The branch of chemistry that studies how fast reactions occur and the steps by which they take place is chemical kinetics.
Missing Word
kinetics
Chemical kinetics deals with reaction rates, the factors that affect them, and reaction mechanisms. It complements thermodynamics, which tells whether a reaction can occur but not how fast.
Category: Reaction Rates
Question 2
The instantaneous rate at a given moment equals the slope of the line tangent to the concentration-versus-time curve at that moment.
Missing Word
tangent
A tangent line touches the curve at a single point, and its slope gives the rate at exactly that time. The slope of a line between two separate points gives an average rate instead.
Category: Reaction Rates
Question 3
When a reaction's rate stays the same no matter how much the concentration of reactant A changes, the exponent on [A] in the rate law is zero.
Missing Word
zero
A reaction that is zero order in A has [A] raised to the power zero, which equals 1, so changing [A] does not affect the rate. A common example is a reaction limited by the available surface of a catalyst.
Category: Rate Laws
Question 4
After surface reaction on a metal such as nickel or platinum, the products are released from the metal in a final step called desorption.
Missing Word
desorption
Desorption is the release of adsorbed molecules from a surface. Once the products desorb, the surface sites are free to adsorb new reactant molecules, so the solid catalyst keeps working.
Category: Catalysis
Question 5
The model stating that molecules react only when they meet with enough energy and in a suitable alignment is collision theory.
Missing Word
collision
Collision theory explains rates in terms of how often particles collide and what fraction of those collisions have enough energy and the right geometry to form products.
Category: Collision Theory
Question 6
Besides colliding with enough energy, reacting molecules must have the proper orientation, so that the atoms that will bond come into contact.
Missing Word
orientation
Orientation describes how the molecules are positioned relative to each other when they collide. For example, CO must strike O2 with its carbon end, not its oxygen end, to form CO2.
Category: Collision Theory
Question 7
The equation k = A e^(-Ea/RT), which relates a rate constant to the activation energy and the temperature, is the Arrhenius equation.
Missing Word
Arrhenius
The Arrhenius equation shows that k grows exponentially with temperature and shrinks as the activation energy rises. Its linear form lets the activation energy be found from a plot of ln k against 1/T.
Category: Collision Theory
Question 8
A species that is produced in one elementary step and consumed in a later one, so that it does not appear in the overall balanced equation, is called an intermediate.
Missing Word
intermediate
An intermediate forms and is then used up during a reaction, such as the oxygen atom in the two-step decomposition of ozone. Unlike a catalyst, it is not present at the start.
Category: Reaction Mechanisms
Question 9
The number of reactant particles that take part in an elementary reaction is that reaction's molecularity.
Missing Word
molecularity
Molecularity applies only to elementary steps and can be one, two or rarely three particles. For an elementary step, the molecularity equals the overall order of that step's rate law.
Category: Reaction Mechanisms
Question 10
An elementary step in which a single molecule breaks apart or rearranges, such as O3 -> O2 + O, is described as unimolecular.
Missing Word
unimolecular
A unimolecular step has one reactant particle, so its rate law is first order: rate = k[A]. The decomposition of cyclobutane into ethylene is another example.
Category: Reaction Mechanisms
Question 11
A substance that speeds up a reaction by providing a pathway with a lower energy barrier, and that is regenerated by the end of the reaction, is a catalyst.
Missing Word
catalyst
A catalyst may react in one step and be re-formed in a later step, so it is not consumed overall. It speeds the reaction without changing its enthalpy change.
Category: Catalysis
Question 12
In heterogeneous catalysis, the first step is adsorption, in which reactant molecules attach to the surface of the solid.
Missing Word
adsorption
Adsorption is the binding of molecules to a surface, different from absorption, in which a substance is taken into the interior of another. After adsorption, the reactants are activated, react, and the products leave the surface.
Category: Catalysis
Question 13
Adding hydrogen across the double bond of ethylene on a nickel surface to form ethane is called hydrogenation.
Missing Word
hydrogenation
Hydrogenation is the addition of hydrogen to a molecule with a multiple bond. With ethylene it is a classic example of a reaction on a solid metal surface.
Category: Catalysis
Question 14
A protein that speeds up a specific biological reaction by binding a reactant at its active site is called an enzyme.
Missing Word
enzyme
Enzymes are biological catalysts, usually proteins, that control the rates of reactions in living things. Each one acts on a particular reactant that fits its active site.
Category: Catalysis
Question 15
In biochemistry, the specific reactant molecule that fits into a protein's active site is called the substrate.
Missing Word
substrate
The substrate binds at the active site to form an enzyme-substrate complex, which then reacts to give products. The fit between active site and substrate explains why enzymes are so selective.
Category: Catalysis