Topic 8 Energetics I Flashcards
What is the enthlapy change of a reaction?
It is the heat change in a reaction at a constant pressure. It is given the symbol delta H and has the units of kJmol-1
What are the standard conditions?
100kPa pressure
298 Kelvin
What are endothermic reactions?
They are reactions that absorb energy from the surroundings.
The products have a higher energy than reactants
Delta H is +
Example would be thermal decomposition
What are exothermic reactions?
They are reactions that release energy to the surroundings
The products are lower in energy than the reactants
Delta H is -
Example would be combustion
What are the 4 types of delta H (enthalpy changes)?
Standard enthalpy change of reaction
Standard enthalpy change of neutralisation
Standard enthalpy change of combustion
Standard enthalpy change of formation
What is the standard enthalpy change of reaction? (delta r H)
The enthalpy change of a reaction according to the molar quantities in the equation under standard equations
What is the standard enthalpy change of neutralisation? (delta neut H)
The enthalpy change when an acid and alkali react to form 1 mole of water under standard conditions
What is the standard enthalpy change of formation? (delta f H)
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 combustion? (delta c H)
The enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H20, under standard conditions
What is bond enthalpy?
It is the amount of energy needed to break 1 mole of a bond type in a molecule in the gaseous state
Bond breaking
To break a bond energy needs to be absorbed
Bonds are broken in the reactants and this is an endothermic process so delta H is +
If more energy is needed to break bonds than energy given out when bonds are formed, the reaction will be endothermic
Bond making
When bonds are formed, energy is released
Bonds are made when products are being produced and this is an exothermic process so delta H is -
If more energy is released when bonds are formed than what was needed to break initial bonds, the reaction will be exothermic
How to calc enthalpy change?
Total energy to break bonds - Total energy released forming bonds
reactants - products
What is Calorimetry?
It is used to work out the enthalpy change of combustion
A fuel is burnt to raise the temp of the water by a specific amount. Generally, we weigh the fuel before and after burning to work out the mass of the fuel burnt
A lid is placed on top to prevent heat loss and shield winds
The energy from the fuel is transferred into the water although some of it is lost to the surroundings too
What is the equation to calculate energy transferred?
q = m c deltaT
q - Heat energy lost or gained (J)
m - mass of water or solution (g)
c - specific heat capacity (4.18)
delta T - change in temp (K)
Then calc moles and then
enthalpy = q/n
