Thermochemistry Flashcards
What is thermochemistry?
Thermochemistry is the study of energy changes, particularly heat, during chemical reactions and physical changes.
Define an exothermic reaction.
An exothermic reaction releases heat into the surroundings, resulting in a negative change in enthalpy (ΔH < 0).
Define an endothermic reaction.
An endothermic reaction absorbs heat from its surroundings, resulting in a positive change in enthalpy (ΔH > 0).
What is enthalpy (H)?
Enthalpy is a state function that represents the total heat content of a system at constant pressure.
Write the formula used to calculate the enthalpy change of a reaction.
ΔH = H(products) – H(reactants).
What is Hess’s Law?
Hess’s Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps, because enthalpy is a state function.
How can Hess’s Law be used to calculate ΔH for a reaction?
By breaking a reaction into a series of steps with known enthalpy changes and then summing these changes to find the overall ΔH.
Define bond energy.
Bond energy is the energy required to break one mole of a particular bond in a gaseous substance.
How can bond energies be used to estimate the enthalpy change of a reaction?
By subtracting the total energy required to break bonds in the reactants from the total energy released in forming bonds in the products.
What is the standard enthalpy change of formation (ΔH_f°)?
It is the enthalpy change when one mole of a compound is formed from its elements in their standard states.
What does a negative ΔH indicate about a reaction?
A negative ΔH indicates that the reaction is exothermic, releasing energy as heat.
What does a positive ΔH indicate about a reaction?
A positive ΔH indicates that the reaction is endothermic, absorbing energy as heat.
Explain the principle behind calorimetry.
Calorimetry measures the heat exchanged during a reaction by monitoring temperature changes in a known mass of a substance with a specific heat capacity.
Write the calorimetry equation.
q = mcΔT, where q is heat energy, m is mass, c is specific heat capacity, and ΔT is the temperature change.
What is meant by a state function?
A state function (like enthalpy) depends only on the state of the system and not on the path taken to reach that state.
How do catalysts affect the energy profile of a reaction?
Catalysts lower the activation energy of a reaction without changing the overall ΔH, thereby increasing the reaction rate.
What is combustion in thermochemistry?
Combustion is an exothermic reaction in which a substance reacts rapidly with oxygen to release heat and light.
“Define activation energy.”
Activation energy is the minimum energy required to initiate a chemical reaction.
How can a reaction’s energy profile be represented?
An energy profile graph plots the energy of the system against the reaction progress, showing the activation energy and overall ΔH.
What are standard conditions in thermochemistry?
Standard conditions usually refer to 25°C (298 K) and 1 atm pressure, under which standard enthalpy changes are measured.
