Enthalpy Change Flashcards
What is enthalpy change?
The amount of heat energy transferred during a chemical reaction at a constant pressure.
units for ΔH
Kj mol-1
What are the standard conditions?
100kPa and 298K
Exothermic
gives out energy
ΔH is negative
Give an example of exothermic reaction:
- combustion of methane
Endothermic reactions
Absorb energy
ΔH is positive
What reaction is bond breaking?
endothermic- you need energy to break bonds
What reaction is once the bonds are formed?
Exothermic as energy is released.
What is bond enthalpy?
Bond enthalpy is the energy required to break bonds.
Define standard enthalpy of formation
the enthalpy change when 1 mole of a compound is formed from its elements in their standard state under standard conditions.
Define standard enthalpy of combustion.
the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
Define specific heat capacity
The amount of energy required to raise the temperature of 1g by 1k.
If the surroundings have a lower specific heat capacity what happened
An exothermic reaction will cause a bigger increase in the temperature of the surroundings
What are the units for specific heat capacity
Jk-1g-1
Equation for enthalpy change
Q= mcAt
Q- amount of heat energy given out or taken in (J)
M- mass of the surrounding (g)
c- specific heat capacity
If the reaction is exothermic
We add a negative sign at the front
If the reaction is endothermic
Add a + sign to the front
Simple flame calorimetry
Burn water using a flame.
Measure the temperature change of the water
What are some problems with the simple flame calorimeter
Some of the energy will be lost.e.g the water will give off vapours into the air.
To prevent this, increase the mass of the water but this can cause such a small change in temperature.
How do you find the molar enthalpy?
Divide the enthalpy change by the moles
What element are found as gases under standard state?
Nitrogen
oxygen
chlorine
fluorine
hydrogen
And all noble gases
What elements are found as liquid under their standard state?
Bromine and mercury
If an element is under standard state what is their enthalpy change?
Always zero!
When a metal reacts with oxygen, what will it form?
Metal+ oxgyen——> metal oxide
Define enthalpy of combustion
The standard enthalpy of combustion is when one mole of a substance is fully burned in oxygen under standard conditions with all reactants and products under their standard state
Define Hess’s law
The enthalpy change in a reaction at constant pressure is independent of the route taken
Arrows for enthalpy of formation
The both face upwards so arrow one needs to be flipped
How is an exothermic reaction shown from a thermometer?
A rise in temperature
How is a endothermic reaction shown by a thermometer?
Temperature drops
