7. Energetics Flashcards
Describe the energy changes that take place when bonds are broken.
Energy must be put in to break bonds
What is thermochemistry?
The study of heat changes during chemical reactions.
Describe the energy changes that take place when bonds are formed.
Energy is given out
If at the end of a reaction, energy has been given out, what is he reaction called?
Exothermic
If at the end of a reaction, energy has been taken in, what is he reaction called?
Endothermic
Give an example of an exothermic reaction
Neutralising an acid with an alkali.
Give an example of an endothermic reaction
The thermal decomposition of calcium carbonate (limestone) to calcium oxide and carbon dioxide. (It needs heat to proceed).
Give a reversible reaction
CuSO4.5H2O CuSO4 + 5H2O
Blue copper sulfate anhydrous copper sulfate + water
—> endothermic (must heat the blue copper sulfate first)
<— exothermic
What is always true for an endothermic reaction?
It is always the case that a reaction that is endothermic in one direction is exothermic in the reverse direction and visa versa.
What unit is energy given out in reactions measured in?
Kilo joules per mole, kJ mol^-1
What is one important application of the study of thermochemistry?
It enables us to compare the efficiency of different fuels.
What are potential replacements for crude oil?
Ethanol and methanol - made form plant material
Hydrogen - made from the electrolysis of water
What is another name for the energy given out per gram of fuel burned?
The energy density
What is the problem with using hydrogen?
Hydrogen is a gas, it takes up a lot of space
What do chemists often use to maintain constant temperature?
Open flasks - there are slight variations form day to day but they only cause a small source of systematic error.
When we measure a heat change at constant pressure what do we call it?
Enthalpy change
What symbol does enthalpy have?
H
What are the standard conditions for measuring enthalpy changes?
Pressure of 100kPa
Temperature of 298K (25 degrees)
How do we get past the confusion of heat changes at a constant temperature?
We don’t think of the reaction as being over until the products have cooled down/heated up to 298K
How much energy do the products end up with in relation to the starting materials in an exothermic reaction?
The products end up with less heat energy than the starting material as they have heated up their surroundings. This means their enthalpy change is negative.
How much energy do the products end up with in relation to the starting materials in an endothermic reactions?
The products end up with more energy than the starting materials. They have taken in heat energy. Therefore the enthalpy change is positive.
What else effects the enthalpy change of a reaction?
The physical states of the reactants and products (e.g heat must be put in to change liquid to gas)
What must always be included in the equations of reactions when calculating enthalpy changes?
State symbol
What is sometimes used to show enthalpy changes?
Energy level diagrams
What are the x and y axis on the enthalpy diagrams?
X: extent of reaction Y: enthalpy
What does a standard exothermic energy level diagram look like?
The reactants are higher than the products; there is a drop down to represent enthalpy change. (Endothermic is the other way round).
What is the general name for the enthalpy change for any reaction?
The standard molar enthalpy change of reaction
Define the standard molar enthalpy of formation.
/\H*f is the enthalpy change when one mole of compound is formed form its constituent elements under standard conditions, all reactants and products in their standard states.
Define the standard molar enthalpy of combustion
/\H*c is the enthalpy change when one mole of compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states.
Define temperature
Temperature is the average kinetic energy of the particles in a system.
Define heat
Heat is a measure of the total energy of all the particles present in a given amount of substance .
Is there any instrument that measures heat directly?
No, there is no instrument that measures heat directly
Practically, how do we measure enthalpy change?
We arrange for the heat to be transferred into a particular mass of a substance, often water.
We need to know the mass of water, specific heat capacity of water and the temperature change to figure out the enthalpy change.
Give the enthalpy change equation.
Enthalpy change =
mass of substance x specific heat capacity x temperature change
q=m x c x /\T
What is the unit of specific heat capacity?
Jg^-1K^-1
Define specific heat capacity.
The amount of heat needed to raise the temperature of 1g of substance by 1K.
When we find an enthalpy change which piece of apparatus do we usually use?
A calorimeter
What is the improved version of the calorimeter called?
The flame calorimeter
How does the flame calorimeter reduce heat loss?
- spiral chimney made of copper
- flame enclosed
- fuel burns in pure oxygen, rather than air.
What is a very accurate version of the calorimeter called?
The bomb calorimeter
It is sealed so does not measure enthalpy but there is a simple relationship between measured energy change and enthalpy change.
What specific heat capacity do we usually give to dilute solutions?
The same one as water.
Why do we sometimes use expanded polystyrene beakers for calorimeters?
They are good insulators and have a low specific heat capacity so they absorb very little heat.
Are neutralisation reactions in solutions exothermic or endothermic?
Exothermic, they give out heat.
What is Hess’s law?
Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
What is Hess’s law a consequence of?
The Law of Conservation of Energy
Energy is neither created or destroyed
What does Hess’s law teach us in short?
That the total energy change is the same whichever route we take.
For thermochemical cycles where enthalpy of combustion is used the elements in the bubble are….? The arrows ……
Carbon dioxide, water and sometimes sulfur dioxide. The arrows come into the element bubbles.
The enthalpies of all elements in their standard states are taken as zero. What is the standard state of H2?
Zero, because hydrogen exists as H2 at room temperature and pressure.
What is the most stable carbon allotrope?
Graphite.
It is given ex special state symbol s, graphite so: C(s, graphite) represents graphite.
In thermochemical cycles for enthalpy of formation where do the arrows go?
The arrows go from the elements in the bubble to the equation.
How would you draw an enthalpy diagram?
Draw a line at level 0 to represent the elements
Look up the /\Hf for each compound and draw them on the diagram
/\Hf is the difference in levels taking account of the direction is change. Up is positive and down is negative.
(Put and arrow up to the side saying enthalpy)
What suggests that we can assign a definite amount of energy to a particular bond?
The graph of ‘/\H*f’ against ‘number of carbon atoms in the molecules’ is a straight line. These two values increase proportionally. With every new carbon added a CH2 group is aded. There is one extra C-C bond in the molecule and two extra C-H bonds.
What do we use in bond enthalpies?
An average value, the same bond in different molecules may have a slightly different Bon enthalpy.
The amount of energy given to the bond is the mean bond enthalpy.
We get the same amount of energy out when we make a bond as we put in when we…
…break a bond
Why will calculations using mean bond enthalpies only give approximate answers?
Because they are averages - bond enthalpy changes in certain molecules.
Why do we use mean bond enthalpies?
They are useful, quick and easy to use. They can also give an approximate value for a compound that has never been made.
Give an example of a mean bond enthalpy.
H-H bond energy is the energy required to separate the two atoms in a hydrogen molecule is the gas phase into separate gaseous atoms.
You can calculate enthalpy changes of reaction using mean bond enthalpies. Explain how.
Draw out the displayed formula of all the molecules. Count the bonds and add up energy taken to break bonds, and energy taken to make bonds.
If more energy was put in (energy taken to break) than given out (energy given from formation) the enthalpy change is positive.
Eg if endothermic: +
If exothermic: -
Do thermochemical cycles or mean bond enthalpies give the most accurate values?
Thermochemical cycles do - the enthalpies of formation have been obtained from the actual compounds involved.
