Enthalpy changes Flashcards
Define enthalpy
Enthalpy is the measure of heat energy in a chemical system.
Define enthalpy change
Difference in enthalpy between the products and reactants of a chemical reaction. ΔH = H(products) - H(reactants)
Define Exothermic enthalpy changes
Energy (chemical) is transfered from the system to the surroundings (in the form of heat).
Define endothermic enthalpy changes
Energy (heat) is transfered from the surroundings to the system (in the form of chemical energy).
Define activation energy
The minimum energy required for a reaction to take place.
What happens to the temperature of the surroundings in an endothermic process?
tempertaure of the surroundings decreases
What happens to the temperature of the surroundings in an exothermic process?
Temperature of the surroundings increases
In an exothermic enthalpy profile diagram are the products higher or lower than the reactants and why?
Lower as the products have lost chemical energy lowering their enthalpy.
In an endothermis enthalpy profile diagram are the products higher or lower than the products and why?
Higher as the products have gained chamical energy increasing their enthalpy.
what are the standard conditions and standard states (4)
- Pressure = 100 KPa
- Temperature = 298 K
- Concentration = 1 moldm -3
- State = physical state of a substance under standard conditions.
Define the enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical reaction.
Under standard conditions, with all reactants and products in their standard states.
Define the enthalpy change of formation
The enthalpy change that occurs when one mole of a substance is formed from its elements.
Under standard conditions, with all reactants and products in their standard states.
Define the enthalpy change of combustion
the enthalpy change that occurs when one mole of a substance reacts completely with oxygen.
Under standard conditions, with all reactants and products in their standard states.
Define the enthalpy change of neutralisation
The enthalpy change that occurs when an acid and a base react completey to form one mole of water.
under standard conditions, with all reactants and products in their standard states.
What equation is used to calculate energy/ enthalpy change
q= m c ΔT
Describe the process to find the enthalpy change of combustion
write this down
- Measure a volume of water into a beaker (200ml).
- Record the intial temperature of the water in the beaker by placing a thermometer in the beaker.
- Add combustable fuel (methanol) to a spirit burner and measure its weight.
- Place the spirit burner under the beaker of water.
- Light the spirit burner and stir the water using the thermometer.
- After 3 minutes measure the temperature of water, then extinguish the flame.
- Measure the mass of the spirit level.
- calculate the moles of fuel burnt by dividing the mass lost (initial - final) by the mr of the fuel.
- calculate the energy transfered to the water, q = m c ΔT.
- Divide the enregy gained by water, by the moles of fuel used to get a value of kJ mol -1, enthalpy of combistion value.
What Inaccuracies are there when finding the enthalpy of combustion?
- Heat may be lost to surroundings other than the water, such a the beaker or air.
- Incomplete combustion may occur.
- Fuel (e.g. methanol) may evaporate.
- Conditions may not be standard.
Describe how to determine the enthalpy change of reaction.
- Place one solution into a polystyrene cup.
- Add a thermometer to the cup and measure the initial temperature of the solution.
- Add a excess of powder or solution into the cup.
- Stir the solution.
- Measure the temperature regularly recording the maximum temperature.
- Use q = m c ΔT to calculate the energy produced.
- Calculate the moles of the initial solution in the cup (not excess solution or powder)
- Divide the energy transfered by the moles of solution/ substance not in excess to get a enthalpy of reaction.
Describe how to determine the enthalpy change of neutralisation.
- Place one solution into a polystyrene cup acid or base (must be the one of known mass or conc and vol).
- Add a thermometer to the cup and measure the initial temperature of the solution.
- Add a excess of the other solution into the cup.
- Stir the solution.
- Measure the temperature regularly recording the maximum temperature.
- Use q = m c ΔT to calculate the energy produced.
- Calculate the moles of the water produced by the reaction.
- Divide the energy produced by the reaction, by the mols of water produced, to get the enthalpy of neutralisation.
Define average bond enthalpy
The energy required to break one mole of a specific type of bond in a gaseous molecule.
why is there an average bond enthalpy?
The actual bond enthalpy can vary in different chemical enviroments of different molecules. The average bond enthalpy is calculated from actual bond enthalpies.
