energetics

• What does the symbol Delta H represent?

  Delta H represents the enthalpy change in a chemical reaction.
• What are the standard conditions for measuring enthalpy change?

  Standard conditions are 100 kilopascals of pressure and 298 Kelvin (25°C).
• What are the differences between endothermic and exothermic reactions?

  • Endothermic reactions absorb energy from the surroundings and feel colder.
  • Exothermic reactions release heat energy to the surroundings and feel warmer.
• What is the enthalpy change for the decomposition of calcium carbonate?

  The enthalpy change is +178 kJ per mole.
• How does the energy profile of an endothermic reaction differ from that of an exothermic reaction?

  In an endothermic reaction, reactants are lower in energy than products, while in an exothermic reaction, reactants are higher in energy than products.
• What is an example of an exothermic reaction?

  The combustion of ethane in oxygen is an example of an exothermic reaction.
• What happens to energy when bonds are broken and formed during a chemical reaction?

  Energy is absorbed when bonds are broken and released when bonds are formed.
• How can you determine if a reaction is endothermic or exothermic based on bond energies?

  If more energy is needed to break bonds than is released when forming new bonds, the reaction is endothermic; otherwise, it is exothermic.
• What is mean bond enthalpy?

  Mean bond enthalpy is the average energy required to break a bond or the energy released when the bond is formed.
• How do you calculate the mean bond enthalpy of CH bonds in methane?

  Divide the total energy required to break all CH bonds by the number of CH bonds.
• What is the equation for calculating the energy change in a reaction using bond enthalpies?

  The energy change is the total energy needed to break the bonds minus the total energy released from forming them.
• How do you calculate the energy change for the combustion of methane?

  Calculate the total energy needed to break the bonds and subtract the total energy released from forming the products.
• What is calorimetry and how is it used in chemistry?

  • Calorimetry measures the heat transfer during chemical reactions.
  • It helps determine the enthalpy change of combustion of fuels.
• What is the formula used to calculate energy in a calorimetry experiment?

  The formula is \( Q = mc\Delta T \).
• In the calorimetry equation \( Q = mc\Delta T \), what does each variable represent?

  Q is energy, m is mass, c is specific heat capacity, and \( \Delta T \) is the temperature change.
• How do you calculate the energy transferred when heating water in a calorimetry experiment?

  Multiply the mass of water by the specific heat capacity and the temperature change.
• How do you calculate the number of moles of ethanol used in a calorimetry experiment?

  Divide the mass of ethanol by its molar mass (Mr).
• How do you calculate the enthalpy change from the energy transferred and the number of moles of fuel used?

  Divide the energy transferred by the number of moles of fuel used.
• What is the purpose of using a polystyrene cup in calorimetry experiments?

  • To minimize heat loss to the surroundings.
  • To contain the solution being heated.
• What type of reactions are typically measured in calorimetry experiments?

  Calorimetry experiments typically measure acid-base reactions.
• Why is it important to measure the temperature before and after adding reactants in calorimetry?

  To determine the temperature change and calculate the energy transferred.
• What is the formula for calculating entropy change?

  Entropy change is calculated as energy divided by moles minus 14212.
• Why is the entropy change negative in the given example?

  It is negative because the reaction is exothermic, producing heat energy.
• What is the entropy change of the fuel in the example?

  The entropy change of the fuel is $-364.5 \text{ kJ/mol}$.
• What is the purpose of a polystyrene cup in a calorimetry experiment?

  • To prevent heat loss to the surroundings
  • To contain the solution being measured
• What is the first step in a calorimetry experiment with a solution?

  The first step is to add the acid and measure the initial temperature.
• Why do we typically use acid-alkali reactions in calorimetry experiments?

  They show a reasonable temperature change.
• What was the initial temperature of the hydrochloric acid in the example?

  The initial temperature was $20^\circ C$.
• How much did the temperature rise after adding sodium hydroxide?

  The temperature rose by $6^\circ C$.
• How do you calculate the energy change in a calorimetry experiment?

  Use the formula $Q =$$mc\Delta T$, where $m$ is the mass of the liquids, $c$ is the specific heat capacity, and $\Delta T$ is the temperature change.
• What is the assumed density of the liquids in the calorimetry experiment?

  The assumed density is $1 \text{ g/cm}^3$.
• How do you calculate the number of moles of hydrochloric acid?

  Calculate moles using the formula: moles = concentration × volume.
• What is the concentration of hydrochloric acid used in the example?

  The concentration is $1 \text{ M}$ (1 mole per dm³).
• How do you convert volume from cm³ to dm³?

  Divide the volume in cm³ by $1000$.
• What is the calculated entropy change of neutralization for hydrochloric acid?

  The entropy change is $-5016 \text{ kJ/mol}$.
• What is Hess's Law?

  • The total energy change of a reaction is independent of the route taken.
  • It allows calculation of energy changes that cannot be measured directly.
• Who formulated Hess's Law?

  Hess's Law was formulated by Germain Hess.
• What are the two types of cycles in Hess's Law?

  The two types of cycles are formation cycles and combustion cycles.
• What is the significance of balancing in Hess's cycles?

  It is important to ensure that all elements and compounds are balanced in the cycle.
• How do you determine the enthalpy of formation in a cycle?

  Substitute the formation data into the cycle and multiply by the number of moles.