Reaction Rates and Catalysis

• Catalysts accelerate reactions, such as platinum catalyzing hydrogen peroxide decomposition, while some catalysts slow reactions down.
• Reactions are categorized based on phase.

Homogeneous Reactions

• Occur entirely within a single phase.

Heterogeneous Reactions

• Occur in multiple phases, like on the surface of a solid catalyst.
• Reaction rate reflects the speed at which a reaction proceeds.
• Molecularity represents number of molecules or reactants involved in elementary reaction

Simple Reaction Dynamics

• Reactant A decreases, while product B increases over time (A → B).
• Reaction rate quantifies concentration change of reactants or products per unit of time.
• Rate may equal the disappearance rate of A or the appearance rate of B.
• Square brackets denote concentration.
• Negative signs indicate decreasing reactant concentration, while positive signs indicate increasing product concentration.

Rate Laws

• At a fixed temperature, the reaction rate depends on reactant concentrations.
• Relationship is determined by measuring the reaction rate with varying initial reactant concentrations.
• Reaction rate is directly proportional to concentration.
• Each concentration term is raised to a power.
• Rate is proportional to (A)ⁿ.
• Reaction rate with respect to a reactant is determined by maintaining other reactant concentrations constant.
• Rate expression relates reaction rate to concentration, which is the rate law.
• The power, or exponent, of a concentration term in the rate law is usually a small whole number, integer, or fraction and the proportionality constant of the reaction.

Reaction Orders

• Reaction order is the sum of the powers of concentrations in the rate law.
• Consider a reaction with rate law: Rate = k[A]ᵐ[B]ⁿ, the order of reaction is m+n.
• Reaction order can also be defined with respect to a single reactant.
• Overall reaction order (m+n) can be 1, 2, 3, or fractional.

Rate Law Examples

• Given rate laws, the reaction can be classified.
• If m+n = 1, First order,.
• If m+n = 2, Second order reactions

Zero Order Reactions

• Do not depend on reactant concentration.
• Reactant concentration isn't included in the rate law example: NO₂ + CO → NO + CO₂ at 200°C.
• The rate does not depend on CO concentration, so is excluded from the rate law, its power understood to be zero.

Molecularity

• Defined by number of reactant molecules involved in an elementary reaction, for example: molecularity = 1+1 = 2
• Differences Between Reaction Order and Molecularity, consider:
  • Reaction order is experimentally determined, can have fractional.
  • It can change with Invariance of reaction Conditions such as Temp, pressure, Conc.
  • Molecularity is theoretical is a whole number with at least No Zero Value

Reaction Rate Calculations

• Reaction rate is the change in concentration of reactant or product over time, which is written as rate = -Δ[reactant]/Δt = -Δ[product]/Δt.
• The concentration is monitored to define functions of the time of reactions.

Reaction Using Molecular Bromine and Formic Acid

• Molecular bromine reacts with formic acid, and the average rate can be written with this information as follows
• Average rate = –Δ[Br₂] / Δt = –([Br₂]final – [Br₂]initial) / (tfinal – tinitial).
• Molecular Bromine is reddish brown in Colour, all the other Species in the reaction are Colourless.
• Bromine's decreasing concentration is monitored using a spectrometer, with the loss of and the colour fading.
• The concentration is doubled at t = 50s also double the rate at t = 250s

More on Reaction Rates:

• Therefore, if the rate is Rate=K[Br₂] then conc is directly proportional
• Where the term, K is known as rate, Constants, be cause the rate of reaction has then Chit of 1/s and [Br₂]
• [Br2] in Capital M Conder), it is Important to know that K is not affected by Commina conc,
• the rate is greater ata and Smaller ata smaller highter conc cong of bromine
• provided the temp. dues not Change, and that is the of the the ration of rate divided by bromine cone

Reaction Rate and Stoichiometry

• Reaction rates can decrease as the concentration decreases and increase in Concentration as product concentration increase.
• The change is best expressed in terms like d/dt, as the rate is given as R = -1/2 d[A]/dt = d[B]/t
• For more complex vead reactions given as a A + b B > c C + d D, the rate is given as R = -1/a Δ[A]/Δt = -1/b Δ[A]/Δt = 1/c Δ[C]/Δt

Explore reaction rates, catalysts, and reaction dynamics. Understand how catalysts speed up or slow down reactions and the differences between homogeneous and heterogeneous reactions. Learn about molecularity, rate laws, and how to quantify reaction speed using concentrations.