AS - Energetics Flashcards
What is enthalpy change? What are the units?
The heat energy transferred in a reaction at constant pressure.
Units are KJ mol-1
What are standard conditions?
100kPa and a stated temperature (usually 298K)
What is an exothermic reaction? What is the enthalpy change for an exothermic reaction?
A reaction that gives out energy where enthalpy change is negative.
What is an endothermic reaction? What is enthalpy change for an endothermic reaction?
A reaction that absorbs energy. Enthalpy change is positive.
Is bond breaking endothermic or exothermic and why?
Endothermic as energy is required to break bonds.
Is bond making endothermic or exothermic and why?
Exothermic because energy is released when bonds are made.
Define standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
Define standard enthalpy of combustion.
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions.
Define the term mean bond enthalpy.
The average energy required to break a certain type of bond, over a range of compounds.
What is bond enthalpy?
The energy required to break bonds.
Why are mean bond enthalpies always positive?
Because bond breaking is always endothermic.
What does the energy required to break a bond depend on?
The environment it is in.
In terms of energy absorbed and released, what basic equation denoted the enthalpy change of a reaction?
Total energy absorbed - total energy released
Why are enthalpy changes calculated using mean bond enthalpies not exact?
Because mean bond enthalpies are average values.
Are enthalpy change values from mean bond enthalpies more or less exact than those calculated using Hess’ law?
Less exact than Hess’ law.
How can calorimetry be used to find enthalpy changes?
You can find out how much heat is given out by a reaction by measuring the change in temperature using a calorimeter.
As the fuel burns, it heats the water. You can work out the heat energy that has been absorbed by the water if you know the mass of the water, the temperature change and the specific heat capacity of water.
What is the formula to calculate an enthalpy change from a calorimetry experiment? What does each letter mean and what are the units?
q=mc(deltaT)
Where:
q = heat lost or gained in joules (this is the same as enthalpy change is the pressure is constant) m = mass of water in the calorimeter in grams c = specific heat capacity of water in joules per gram per Kelvin DeltaT = the change in temperature of the water in Kelvin
What does the specific heat capacity of water mean?
The amount of heat energy it takes to raise the temperature of 1g of water by 1K.
Name two experimental problems that apply to all types of calorimetry.
- Some heat will be absorbed by the container, rather than going towards heating up the water.
- Some heat is always lost to the surroundings during experiment, no matter how well the container is insulated.
Name two experimental problems that only apply to flammable liquid calorimetry.
- Some combustion may be incomplete - which means less energy will be given out.
- Some of the flammable liquid may escape by evaporation as they’re usually quite volatile.
What is Hess’ law?
The total enthalpy change of a reaction is independent of the route taken.
What is the enthalpy change of formation for all elements?
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