Energetics Flashcards
What is the conservation of energy?
Energy cannot be created or destroyed only stored or transferred
Chemical bonds are the forces of attraction the bind atoms together. They hold a potential energy.
Define enthalpy
The thermal energy that is stored in a chemical system.
What are the standard conditions for enthalpy reactions?
Pressure: 100KPa
Temperature: 298K
What is the system and what are the surroundings?
System- chemical reactions (atoms and bonds)
Surroundings- everything else
Define enthalpy change
The difference between the enthalpy of the products and the reactants
Change in enthalpy= Hproducts - Hreactants
What are exothermic reactions?
When the enthalpy of the products is lower than the enthalpy of the reactants and therefore heat energy has been lost to the surroundings
What are endothermic reactions?
When the enthalpy of the products is greater than the enthalpy of the reactants and therefore energy has been absorbed by the reactants to form bonds.
What is activation energy?
The minimum energy require to break bonds in the reactants for a reaction to take place
What is Hess’s law?
The enthalpy change of a reaction is independent of the route taken. It relies only on be initial and final states of the reaction.
Define the enthalpy change for a reaction.
The enthalpy change when the amounts of reactants show in the equation react under standard conditions to give their products in their standard states.
Define enthalpy change of formation.
The enthalpy hangs when 1 mole of a compound is formed from its elements in their standard states under standard conditions
Define the enthalpy change of combustion
The enthalpy change when 1 mole of a substance is burnt completely in Excess oxygen under standard conditions
Define enthalpy of change of neutralisation.
The enthalpy change when solutions of acid and alkali react together under standard conditions to produce 1 mole of water.
Define the enthalpy change of solution.
The enthalpy change when 1 mole of solute dissolved completely in sufficient solvent to form a solution in which the ions are far enough apart to not interact with each other.
Define the mean bond enthalpy
The mean /average energy required for 1 mole of a given type of gaseous bond to undergo homolytic fission
How do you calculate enthalpy?
q=mc(change in temperature)
What is calorimetry?
The study of chemical energy in a chemical reaction
Explain the coffee cup calorimeter process.
- Add a measured mass of the first reactant (if solid then dissolve in the appropriate solvent)
- Add the second reactant
- Monitor the temperature every couple of minutes (not the minute the second reactant is added)
- Play a graph.
Explain the spirit burner calorimeter process.
- Weigh the spirit burner containing the test liquid.
- Add a know volume of water to the copper can
- Use the thermometer to record the waters initial temperature
- Light the spirit burner
- Continuously stir the water
- After a few mins extinguish the burner and immediately re-weigh the burner.
- Calculate mass of fuel burnt
- Measure final temperature of the water
What are the possible errors and assumptions made when doing calorimetry (spirit burner)
Some energy produced from burning the fuel is no transferred to the water (instead the air or copper tin)
Incomplete combustion of fuel may occur
Conditions are not standard
Some alcohol or water may evaporate
Explain in terms of bond energy an endothermic reaction
More energy is needed to break bonds in the reactants that is released when bonds are formed in the products so the products have a higher enthalpy and H is positive
Explain in terms of bond energy an exothermic reaction
More energy is given out when the bonds in the products are formed than is absorbed when the bonds in the reactants are broken
So reactants have a higher enthalpy
What is the equation for enthalpy change?
Enthalpy change = enthalpy of the products - enthalpy of the reactants
How do you work out the enthalpy change of combustion from calorimetry?
- Find Q (energy in joules)
- Find moles of fuel burnt etc (mass of Mr)
- Enthalpy change is Q / n (in joules)
- Convert to KJ
- state whether it is exo or endothermic (+/-)
Explain why calculate mean bond enthalpies are different to those calculated form experimental formation?
The mean bond enthalpy are the average enthalpy from a range of compounds and therefore will not be exact
How do you calculate the enthalpy of a reaction from the mean bond enthalpies?
Enthalpy = SUM(bonds broken in reactants)-SUM(bonds formed in the products)
