Section 4 - Pgs 59-63 Flashcards
Does breaking bonds require energy to be supplied or is energy released?
Energy has to be supplied to break bonds. (ENDOTHERMIC)
Does forming bonds require energy to be supplied or is energy released?
Energy is released when new bonds are formed. (EXOTHERMIC)
What is an exothermic reaction?
One which gives out energy to the surroundings, usually in the form of heat.
How can an exothermic reaction be detected?
By a rise in temperature (of the surroundings).
What causes an exothermic reaction to be exothermic?
The energy released in bond formation is greater than the energy required to break the original bonds.
What is an endothermic reaction?
One which takes in energy from the surroundings, usually in the form of heat.
How can an endothermic reaction be detected?
By a fall in temperature (of the surroundings).
What causes an endothermic reaction to be endothermic?
The energy required to break the original bonds is greater than the energy released in bond formation.
What is enthalpy change?
The overall change in energy in a reaction (per mole).
What is the symbol for enthalpy change?
ΔH
What is the unit for enthalpy change (ΔH)?
kJ/mol (Kilojoules per mole of the reactant)
When is enthalpy change (ΔH) positive and why?
When the reaction is endothermic - because the reaction takes IN energy.
When is enthalpy change (ΔH) negative and why?
When the reaction is exothermic - because the reaction is giving energy OUT.
What is an energy level diagram?
A diagram used to show how the energy in a reaction changes as the reaction progresses.
What is on the x and y axis of an energy level diagram?
x axis - Progress of reaction
y axis - Energy
What does the height from the reactants to the top of the curve represent on an energy level diagram?
The activation energy.
What is activation energy?
The minimum energy required to break the original bonds in the reactants (i.e. How much energy in needed for the reaction to happen).
In an exothermic energy level diagram, are the products higher or lower than the reactants and why?
Lower - because energy is given out from the reaction, reducing how much energy is in the reaction.
(Diagram pg 60)
In an endothermic energy level diagram, are the products higher or lower than the reactants and why?
Higher - because energy is taken in by the reaction, increasing how much energy is in the reaction.
(Diagram pg 60)
What does the height from the reactants to the products represent on an energy level diagram?
The enthalpy change.
What can lower the activation energy of a reaction?
A catalyst.
How does a catalyst work?
It provides an alternative pathway for the reaction with a lower activation energy.
How is a catalyst represented on an energy level diagram?
A lower curve which reaches the products line earlier.
Does a catalyst change the overall energy change for a reaction?
No, it remains the same.
What is a catalyst?
A substance that can speed up a reaction (without being used up itself) by providing an alternative route to the product which has a lower activation energy.
What is a bond energy?
- The amount of energy that is required to break a certain bond or is released by its formation.
- It is specific to each bond (e.g. H-H has a specific bond energy.
Are bond energies for a specific bond always the same?
No, they can vary depending on the compound they are in. (In the exam you are provided with an average though).
What are bond energies used for?
Calculating enthalpy change of a reaction.
What is the equation for calculating enthalpy change from bond energies?
Enthalpy Change = Total Energy to Break Bonds - Total Energy Released in Making Bonds
Calculate the enthalpy change for: H2 + Cl2 -> 2HCl Bond energies: H-H: +436kJ/mol Cl-Cl: +242kJ/mol H-Cl: +431kJ/mol
1) Breaking 1 mole of H-H and 1 mole of Cl-Cl bonds requires:
436 + 242 = 678kJ
2) Forming 2 moles of H-Cl bonds releases:
2 x 431 = 862kJ
3) ΔH = 678 - 862 = -184kJ/mol
4) The ΔH is negative so the reaction much be exothermic.
What is calorimetry?
The determining of energy change in a reaction by measuring heat exchanges with with surroundings.
How is calorimetry carried out in dissolving, displacement and neutralisation reactions?
The reagents should be in solution.
1) The temperature of the reagents is measured (making sure it is the same).
2) The reagents are mixed in a polystyrene cup.
3) Heat loss can be reduced by putting the cup in a beaker of cotton wool and putting a lid on the cup.
3) The temperature of the products is measured after the reaction (when the temperature reaches its highest or lowest).
4) Temperature change is calculated and, from this, enthalpy change is calculated.
In calorimetry, how do you know when the reaction has finished?
You measure the highest (exothermic) or lowest (endothermic) temperature it reaches.
How is calorimetry carried out in combustion reactions?
1) A set amount of water is put into a copper can and its temperature is recorded.
2) The spirit burner and lid is weighed.
3) The spirit burner is put under the copper can and lit.
4) Draughts should be reduced using a screen to maximise energy transfer.
5) The water is heated, stirring, until the temperature reaches an approximate set temperature (e.g. 50*C).
6) The flame is put out using the lid and the precise temperature of the water is measured.
7) The spirit burner and lid is weighed again.
8) Enthalpy change is calculated.
What is the equation for energy change (used in enthalpy calculations)?
Energy Change = Mass of Solution x Specific Heat Capacity (4.18J/g/*C) x Change in Temperature
Q = m x c x ΔT
When calculating energy change, what mass is used (as ‘m’)?
- If a solid is dissolved in another or if some water is heated by a fuel (i.e. when there is only one liquid involved), the mass of the solution is used.
- If two solutions are mixed, it is the sum of their two masses.
(i. e. Just use the heat of the thing that is being heated up)
How can you calculate the molar enthalpy change of a reaction?
1) Find Q (energy change) using the mass of the solution (where 1cm3 = 1g), the specific heat capacity (4.18J/g/*C) and the change in temperature of the solution. (Q = m x c x ΔT)
2) This gives you the overall energy change for the reaction.
3) Work out the moles of the fuel used or of one of the reactants (if it is a dissolving, displacement or neutralisation reaction) depending on what the question asks.
4) For solids: Moles = Mass / Mr
For solutions: Moles = (Volume / 1000) x Concentration
5) Find ΔH by dividing the energy change by the moles. (ΔH = Q / moles)
6) Remember to change ΔH to a negative value if the reaction is exothermic!
What is the value for specific heat capacity of water?
4.18*C
What is specific heat capacity?
The amount of energy required to heat 1g of a certain solution up by 1*C.
