Chapter 8 - Chemical energies (L) Flashcards
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)
Why do we take a mean bond enthalpy for the same type of bond (e.g. C-H bonds)?
Because different C-H bonds will have different individual bond enthalpies
In calorimetry, what is the equation to work out heat energy change (Q)?
Q= m x c x ∆T
What 3 things do you need to know to work out q (heat energy change)?
Mass of solution (m)
Specific heat capacity (c)
Change in temperature (∆T)
What is the equation to work out enthalpy change after working out Q?
Enthalpy change = energy change / moles
What are possible sources of error in working out the enthalpy of combustion?
- Some heat energy produced would be transferred to the air and the copper can and not the water
- Some incomplete combustion may occur, meaning the substance is not completely burned in oxygen
How do you conduct an experiment to work out the enthalpy of combustion for ethanol?
- Add known volume of water to copper can and measure temperature of water before and after
- Weigh the spirit burner containing ethanol before and after
- Work out temperature change and mass of ethanol burned
How do you conduct an experiment to work out the enthalpy of neutralisation?
- Add 25cm³ of 1 mole/dm³ acid into a polystyrene cup, and measure the temperature of the acid
- Measure the temperature of the alkali, and then add 25cm³ of the alkali to the acid
- Stir the solution until the maximum temperature is reached
How do you work out temperature change in enthalpy of neutralisation experiments?
Take the average of the acid and alkali initial temperatures.
Subtract this value from the final solution temperature.
What else must you work out in enthalpy of neutralisation calculations?
The moles, by multiplying the concentration and the volume (in dm)
What uncertainties are there in enthalpy of neutralisation experiments?
- Usual uncertainties of measurement involved with the use of the pipette and thermometer
- Some heat energy would be transferred to the thermometer and cup
Why are state symbols important in standard enthalpy of formation tests?
Because elements must be in their standard states; the value for enthalpy change of formation for gaseous ethanol is different to that of liquid ethanol
Why is Hess’s law often used in standard enthalpy of formation calculations?
As most standard enthalpy of formation cannot be determined experimentally
What does Hess’s law state?
The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.
I.e. going from reactants A -> products B and reactants A -> intermediates -> products B will have the same overall enthalpy change (much like vectors in maths)
