flashcards chapter 9

• What is enthalpy?

  Enthalpy, H, is a measure of the heat energy in a chemical system.

  
  

  Enthalpy is sometimes thought of as the energy stored within bonds.
• What does chemical system refer to?

  The atoms, molecules, or ions making up the chemicals.
• Can enthalpy be measured?

  No, but enthalpy changes can.
• What is the enthalpy change, ∆H?

  The energy transferred in a reaction at constant pressure;

  
  

  Units: KJ mol ⁻¹
• How do you calculate enthalpy change, ∆H?

  ∆H = H(products) H(reactants);

  
  

  Enthalpy change can be positive or negative.
• What is the law of conservation of energy?

  Energy cannot be created or destroyed;

  
  

  Heat energy is transferred between systems (chemicals) and surroundings (apparatus)
• What does ∆H° show?

  Standard enthalpy change;

  
  

  enthalpy change under standard conditions;

  
  

  Shows the measurements were made under standard conditions and the elements were in their standard states;

  
  

  Standard pressure = 100kPa;

  
  

  Standard temperature = 298K
• What does this mean for chemical reactions?

  When a chemical reaction involving an enthalpy change takes place, heat energy is transferred between the system and the surroundings.
• What are the system and surroundings?

  System is the chemicals - reactants and products.

  
  

  Surroundings are the apparatus, e.g thermometer, the laboratory and everything that's not the chemical system
• What two directions can energy transfer be in?

  From the system to the surroundings = exothermic.

  
  

  From the surroundings to the system = endothermic.
• What are enthalpy profile diagrams?

  Diagrams that show the relative enthalpies of the reactants and products and the enthalpy change.
• What is an exothermic reaction?

  Energy transferred from the system to the surroundings;

  
  

  chemical system releases heat energy to surroundings;

  
  

  Any energy loss by the chemical system is balanced by the same energy gain by the surroundings.

  
  

  ∆H is negative.

  
  

  Temperature of the surroundings increases as they gain energy.

  
• Examples of exothermic reactions?

  Oxidation is usually exothermic, the combustion of a fuel like methane;

  
  

  Oxidation of carbohydrates
• What is an endothermic reaction?

  Energy is transferred to the system from the surroundings;

  
  

  The chemical system takes in energy from the surroundings.

  
  

  Any energy gain by the system is balanced by the same energy loss by the surroundings.

  
  

  ∆H is positive.

  
  

  The temperature of the surroundings decreases as they lose energy.

  
• Examples of endothermic reactions?

  The thermal decomposition of calcium carbonate;

  
  

  The main reactions of photosynthesis
• What is the activation energy, Ea?

  The minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction
• How can activation energy affect the rate of reaction?

  -In general, reactions with small activation energies take place very rapidly, because the energy needed to break bonds is readily available from the surroundings.

  
  

  -Very large activation energies may be such a large energy barrier that a reaction may take place extremely slowly or even not at all.
• What does a standard enthalpy change refer to?

  An enthalpy change under standard conditions.

  
• What are the standard conditions?

  Standard pressure = 100kPa (close to a pressure of one atmosphere, 101kPa).

  
  

  Standard temperature = usually 298K (25 degrees Celsius).

  
  

  Standard concentration = 1 moldm⁻³

  
  

  Standard state = physical state of a substance under standard conditions.
• What is the enthalpy change of reaction?

  The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.

  
• What is the enthalpy change of formation?

  The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

  
• What is the enthalpy change of formation for an element?

  0 kJmol⁻¹
• What is the enthalpy change of combustion?

  The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
• What is the enthalpy change of neutralisation?

  The enthalpy change for the formation of 1 mole of water from the reaction of an acid and an alkali, under standard conditions, with all reactants and products in their standard states

  
  

  H+(aq) + OH-(aq) → H2O(l)
• How do you calculate an energy change?

  q (in Joules) = mc∆T
• What three quantities are needed to calculate energy change?

  Mass (of the surroundings);

  
  

  Specific heat capacity;

  
  

  temperature change
• What is the mass (of the surroundings)?

  Measured by weighing.

  
  

  The mass that changes temperature

  
  

  Units: g
• What is the specific heat capacity of the surroundings?

  The specific heat capacity is the energy needed to raise the temperature of 1g of a substance by 1K.

  
  

  c = 4.18 J g⁻¹K⁻¹
• What is the temperature change of the surroundings?

  Determined from the thermometer readings.

  
  

  T(final) - T(initial).
• Calculation of the enthalpy change of combustion? Example


  part 1Calculate q=mc∆T in joules;

  
  

  Then convert to KJ;

  
  

  Then divide by the number of moles of the substance burnt;

  
  

  Combustion is exothermic, so add a negative sign (neutralisation is also exothermic)

  
• Calculation of the enthalpy change of combustion? Example

  part 2

  
• How can you determine an enthalpy change of combustion experimentally?

  Using:

  
  

  Calorimetry;

  
  

  Spirit burners
• How do you use a spirit burner?

  Measure out a volume of water into a beaker (e.g. 150cm^3). Record the initial temperature.

  
  

  Add your fuel to the spirit burner. Weigh the spirit burner containing the fuel.

  
  

  Place the spirit burner under the beaker. Light the burner and burn the fuel whilst stirring the water with the thermometer;

  
  

  As fuel burns, it heats the water

  
  

  After about three minutes extinguish the flame. Immediately record the maximum temperature reached by the water.

  
  

  Re-weigh the spirit burner containing the methanol. Assume that the wick has not been burnt.
• What are the reasons for an inaccurate enthalpy change of combustion experimental value?

  Heat loss to the surroundings other than the water - this includes the beaker but mainly the air surrounding the flame.

  
  

  Incomplete combustion of methanol = there may be some incomplete combustion, with carbon monoxide and carbon being produced instead of carbon dioxide. You would see carbon as a black layer of soot on the beaker.

  
  

  Evaporation of methanol from the wick = the burner must be weighed as soon as possible after extinguishing the flame. Otherwise some methanol may have evaporated from the wick. Spirit burners usually have a cover to reduce this error.

  
  

  Non-standard conditions = the conditions for this experiment are unlikely to be identical to standard conditions.

  
  

  All but the last of these would lead to a value that is less exothermic than expected.
• How could you minimise these errors?

  Use of draught screens and an input of oxygen gas could minimise errors from heat loss and incomplete combustion.
• How can you determine an enthalpy change of neutralisation or displacement experimentally?

  Add acid;

  
  

  Measure temperature;

  
  

  Then add alkali, stir and measure temperature change;

  
  

  Heat given out calculated using q = mc∆T
• How can you determine an enthalpy change of reaction?

  Many reactions take place between two solutions, or between a solid and a solution.

  
  

  The enthalpy change of these reactions can be determined using plastic cups made of polystyrene foam.

  
  

  When carrying out reactions between aqueous solutions, the solution itself is the immediate surroundings.

  
  

  Any energy change is between the chemical particles and water molecules in the solution.

  
  

  A thermometer in the solution will record any temperature change, allowing the heat energy change to be calculated.
• Calculation of the enthalpy change of reaction? Example

  
• Why use polystyrene cups?


  Cheap, waterproof and light weight, and offer some insulation against heat loss to the surroundings.
• Define average bond enthalpy?

  The energy required to break one mole of a specified type of bond in a gaseous molecule.