Chemistry

• What is the difference between an exothermic and endothermic reaction?
• Endothermic reaction


  More energy is needed to break the bonds of the reactants than is released when the bonds of the products form
• Exothermic reaction


  More energy is released when the products form than is needed to break the bonds in the reactants
• Collisions in Reactions

  1. Collision

  2. Activation complex

  3. Products
• Collision Theory


  For a chemical reaction to take place, two particles must collide. After the collision, an unstable activation complex forms that breaks apart into the products
• No reaction could occur if the particles don’t collide with enough energy or the reactants are in the wrong orientation for new bonds to form
• Activation Energy

  The minimum energy colliding particles need to react
• System


  The part of the universe where you are focusing your attention
• Surroundings


  Everything else outside the system
• During any chemical or physical process, the energy of the universe remains unchanged
• If the energy of the system increases during a process


  The energy of the surroundings must decrease by the same amount and vice versa
• Heat


  Energy that moves from one object to another due to a temperature difference
• Thermochemistry


  Looks at the flow of heat between the system and its surroundings
• Units of Heat flow

  • calorie
  • joule
• A calorie (cal) is defined as the quantity of heat needed to raise the temperature of 1 g of pure water by 1 °C
• The joule (J) is the SI unit of energy. One joule of heat raises the temperature of 1 g of pure water by 0.2390 °C
• You can convert between calories and joules using the following relationships: 1 J = 0.2390 cal, 4.184 J = 1 cal
• Calorimetry


  The measurement of the heat flow into or out of a system for chemical and physical processes
• Calorimeter


  The insulated device used to measure the absorption or release of heat in chemical or physical processes
• Heat Capacity


  The amount of heat needed to increase the temperature of an object exactly 1 °C
• Specific Heat Capacity

  The amount of heat it takes to raise the temperature of 1 g of the substance by 1 °C
• The greater the mass of the object, the greater its heat capacity
• The heat capacity of an object depends on both its mass and its chemical composition
• Different substances with the same mass may have different heat capacities
• On a sunny day, a 20-kg puddle of water may be cool while a 20-kg iron sewer cover may be too hot to touch
• If both the water and the iron absorb the same amount of radiant energy from the sun, the temperature of the water changes less than the temperature of the iron in the same amount of time because water has a larger specific heat capacity
• Calculating Specific Heat

  To calculate the specific heat (C) of a substance, you divide the heat input by the mass of the substance times the temperature change. The change in temperature is calculated by taking the initial temperature away from the final temperature. Heat may be expressed in terms of joules or calories. Therefore, the units of specific heat are either J/(g.°C) or cal/(g.°C). C = q / (mass x ΔT) = Heat (J or cal) / mass (g) x change in temperature (°C)
• Enthalpy

  The enthalpy (H) of a system accounts for the heat flow of the system at constant pressure. The change in enthalpy (ΔH) of a reaction can be determined by measuring the heat absorbed or released by a reaction at constant pressure. The terms heat and enthalpy change can be used interchangeably if the reaction is at constant pressure. Therefore q = ΔH. The heat absorbed by the surroundings (qsurr) is equal to, but has the opposite sign of, the heat released by the system (qsys). Therefore, the enthalpy change for the reaction (ΔH) can be written as follows: qsys = ΔH = - qsurr = -m x C x ΔT The sign for ΔH is positive for an endothermic reaction and negative for an exothermic reaction
• In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product. A chemical equation that includes the enthalpy change is called a thermochemical equation. The heat of reaction is the enthalpy change for the chemical equation exactly as it is written, usually reported as ΔH
• In this and other exothermic processes, the chemical potential energy of the reactants is higher than the chemical potential energy of the products
• The amount of heat absorbed or released during a reaction depends on the number of moles of reactants involved. This reaction of 1 mol of CaO with 1 mol of H2O releases 65.2 kJ of heat. Therefore, the reaction of 2 mol of CaO with 2 mol of H2O releases 130.4 kJ of heat
• Bond Enthalpy

  Bond enthalpy is the amount of energy needed to break the bonds in one mole of a substance. Enthalpy changes can be shown in an enthalpy diagram. Energy into the system is positive and out of the system is negative. Since a reaction may involve breaking and forming more than one bond, the enthalpy of the reaction can be calculated using average bond enthalpies
• Hess's Law


  The change in the enthalpy for a reaction is independent of the path taken to accomplish the change
• Converting reactants A directly into products B

  Enthalpy change ΔH_AB
• Reactants A change to intermediate C, which then changes to products B


  1. Overall enthalpy change is still ΔH_AB

  2. ΔH_AC + ΔH_CB = ΔH_AB
• Heat Summation


  1. C(s) + 2H₂O(g) → CO₂(g) + 2H₂(g)

  2. 2H₂O(g) → 2H₂(g) + O₂(g) ΔH = +483.6 kJ/mol

  3. C(s) + O₂(g) → CO₂(g) ΔH = -393.5 kJ/mol

  4. C(s) + O₂(g) + 2H₂O(g) → CO₂(g) + 2H₂(g) + O₂(g) ΔH = +90.1 kJ/mol
• Hess's law of heat summation states that if you add two or more thermochemical equations to give an overall equation, you can also add the enthalpies of reaction to give the overall enthalpy of reaction
• Carbon and water vapor react to form carbon dioxide and hydrogen


  • First, water breaks apart into hydrogen and oxygen
  • Then carbon reacts with oxygen from water to form carbon dioxide
• This reaction is endothermic. The diagram shows us that more energy is needed for the reaction to occur than is released in forming new bonds. This is also shown by the positive delta H value
• Consider the conversion of diamond to graphite

  1. C (s, diamond) → C (s, graphite)

  2. C (s, graphite) + O₂(g) → CO₂(g) ΔH = -393.5 kJ

  3. C (s, diamond) + O₂(g) → CO₂(g) ΔH = -395.4 kJ

  4. CO₂(g) → C (s, graphite) + O₂(g) ΔH = +393.5 kJ

  5. C (s, diamond) → C (s, graphite) ΔH = -1.9 kJ