YR12 - Rate, enthalpy and Hess Cycles
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Define enthalpy change
The difference in enthalpy between the products and reactants in a reaction
Define exothermic reaction
A reaction where heat energy is transferred from the system to the surroundings
Define endothermic
A chemical reaction where energy is absorbed from the surroundings into the system
How should temperatures be recorded?
To 1 decimal place
What is shown in an exothermic bond profile diagram?
The products have less enthalpy than the reactants. More energy is released from making bonds than breaking bonds.
Define activation energy
The minimum amount of energy required to start a reaction
What is shown in an endothermic bond profile diagram?
The products have more enthalpy than the reactants. More energy is required for making bonds than breaking bonds.
How is the activation energy of a backwards reversible reaction calculated?
Go against the arrow and change the sign
Define enthalpy change of reaction
The enthalpy change when the reaction occurs in the molar quantities shown in the equation
Define enthalpy change of formation
The enthalpy change when 1 mole of a compound is formed from it’s elements under standard conditions
Define enthalpy change of combustion
The enthalpy change when 1 mole of a substance completely combusts under standard conditions
Define enthalpy change of neutralisation
The enthalpy change when solutions of acid and alkali react together under standard conditions to produce 1 mole of water
Why can the enthalpy change of formation value differ to values in data booklets?
The reaction was not carried out under standard conditions
What are the steps to calculate any given enthalpy change?
1) Work out the heat energy (q) by q = m x c x delta T 2) Work out the moles of the limiting reactant 3) Work out the enthalpy change (q/n)
What is meant by Hess’ Law?
If a reaction can take place by 2 different routes, the enthalpy change is the same for both routes
When given combustion data, which way do the arrows point and where do the products of combustion go on a Hess Cycle diagram?
Both point downwards, towards the products of combustion
When given formation data, which way do the arrows go in a Hess Cycle?
Both point upwards, from the elements present in the equation to the equation itself
On a Maxwell-Boltzmann curve, what does the area under the curve represent?
The total number of molecules present
How does a change in temperature effect the Maxwell-Boltzmann distribution?
• When temprature increases, the graph shifts right and down. The new line must be above the original. • When temprature decreases, the graph moves left and up.
What should be on the X and Y axes of a Maxwell-Boltzmann distribution?
X axis - Energy Y axis - Number of molecules
Explain the effect of temprature on the Maxwell-Boltzmann distribution.
As temprature increases, the amount of energy avaliable decreases and the curve moves to the right. The amount of particles doesn’t change, however, so the curve gets lower. The area under the curve stays the same.
Describe and explain the effect of increasing concentration on the Maxwell-Boltzmann distribution.
• The hump of the graph moves higher • As concentration increases, amount of particles increase but energy stays the same.
Describe the change to the Maxwell-Boltzmann distrubution when a catalyst is added to the system.
The Ea value moves left and is labelled as Ec.
What are the 3 standard conditions?
• 100 kPa • 298K • Solutions at 1 mol dm⁻¹