chapter 9 Flashcards
describe the conservation of energy
- energy cannot be created or destroyed
- when a reaction involving enthalpy changes takes place, heat energy is transfered between the system and the surroundings
what is enthalpy
- the measure of heat energy present in a chemical system
what is enthalpy change
- the amount of energy released or taken in during a chemical reaction
what is an endothermic reaction
- a reaction which takes in energy from the surroundings
- it has a positive enthalpy value
what is an exothermic reaction
- a reaction which releases energy into the surroundings
- has a negative enthalpy value
what is activation energy
- the minimum energy required for a reaction to take place
what are standard conditions
- 100 kPa
- 298 K
- standard states
what is the standard enthalpy change of a reaction
- the enthalpy change associated with a stated equation in the molar quantities shown in the chemical equation under standard conditions
what is the standard enthalpy change of formation
- the enthalpy change associated with the formation of 1 mole of a compound from its elements under standard conditions
what is the standard enthalpy change of combustion
- the enthalpy changes associated with the complete combustion of 1 mole of a substance under standard conditions
what is the standard enthalpy change of neutralisation
- the enthalpy change associated with the formation of 1 mole of water from a neutralisation reaction between an acid and a base, under standard conditions
describe an experiment to find the enthalpy change
- calorimeter experiment
- measure a volume of water into the calorimeter
- weigh the spirit burner containing the fuel with the lid on
- measure the initial temperature of the water
- place the spirit burner under the calorimeter
- light the spirit burner and heat the water, stirring with the thermometer
- use the lid of the spirit burner to put out the flame
- record the highest temperature reached by the water
- weigh the spirit burner with the lid on and calculate the change in mass
what are the 4 biggest flaws in a calorimetry experiment
- heat loss to the surroundings
- incomplete combustion of the fuel
- evaporation of the fuel
- not performed under standard conditions
what are cooling curves used for
- can be used to increase the accuracy of the heat energy value as it corrects for heat loss during the experiment
what is average bond enthalpy
- the energy required to break one mole of a specified type of bond in a gaseous molecule
- in chapter 5.3 its a measurement of the strength of a covalent bond, the larger the bond enthalpy value, the stronger the bond
what are the limitations of average bond enthalpies
- text book values often use the actual bond enthalpy values, whereas we use the average bond enthalpy values
- the experiment is not performed under standard conditions
what makes a reaction endothermic
- more energy is required to break bonds than released when making bonds
what makes a reaction exothermic
- more energy is released when making bonds than required to break the bonds
what is the calculation for calculating enthalpy changes
with enthalpies of reactants and products
- the sum of the enthalpies in the reactants - the sum of bond enthalpies in products
what is hess’ law
- if a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
