Energetics (4) 🌟 Flashcards
(22 cards)
Why is the amount of energy involved when a chemical reaction takes place important?
- you can measure the energy values of fuels
- you can calculate energy requirements for industrial processes
- helps to predict if a reaction will take place or not
What is thermochemistry?
The study of heat changes during chemical reactions
Give an example of an exothermic reaction
Neutralisation
Give an example of an endothermic reaction
The breakdown of calcium carbonate (limestone) to calcium oxide (lime)
- it needs heat
What is energy measured in?
KJ mol-1
What is an enthalpy change?
When you measure a heat changes during at a constant pressure
What are the standard conditions for measuring enthalpy changes
- pressure= 100kPa
- temperature= 298k (25*C)
Which reaction has a negative enthalpy change?
Exothermic
Which reaction has a positive enthalpy change?
Endothermic
What is the standard molar enthalpy of formation?
The enthalpy change when one mole of substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard state
What is the standard molar enthalpy of combustion?
The enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions , all reactants and products being in their standard states
What is specific heat capacity?
The amount of heat needed to raise the temp of 1g of substance by 1kelvin
It’s units are Jg-1K-1
What is the formula for enthalpy change?
Enthalpy change = mass of substance x specific heat capacity x temperature change
Q = m c t
How would you work out using q=mct
M= mass of water being heated
1. Use calculation
2. Find out the moles of the mass of spirit burner
3. The answer to the calc is the enthalpy change for that amount of moles
4. Convert to dine enthalpy change for one mole.
What is used for the q=mct experiment?
Calorimeter
How are flame calorimeters adapted to reduce heat loss?
- spiral chimney is made of copper
- the flame is enclosed
- the fuel burns in pure oxygen rather than air
How can you allow for heat loss in an experiment?
Plot a cooling curve then Repeat the experiment using the cooling curve
What does Hess’s law state?
That the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products
What is the steps to using Hess’s law in a question?
Draw a box below with the data given products inside and draw the arrows
What does breaking bonds do?
Requires energy
What does bond making do
Releases energy
What is bond dissociation enthalpy?
The enthalpy change required to break a covalent bond with all species in the gaseous state
