Chapter 8 - Chemical energies (L)
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What two components make up chemical energy?
Kinetic energy Potential energy
What is kinetic energy a measure of?
The motion of particles in a substance
What is potential energy a measure of?
How strongly particles in a substance interact with one another (attract and repel)
What is heat energy?
The portion of kinetic and potential energy of a substance that is responsible for the temperature of the substance
What is heating of a substance the result of?
The transfer of heat energy from one system to another, which in turn produces a change in temperature
What is the enthalpy change of a process?
The heat energy that is transferred between system and surroundings
What is an exothermic change?
When heat energy is transferred from the system to the surroundings
What is an endothermic change?
When heat energy is transferred from the surroundings to the system
In a chemical reaction, what does ‘system’ refer to?
The reaction mixture
In a chemical reaction, what does ‘surroundings’ refer to?
Everything outside of the system (which in practice is the air in the room of the reaction)
What is the definition of standard enthalpy change of reaction?
The enthalpy change of a reaction according to the molar quantities in the equation, under standard conditions
What are standard conditions?
100kPa pressure and a stated temperature (often 298K)
At the end of exothermic reactions, which has a greater enthalpy, reactants or products?
Reactants have a greater enthalpy, as during the reaction, heat energy has been transferred from the system to the surroundings
At the end of endothermic reactions, which has a greater enthalpy, reactants or products?
The products will have a greater enthalpy than the reactants, as heat energy has been transferred from the surroundings into the system
In exothermic reactions, is the enthalpy change positive or negative?
Negative
In endothermic reactions, is the enthalpy change positive or negative?
Positive
How do you measure the change in enthalpy?
Hproducts - Hreactants
What is the definition of standard enthalpy of neutralisation?
The enthalpy change when an acid and an alkali react to form 1 mole of water, under standard conditions
What is the definition of standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions
What is the definition of standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions
What type of process is bond breaking?
An endothermic process, as energy needs to be absorbed for the bonds to break
What type of process is bond forming?
Exothermic, as energy is released as a result of bonds being formed
How do you work out enthalpy change from bond energies?
Energy needed to break the bonds - energy released from bond formation
What is bond enthalpy?
The amount of energy required to break 1 mole of a bond type in a molecule (in gaseous state)