Chemistry

• Enthalpy


  The amount of heat absorbed or released by a chemical reaction at constant atmospheric pressure. It is a form of chemical energy.
• Thermochemical equation


  Useful in determining the enthalpies of reaction

  
• Enthalpy of reaction


  ΔH = Hproducts - Hreactants

  
• Enthalpy


  • It is an extensive property, its magnitude is proportional to the amount of reactants and products in the reaction
  • Reversing a chemical reaction results in the same magnitude of enthalpy but of the opposite sign
  • The enthalpy change for a reaction depends upon the state of the reactants and products

  
• Standard molar enthalpy of formation

  The change in enthalpy when a compound is formed from its elements under standard conditions (1 atm, 25°C)

  
• Hess's Law


  The enthalpy of a sum of a series of reactions is equal to the sum of the enthalpies of those reactions

  
• Hess's Law allows the enthalpy of a reaction to be determined without actually performing the reaction
• Chemical kinetics


  The study of the rate of chemical reactions, including the mechanism by which a reaction occurs and the different factors that affect it
• Melting of ice is not a chemical reaction, it is a physical change
• Reaction rate


  The change in the concentration of reactants or products per change in time
• Chemical reaction


  The interaction of two or more substances, resulting in chemical changes in them. Chemical reactions involve rearrangement of the atoms and produce new substances in this process.
• Chemical kinetics


  • Provides information about the speed at which reactions occur
  • Sheds light on the reaction mechanism (exactly how the reaction occurs)

  
• Melting of ice is not an example of chemical reaction
• Topics in chemical kinetics

  • Reaction rate
  • Rate Law
  • Change of concentration with time
  • Rate and temperature
  • Reaction mechanism
  • Catalysis
• Rate of a reaction


  The change in the concentration of reactants or products per change in time
• Understanding reaction rates is significant
• Rate of a reaction

  1. Change in concentration of a reactant or product per unit time

  2. For hypothetical equation: A → products, rate = -Δ[A]/Δt
• By convention, a negative sign is attached on the change in the concentration of reactants, which underscores the fact that the concentration of the reactants decreases as the reaction proceeds
• Reaction rates and stoichiometry

  1. If the ratio of reactants and products is 1:1, the rate of disappearance of reactant is the same as the rate of appearance of product

  2. If the ratio is not 1:1, the rates are proportional to the stoichiometric coefficients

  
• Factors affecting rate of reaction

  • Concentration of reactants (higher concentration increases likelihood of collisions)
  • Temperature (higher temperature increases kinetic energy and collision frequency)
  • Catalysts (speed up reaction by changing mechanism)
  • Nature of reaction (some reactions are naturally faster than others)
  • Pressure (for gas reactions, higher pressure increases collisions)
• Rate law


  A mathematical expression that shows how rate of reaction depends on the concentration of the reactants
• Rate law


  rate = k[A]^x, where k is the rate constant and x is the order of the reaction with respect to A
• Types of rate laws

  • Differential rate law (shows how rate depends on concentrations)
  • Integrated rate law (shows how concentrations depend on time)
• Orders of reaction


  • Zero-order (rate is constant, independent of concentration)
  • First-order (rate doubles when concentration doubles)
  • Second-order (rate quadruples when concentration doubles)
• Half-life


  The time required for one-half of a reactant to react
• Collision model (theory)


  • Molecules can only react if they collide with each other
  • Molecules must collide with the correct orientation and with enough energy to cause bond breakage and formation
• As temperature increases


  Reaction rate increases
• Activation energy


  The minimum amount of energy required for a reaction to occur
• Catalysts


  • Increase the rate of a reaction by decreasing the activation energy
  • Change the mechanism by which the process occurs
• Enzymes


  Catalysts in biological systems, the substrate fits into the active site of the enzyme