Energetics Seneca Flashcards
Define enthalpy change.
- Measure of heat taken in or out of process.
When objects are heated, what do they use the energy for?
- To expand.
Why do we use enthalpy instead of energy?
- Energy relies on fixed volume.
What condition must be constant to measure enthalpy?
- Pressure, which is constant in the atmosphere.
Do exo and endothermic reactions take in or give out heat energy?
- Exo - give out.
- Endo - take in.
Do exo and endothermic reactions have a positive or negative enthalpy change?
- Exo - negative.
- Endo - positive.
Example of an exothermic reaction?
- Combustion.
Example of endothermic reaction?
- Thermal decomposition.
Endothermic making or breaking bonds?
- Breaking.
Exothermic making or breaking bonds?
- Making.
What is enthalpy change of a reaction the sum of?
- Individual bond enthalpies being broken and made during a reaction.
What is the dominating term?
- Whether the reaction requires more energy to make or break bonds, giving the overall enthalpy.
What is a memory tool to remember bond breaking and bond making?
-Bendo-Mexo.
What is the symbol for enthalpy change?
- ΔH.
What are standard conditions?
- 1 bar (100kPa).
- 298 K.
What are the 2 standard enthalpy changes?
- Standard enthalpy of combustion (ΔcHθ).
- Standard enthalpy of formation (ΔfHθ).
Standard enthalpy of combustion (ΔcHθ) meaning?
- Enthalpy change when one mole of substance in its standard state burns completely in oxygen under standard conditions.
Standard enthalpy of formation (ΔfHθ) meaning?
- Enthalpy change when one mole of substance is formed in its standard state from the pure elements in their standard states under standard conditions.
How does bomb calorimetry work?
- Burning a sample of compound in a sealed vessel + measuring temp change.
- Heating water jacket.
Sources of inaccuracy in bomb calorimetry?
- Heat lost to surroundings.
- Incomplete combustion.
- Reactant may evaporate before it combusts.
What is the equation to measure enthalpy change?
- q = m x c x ΔT.
What do the letters represent in the enthalpy change equation?
- q - heat energy.
- m - mass of sample.
- c - specific heat capacity.
- ΔT - temp change.
What are the units of q?
- Joules.
How to gain accurate temperature changes?
- Measure temp loss after reaction is complete by extrapolating line.
Why are the initial and final temperatures on a calorimeter inaccurate?
- Heat is always being lost from the calorimeter.
What is the only case when we can assume that the reaction mixture has the same density or specific heat capacity as water?
- When the question states we can.
What does Hess’ Law mean?
- Enthalpy change of a reaction is independent of the pathway.
- Enthalpy change is the same if its direct or indirect.
What is Hess’ Law useful for?
- Useful for obtaining enthalpy changes that aren’t directly observable.
What is the enthalpy of formation for any element?
- Zero.
What is the meaning of bond enthalpy?
- Enthalpy change when one mole of bonds is broken in the gas phase.
What do mean bond enthalpies mean in molecules that have identical bonds?
- The 2nd bond will require more energy to break than the 1st because the molecule will rearrange after the 1st bond is broken.
- ∴ an average of bond enthalpy values is taken.
What are the 2 main reasons that mean bond enthalpies are inaccurate?
- Bond enthalpies are only valid in gas phase.
- Bond enthalpies depend on the particular molecule, however mean bond enthalpies are quoted generally.
Is bond enthalpy positive or negative?
- Positive.
In standard enthalpy of formation (ΔfHθ) where do the arrows point in Hess’ triangle?
- From the elements in their standard states to the reactants + products.
In standard enthalpy of combustion (ΔcHθ) where do the arrows point in Hess’ triangle?
- Point from the reactants + products to the oxides.
