Enthalpy Flashcards
Define △cH
The enthalpy change for the complete combustion of 1 mole of a substance
Define △neutH
The enthalpy change required for the neutralisation of an acid by an alkali to form 1 mole of H2O
Define △fH
The enthalpy change for the formation of a compound from its constituent elements
Explain the law of conservation
The amount of energy in an isolated system remains the same, energy cannot be created or destroyed only transferred
What energy change is breaking bonds associated with?
- To break bonds, energy is taken in
- Endothermic reaction
What energy change is making bonds associated with?
- To make bonds, energy is released
- Exothermic reaction
What is an endothermic reaction?
- Enthalpy of products > Enthalpy of reactants
- Overall positive enthalpy change
What is an exothermic reaction?
- Enthalpy of products < Enthalpy of reactants
- Overall negative enthalpy change
What are the standard conditions?
100kPa and 298K
How can you calculate enthalpy change from experimental data?
Q=mc△T
What does average bond enthalpy mean?
Mean energy required to break 1 mole of bonds in gaseous molecules
Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?
Bond enthalpies are an average for the same bond across different molecules whereas standard enthalpy of combustion and formation apply to that molecule
What is the method for determination of △cH of a liquid fuel such as methanol?
- Using a measuring cyclinder, measure out 150cm3 of water into beaker, record initial temp of the water
- Add methanol to the spirit burner
- Place the spirit burner under the beaker and weigh it
- Light the burner and burn the methanol
- After about 3 mins extinguish the flame, record max temp reached by water
- Re weigh the spirit burner
How do you calculate the △cH of methanol after the experiment?
- Calculate the energy change q of the water in kJ using q=mct
- Calculate the amount of mol of CH3OH burnt
- Calculate the △cH in kJ by dividing energy change over moles of CH3OH
Why is data book value for △cH of methanol different to the experimental value?
- Heat loss to the surroundings
- Incomplete combustion of methanol
- Evaporation from the wick
- Non standard conditions
What is a simple experiment to determine the enthalpy change of a reaction?
Combining the 2 aq solutions into a polystrene cup and add a thermometer
What are the steps to calculate △rH for a solid and a solution?
- Calculate the energy change in the solution Q=mct
- Calculate the amount in moles of CuSO4 that reacted
- Calculate the △rH by dividing the energy change by the moles of CuSO4
When drawing a hess cycle for standard enthalpy changes of formation what direction are the arrows facing?
Upwards
When drawing a hess cycle for standard enthalpy changes of combustion what direction are the arrows facing?
Downwards
What does activation energy mean?
The minimum energy required for a reaction to take place
The actual conditions for the production of ethanol are 60 atmospheres pressure at 300 °C in the presence of a catalyst. Comment on why these conditions are used
- Too high pressure may be too expensive
- Relatively high temp so reaction isn’t slow without shifting equilibrium to the LHS
It is very difficult to determine the standard enthalpy change of formation of hexane directly. Suggest a reason why.
- Ea too high
- Many different hydrocarbons would form
Why do bond enthalpies have positive values?
Bond breaking is endothermic
State one important manufacturing process in which hydrogen is used
Haber process/ammonia/production of margarine
As the chain length in the alkanes increases, the value of ∆cH becomes more negative. Use your understanding of bond breaking and bond making to explain this trend.
- As you increase the chain length of an alkane there are more C-C and C-H bonds which both take in energy to break
- Since energy is taken in to break bonds the value for enthalpy of combustion will be negative for each bond
- So as you have more bonds the value of combustion becomes more negative
How do you calculate Ea for the reverse reaction?
Ea for forward reaction + enthalpy change
How do you calculate percentage uncertainty?
uncertainty/mass *100
change in temp = uncertainty/mass *2 *100
