C9-10. Enthalpy Changes Flashcards
How do we define enthalpy?
Enthalpy is a measure of the energy stored in a system.
How can we observe enthalpy change?
- Enthalpy changes occur when energy is given out or taken into account system during reaction. It cannot be directly measured.
- We observe it as a temperature change, using a thermometer or temperature probe.
How can we calculate enthalpy change?
- We use Q=mcT to find the total energy change in the system.
- This is then converted to kJ, before being divided by the number of moles of the subject of the calculation to calculate delta-H.
What are the standard conditions for measuring enthalpy changes?
- 100 kPa
- 298 K
- 1 mol dm-3
- Standard states of all reactants and products
Define standard enthalpy change of combustion.
The enthalpy change when 1 mole of a substance undergoes complete combustion under standard conditions.
Define standard enthalpy change of reaction.
The enthalpy change of a reaction in the molar quantities shown in the chemical formula under standard conditions.
Define standard enthalpy change of formation.
The enthalpy change when 1 mole of a substance is formed from its constituent elements under standard conditions.
Define standard enthalpy change of neutralisation.
The enthalpy change when 1 mole of water is produced during a reaction between an acid and base under standard conditions.
What is an exothermic reaction?
Exothermic reactions occur when the products formed have less energy than the reactants used, since energy is transferred to the surroundings.
What is an endothermic reaction?
Exothermic reactions occur when the products formed have greater energy than the reactants used, since energy is transferred from the surroundings.
How do we use cooling curves?
- Plot 2 lines of best fit: one for the starting temperature, and one for the temperature change.
- Extrapolate the latter to the time at which the reaction was initiated.
- Find the difference between this point and the initial temperature.
How can we practically measure enthalpy changes?
- We use an insulated reaction vessel containing reacting substances and a thermometer.
- We record temperature every minute until it is stable, before initiating the reaction and continuing to record temperature.
- We then use cooling curves to determine max. delta-H.
Why do we need to use cooling curves?
They allow us to estimate the enthalpy change at the instant of the reaction, at which point negligible energy has been lost to the surroundings.
What sources of error do cooling curves let us account for?
- Evaporation of the reacting substance
- Loss of thermal energy to the surroundings
What can often affect measured/estimated enthalpy change?
- Evaporation of reacting substances
- Loss of thermal energy to the surroundings through lack of insulation
- Other reactions occurring with different enthalpies such as incomplete combustion
What is the activation energy of a reaction?
The minimum energy required to enable to a reaction to take place. This is attained when the energy input is sufficient to break the bonds in the reactants.
Is bond breaking exo- or endo-thermic?
Energy is required to break bonds, therefore bond breaking is an endothermic process.
Is bond formation exo- or endo-thermic?
Energy is released when bonds are formed, therefore bond formation is an exothermic process.
How can we use bond enthalpies to estimate enthalpy change?
(sum of bonds broken) - (sum of bonds formed) = enthalpy change.
Why is the use of bond enthalpies of calculate enthalpy change only an estimation?
- The values we are provided are average bond enthalpies.
- These are an average value taking into account the enthalpy changes of the bond in all molecules involving the named bond.
- These vary between molecules.
Why is it difficult for us to measure enthalpies accurately or easily?
- The activation energy may be high.
- The reaction rate may be very slow.
- Multiple reactions could be occurring at once, especially during the formation of hydrocarbons.
Summarise Hess’ law.
- This states that the overall enthalpy change for a reaction is independent of the route taken, in a closed system.
- I.e. the enthalpy change of the direct route X>Y equals that of the indirect route X>Z>Y.
How can we use Hess cycles to determine overall enthalpy changes using enthalpy change of formation?
- Using formation, overall enthalpy change of X>Y would equal that of (Z>Y) - (X>Z).
- We know this because the arrows we draw point upwards from the elements.
How can we use Hess cycles to determine overall enthalpy changes using enthalpy change of combustion?
- Using combustion, overall enthalpy change of X>Y would equal that of (X>Z) - (Z>Y).
- We know this because the arrows we draw point downwards towards the elements.
How do we use Hess’ law to calculate overall enthalpy change for a reaction?
- If we are given known enthalpy changes of combustion/formation of an indirect reaction pathway, we can combine these algebraically to produce an overall enthalpy change.
- For enthalpy of formation: (Z>Y) - (X>Z) = (X>Y)
- For enthalpy of combustion: (X>Z) - (Z>Y) = (X>Y)
What happens to the temperature of the surrounding in…
- exothermic reactions
- endothermic reactions
- Temperature increases. Energy is transferred to the surroundings from the system.
- Temperature decreases. Energy is transferred from the surroundings to the system.
Why do reactions involving small activation energies often take place very readily?
The energy required to surpass this activation energy and break bonds is readily available from the surroundings alone.
What is the standard enthalpy change of formation of an element from itself?
This is always 0 kJ mol-1, since no change occurs when forming one mole of an element from itself. This is important in Hess cycles, where this idea comes up regularly.
What state do all species need to be in when determining enthalpy changes from average bond enthalpies? Why is this?
All species must be in the gaseous state. This is because average bond enthalpy is the energy required to break one mole of a specific bond in a gaseous molecule.
Define specific heat capacity.
- This is the energy required to raise the temperature of 1g of a substance by 1 K.
- Note that this differs from the definition of SHC in Physics.
- SHC is given by the letter c.
What assumptions can we make when completing calorimetry calculations?
- We can assume that the density of water in a calorimeter is 1 g cm-3.
- We can also assume that the specific heat capacity of water in a calorimeter is 4.18 J g-1 K-1.
