1.4 - Energetics Flashcards
What is the definition for ‘enthalpy change’?
Enthalpy change is the heat energy transferred in a reaction at a constant pressure. The units are kJ/mol
What are standard conditions?
1 atm in pressure and at a stated temperature
What is an exothermic reaction?
Exothermic reactions give out energy to their surroundings so the temperature in the reaction increases. The enthalpy change will be negative as the products end up with less energy than the reactants
What is an endothermic reaction?
Endothermic reactions take in their energy from their surroundings so the temperature in the reaction usually falls. The enthalpy change will be positive as the products end up with more energy than the reactants
What is ‘bond enthalpy’?
Bond enthalpy is the energy needed to break a bond
Is bond-breaking endothermic or exothermic?
Energy is needed to break bonds, so it is endothermic
Is bond-making endothermic or exothermic?
Energy is released when bonds are formed, so it is exothermic
What is ‘mean bond enthalpy’?
The average energy needed to break a certain type of bond over a range of compounds. This is because the energy required to break a certain type of bond can change depending on where it is
What is the equation that links together enthalpy change of a reaction, total energy released and total energy absorbed?
Enthalpy Change of a Reaction = Total Energy Absorbed (Reactants) - Total Energy Released (Products)
What is the definition for the ‘Standard Enthalpy Change of Formation’?
The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
What is the definition for the ‘Standard Enthalpy Change of Combustion’?
The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states
What is the definition for the ‘Standard Enthalpy Change of Reaction’?
The standard enthalpy change of a reaction is the enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under standard conditions with all reactants and products in their standard states
What is a ‘calorimetry’ experiment?
Experiments that measure the heat given out by reactions
Why are calorimetry experiments hard to get an accurate result?
Some heat is always lost to the surroundings
Some of the combustion may be incomplete, which means less energy is given off
Flammable liquids are also quite volatile and so you may lose some fuel by evaporation
How is an accurate temperature change calculated?
1) During the experiment, record the temperature at regular intervals, beginning a couple of minutes before the reaction begins
2) Plot a graph of the results
3) Draw two lines of best fit; one going through the points before the reaction and one going through the points after the reaction
4) Extend both lines so that they both pass the time when the reaction started
5) The distance between the two lines at the time the reaction started is the accurate temperature change
