Energetics Flashcards
What is the definition for enthalpy change?
The heat energy transferred in a reaction at constant pressure
What are the units of enthalpy change?
kjmol-1
What does an exothermic reaction do?
Give out energy
Is delta H positive or negative for an exothermic reaction?
Negative
What does an endothermic reaction do?
Reactions absorb energy
Is delta H positive or negative for an endothermic reaction?
Positive
Is bond breaking exothermic or endothermic?
Endothermic as energy is required
Do stronger bonds take more or less energy to break?
More
Is bond making endothermic or exothermic?
Exothermic as energy is released
What is bond enthalpy?
The energy required to break bonds
What is mean bond enthalpy?
The average energy needed to break a certain type of bond, over a range of compounds
What is the equation for enthalpy change of reaction?
Enthalpy change of reaction = total energy absorbed - total energy released
What is standard enthalpy of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What is the standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
What does calorimetry allow us to do?
Find out how much heat is given out by a reaction by measuring a temperature change
How does calorimetry work>
To find enthalpy of combustion of a flammable liquid, you burn it in calorimeter
As fuel burns, it heats water
You can work out the heat energy absorbed by the water if you know the mass of the water, the temperature change and specific heat capacity of water
Ideally, all the heat given out by the fuel as it burns would be absorbed by the water - allowing you to work out the enthalpy change of combustion exactly
Why is it hard to get an accurate result with calorimetry?
You always lose heat to the surroundings
When you burn a fuel, some of the combustion may be incomplete meaning less energy is given out
Flammable liquids are often quite volatile too, so you may lose some fuel to evaporation
How do you find the enthalpy change for a neutralisation reaction?
Add known volume of acid to insulated container and measure temp
Then add a known volume of alkali and record the temp of the mixture at regular intervals over a period of time (stir solution to ensure evenly heated)
Find the temp change for the experiment. Use it to calc enthalpy change of reaction
How can you use a graph to find accurate temperature change?
During experiment, record the temperature at regular intervals, beginning a couple of minutes before starting reaction
Plot a graph of results. Draw two lines of best fit: one through the points before the reaction started and one through the points after it started
Extend both lines so they both pass the time when the reaction started
The distance between the two lines at the time the reaction started before any heat was lost is the accurate temperature change for the reaction
What is the formula for calculating enthalpy change from a calorimetry experiment?
q=mcdeltaT
What is Hess’s Law?
The total enthalpy change is independent of the route taken
How do you calculate rate of reaction?
Amount of reactant used or product formed / time
A reaction won’t take place between two particles unless:
-They collide in the right direction - they need to be facing each other in the right way
-They collide with at least a certain minimum amount of kinetic energy
What is activation energy?
The minimum amount of kinetic energy that particles need to react
