Topic 8 Flashcards
What is an exothermic process
Where heat energy is transferred from the system to the surroundings
What is an endothermic process
Where heat energy is transferred from the surroundings to the system
What are the standard conditions
100KPa, 298K
What is the ‘standard enthalpy change of reaction’
it is the enthalpy change measured at 100kPa and a stated temperature, usually 298K, when the number of moles of substances in the equation as written react
For an exothermic reaction is the enthalpy change positive or negative
Negative
For an endothermic reaction is the enthalpy change positive or negative
Positive
What is the standard enthalpy change of combustion
It is the enthalpy change measured in 100KPa and a specific temperature, usually 298K, when one mole of a substance is completely burned in oxygen
What experiment is used to measure the enthalpy change of combustion in a liquid
A known mass of liquid is burned and the heat energy produced is used to heat a known volume of water
What equation is used to calculate the enthalpy change of combustion
Q=mc,change of temp
m= the mass of the water
c= The specific heat capacity of water
(4.18)
What are some possible sources of error when experimenting the enthalpy change of combustion
- Some of the heat energy produced is transferred to the air and not the water
- The conditions are not standard
- Some of the ethanol may not burn completely to form carbon dioxide and water
What is the standard enthalpy change of neutralisation
It is the enthalpy change measured at 100KPa and a specified temperature, usually 298K, when one mole of water is produced by the neutralisation of an acid with an alkali
Why do the combustion of alcohols release less energy than the combustion of alkanes
- Alcohols already contain oxygen
- Alcohols are already partially oxidised
- Alcohols contain fewer carbon and hydrogen atoms
What in baking is sodium hydrogencarbonate used for
Used in baking powder, carbon dioxide is released causing cakes to rise
How can the experiment to record temperature change in a polystyrene cup be change to an alternative method to find a more accurate value for temperature change
- Record the temperature at timed intervals after adding the solid
- Plot the temperature values on graph
- Extrapolate the line of best fit to the time of adding the solid
If anhydrous copper(II) sulfate is pale blue rather than white, how does this affect the standard enthalpy change value
- the pale blue colour shows that it is partially hydrated
- therefore as it is less anhydrous it reacts with less water
- therefore less heat energy is released during the experiment
What are the reasons reactions are classified as combustion reactions
- oxygen is a reactant
- the reactions are shown as exothermic
What are the reasons why some reactions can not represent a standard enthalpy change of formation
- They have more than one product
- Two moles of the product are formed
What physical property should be kept constant when measuring an enthalpy change
Pressure
Why would using an old sample of hydrated sodium carbonate change the enthalpy change
- The enthalpy change of solution would be lower, less endothermic
- as anhydrous sodium carbonate released energy with water
- less energy is needed to separate the water molecules from the ions
