Energetics I Flashcards
(18 cards)
What is enthalpy?
Enthalpy is the measure of energy in a chemical system.
In an exothermic reaction energy is transferred…
from the system to the surroundings.
In an endothermic reaction energy is transferred…
From the system to the surroundings.
Is enthalpy change positive or negative for an exothermic reaction?
Negative
Is enthalpy change positive or negative for an endothermic reaction?
Positive
Define activation energy.
The minimum energy required for a reaction to take place.
What are the standard conditions for standard enthalpy change?
101kPa
298K (25C)
1 mol dm-3
What is the standard enthalpy change of combustion?
When one mole of a substance reacts completely with oxygen under standard conditions.
What is the standard enthalpy change of neutralisation?
When one mole of water is formed in a neutralisation reaction under standard conditions.
What is the standard enthalpy change of formation?
When one mole of a compound is formed from its elements under standard conditions.
What is the standard enthalpy change of reaction?
When a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions.
Outline an experiment you could use to determine the standard enthalpy change of combustion.
Use a balance to measure the starting mass of spirit burner and fuel.
Use a thermometer to measure the start temperature of the water.
Stir the water with the calorimeter to ensure the thermal energy is distributed.
Measure final temperature of water and final mass of spirit burner and fuel.
Give some reasons as to why a combustion experiment may not give the most accurate result for enthalpy change.
-if the unlit spirit burner is left uncapped then fuel can evaporate
-a lot of the heat energy released by the fuel does not pass into the water and is lost to the surroundings
-the fuel might not all have undergone complete combustion
-the experiment may not have been carried out under standard conditions
Outline an experiment that could be used to calculate the standard enthalpy change of reaction.
-Weigh out magnesium powder by working out how many moles we need. we want copper in excess.
-we need to take temperature readings every 30 seconds for 5 minutes
-to calculate the temperature change we need to plot the results on a graph and draw a line back from the cooling part of the curve and a vertical line from the start of the reaction.
Define average bond enthalpy.
The energy required to break one mole of a specified type of bond in a gaseous molecule.
What does Hess’ law state?
If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route.
