Enthalpy Changes Flashcards
(18 cards)
Define enthalpy change
The amount of heat energy transferred during a chemical reaction at constant pressure
How do you know if your reaction is exothermic?
The change in heat enthalpy is negative
In an exothermic reaction, energy is transferred from the system to the surroundings
How do you know is your reaction is endothermic?
The change in heat enthalpy is positive
In an endothermic reaction, energy is transferred from the surroundings to the system
What are common examples of exothermic reactions?
Combustion
Respiration
Neutralisation
What are common examples if endothermic reactions?
Photosynthesis
Thermal Decomposition
Define activation energy
The minimum energy required for a reaction to take place by breaking bonds in reactants
What are the standard conditions for temperature,pressure,concentration?
Temperature: 298K
Pressure: 100kPa
Concentration: 1.00 moldm-3
Define the standard enthalpy change of a reaction. Is it exothermic or endothermic?
The enthalpy change for a given reaction in the molar quantities shown in the equation
It can be either endothermic or exothermic
Define standard enthalpy of combustion. Is it exothermic or endothermic?
The enthalpy change when one mole of a substance completely combusts at 298K and 100kPa
It is an exothermic reaction
Define standard enthalpy of formation. Is it exothermic or endothermic?
The enthalpy change when one mole of a compound is formed from its elements at 298K and 100kPa
It can be either exothermic or endothermic
Define standard enthalpy change of neutralisation
Is it exothermic or endothermic?
The enthalpy change when one mole of H2O is formed from reaction of H+ and OH- at 298K and 100kPa
It is an exothermic reaction
What is the equation for calculating energy change?
Q=mc🔺T
Q is energy change in Joules
M is mass of solution in grams
C is specific heat capacity
T is change in temperature
What is the equation for calculating enthalpy change?
🔺H=q/n
H is enthalpy change in kjmol-1
q is energy change in kilojoules
n is moles of reactant
Why might the calculated value for enthalpy change be different from the data book value?
Heat Loss
Incomplete Combustion
What are examples of sources of error in combustion calorimetry?
Data book value uses standard condition values
Evaporation of alcohol from wick
Evaporation of water from beaker
How can we minimise combustion calorimetry errors?
Ensure a plentiful supply of oxygen
Add a lid to the beaker
Use a copper can instead of a beaker
Cover wick when not burning
What are sources of error in calorimetry (reaction in solution)?
Heat loss to surroundings
Heat gain from surroundings
Incomplete reaction
Water evaporates from the beaker
Specific Heat Capacity of solution is not the same as water
How can we minimise errors in calorimetry (reaction in solution)?
Use a polystyrene cup
Ensure standard conditions
Use a lid
