pathway and the temperature
What does "x" and "y" represent in the Rate Law equation?
The reaction order for reactants A and B: -must be determined experimentally -shows relationship between change in concentration of a reactants and the change in the reaction rate -usually integers (either 0,1,2)
How is it possible for a very spontaneous reaction to seemingly not occur?
The spontaneity of a reaction is unrelated to its kinetics. Thus, a spontaneous reaction can proceed very slowly and be unobservant.
Does an endothermic reaction get hot or cold?
The surroundings become cooler but the product it self becomes warmer (absorbing heat).
Does an exothermic reaction get hot or cold?
The surroundings of an exothermic reaction become hot. However, the product itself becomes cooler.
Is an endothermic reaction uphill or downhill?
Uphill
In what situations does entropy increase?
When substances go from: -Solid to liquid -Liquid to vapor -Solvent/solute to Solution -Adding heat -Increasing moles of a gas
In what instances is a molecule 0th order in a reaction mechanism?
When the molecule/substance does not take part until after the RDS
What is the calculation to determine the change in Free energy (delta G)?
deltaG=(deltaH)-(temperature)(deltaS) -Temperature is calculated in Kelvin
What is the mathematical equation associated with reaction rates?
rate= delta[products]/(delta T) = delta[reactants]/(delta T) Where [] = concentration in moles/liter
