energetics Flashcards
What is thermochemistry?
The study of heat changes during chemical reactions.
What happens when chemical bonds break?
Energy is required to break bonds.
What occurs when chemical bonds form?
Energy is released to form bonds.
What is the energy change in an exothermic reaction?
Energy change is negative.
What is the energy change in an endothermic reaction?
Energy change is positive.
What typically happens to the temperature in an exothermic reaction?
Temperature usually increases.
What typically happens to the temperature in an endothermic reaction?
Temperature usually decreases.
What is enthalpy (ΔH)?
The measure of the heat content of a substance measured in KJmol-1.
What is molar enthalpy change?
The energy change per mole of substance.
What is the standard pressure for measuring enthalpy changes?
100 kPa.
What is the standard temperature for measuring enthalpy changes?
298K (around 25°C).
What is the enthalpy of formation (ΔHfθ)?
The enthalpy change when one mole of a substance is formed from its elements in their standard states.
What is the enthalpy of combustion (ΔHCθ)?
The enthalpy change when one mole of a substance is completely burned in oxygen.
What is the enthalpy of neutralization (ΔHneutθ)?
The enthalpy change when one mole of water is formed in a reaction between an acid and an alkali.
What is calorimetry?
A method to calculate the energy stored in a substance by measuring heat exchange during a reaction.
What is the equation for calculating energy change?
q = mcΔt.
In calorimetry, what is the specific heat capacity of water?
4.18 J g-1 °C-1.
What should be assumed about the density of solutions in calorimetry?
Assume all have the same density as water, 1 cm3 = 1 g.
What is Hess’s Law?
The overall enthalpy change for a chemical reaction is independent of the route taken.
What is the enthalpy change for a reaction calculated using Hess’s Law?
ΔH3 = ΔH1 + ΔH2.
What is the bond dissociation enthalpy?
The enthalpy change when one mole of covalent bonds is broken in the gaseous state.
What must be done to calculate the enthalpy change per mole?
Use the equation ΔH = q/n.
What is the relationship between the energy of products and reactants in an endothermic reaction?
Energy of the products is higher than the energy of the reactants.
What is the relationship between the energy of products and reactants in an exothermic reaction?
Energy of the reactants is higher than the energy of the products.
What happens to the temperature of surroundings in an exothermic reaction?
Temperature of the surroundings increases.
What happens to the temperature of surroundings in an endothermic reaction?
Temperature of the surroundings decreases.
What is the significance of standard states in enthalpy measurements?
They ensure that reactions have gone to completion before measuring energy change.
What are the assumptions made in simple calorimetry?
- The liquid in the reaction mixture is mostly water
- The liquid containing the fuel has a specific heat capacity of 4.2 J K-1 g-1
- The reaction mixture has a density of 1 g cm-3
- All heat energy is absorbed by the water.
What is bond dissociation enthalpy?
The enthalpy change when one mole of covalent bonds is broken with all species in the gaseous state.
This definition highlights the specific conditions under which bond dissociation enthalpy is measured.
What is mean bond enthalpy?
The average enthalpy change when one mole of a specified covalent bond is broken, averaged out across the range of compounds.
Mean bond enthalpy provides a generalized value for bond strengths across different molecules.
How do you calculate the total enthalpy change using mean bond enthalpies?
𝛴𝜟 breaking - 𝛴𝜟 making = 𝛴𝜟 total.
This equation summarizes the process of calculating the enthalpy change for a reaction based on bond energies.
What type of reaction has a net positive enthalpy change?
Endothermic reactions.
Endothermic reactions absorb heat, resulting in an increase in enthalpy.
What type of reaction has a net negative enthalpy change?
Exothermic reactions.
Exothermic reactions release heat, leading to a decrease in enthalpy.
What process is associated with breaking bonds?
Endothermic process.
Breaking bonds requires energy input, hence it is endothermic.
What process is associated with the formation of bonds?
Exothermic process.
The formation of bonds releases energy, making it exothermic.
Can bond enthalpies be used with substances in states other than gaseous?
No, this method works only with gaseous substances.
Bond dissociation energies are specifically measured for gaseous molecules.
What can knowing bond enthalpies help identify?
Mistakes when calculating the enthalpy change of a reaction.
Understanding bond enthalpies can improve accuracy in thermodynamic calculations.
What is the relationship between bond enthalpy and data book averages?
Bond enthalpy values differ from the averages listed in data books.
Data book values are often averages that may not reflect specific bond strengths in a given context.
Fill in the blank: Breaking bonds is an _______ process.
endothermic.
This highlights the energy requirement for bond dissociation.
Fill in the blank: The formation of bonds is an _______ process.
exothermic.
This emphasizes the energy release during bond formation.
