Energetics Flashcards
Is it exothermic or endothermic if energy is given out and bonds are made?
Exothermic
Is it exothermic or endothermic if energy is taken in to break bonds?
Endothermic
What is heat enthalpy?
It is a measure of the heat content of a substance or system and it is measured in kJmol-1
What are the standard conditions when measuring enthalpy change?
-298K
-100kPa
What happens during an exothermic reaction?
- Surrounding get warmer and the products have less energy than the reactants.
- Enthalpy / heat change is negative
What happens during an endothermic reaction?
- Surroundings get cooler and the products have more energy than the starting materials
-Enthalpy/ heat change is positive
How do we measure enthalpy changes?
Using a calorimeter
What are the four processes we measure enthalpy changes for?
-Dissolving
-Neutralisation
-Formation
-Combustion
What does a simple calorimeter consist of?
Sprirt burner , clampstand and thermometer
What is the equation for calculating energy change ?
q=mcΔT
=Mass ( of water or solution) x specific heat capacity x change in temp
What is the specific heat capacity of water?
4.18Jg-1k-1
When do we add both masses of solution and solid?
Dissolving
Why don’t we record the temperature at 4 minutes?
Because we add reagent instead
How do we improve the calorimetry experiment?
- Temp on thermometer does not instantly peak
- Time is needed for the thermometer to respond to changes in temp and during this time temperature is being lost to the surroundings
How to work out enthalpy change in combustion reactions?
use q=mass x shc x temp change
- convert to KJmol-1 and divide by moles of fuel
How to work out enthalpy change in neutralisation reactions?
- Use q= mass x shc x temp change
-convert to kJmol-1 - Add both masses and take 1cm3 to be equal to 1g
- Use moles of limiting reactant
How to work out enthalpy changes in displacement reactions?
Use q= mass x shc x temp change
-Use moles of limiting reactant
How do you convert Joules to Kilojoules?
/1000
What is a Hess cycle?
When a reaction can occur by more than 1 route, the overall enthalpy change is independent of the route taken.
What is the definition of Standard Enthalpy change 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 change of formation of an element that is in its standard state?
0 (unless they are not in their standard state)
What is the definition of the Standard Enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions
