Exothermic: chemical reaction in which heat energy is given out.
Endothermic: chemical reaction in which heat energy is taken in.
(So, in an exothermic reaction the heat exits from the chemicals so temperature rises)
Calorimetry allows for the measurement of the amount of energy transferred in a chemical reaction to be calculated.
EXPERIMENT1: Displacement, dissolving and neutralisation reactions
Example: magnesium displacing copper from copper(II) sulfate
Method:
| Initial temp. of solution (oC) | Maximium temp. of solution (oC) | Temperature rise (oC) |
|---|---|---|
| 24.2 | 56.7 | 32.5 |
Note: mass of 50 cm3 of solution is 50 g
The cup used is polystyrene because:
polystyrene is an insulator which reduces heats loss
EXPERIMENT2: Combustion reactions
To measure the amount of energy produced when a fuel is burnt, the fuel is burnt and the flame is used to heat up some water in a copper container
Example: ethanol is burnt in a small spirit burner
Method:
| Mass of water (g) | Initial temp of water (oC) | Maximum temp of water (oC) | Temperature rise (oC) | Initial mass of spirit burner + ethanol (g) | Final mass of spirit burner + ethanol (g) | Mass of ethanol burnt (g) |
|---|---|---|---|---|---|---|
| 100 | 24.2 | 54.2 | 30.0 | 34.46 | 33.68 | 0.78 |
The amount of energy produced per gram of ethanol burnt can also be calculated:
Calorimetry allows for the measurement of the amount of energy transferred in a chemical reaction to be calculated.
EXPERIMENT1: Displacement, dissolving and neutralisation reactions
Example: magnesium displacing copper from copper(II) sulfate
Method:
| Initial temp. of solution (oC) | Maximium temp. of solution (oC) | Temperature rise (oC) |
|---|---|---|
| 24.2 | 56.7 | 32.5 |
Note: mass of 50 cm3 of solution is 50 g
EXPERIMENT2: Combustion reactions
To measure the amount of energy produced when a fuel is burnt, the fuel is burnt and the flame is used to heat up some water in a copper container
Example: ethanol is burnt in a small spirit burner
Method:
| Mass of water (g) | Initial temp of water (oC) | Maximum temp of water (oC) | Temperature rise (oC) | Initial mass of spirit burner + ethanol (g) | Final mass of spirit burner + ethanol (g) | Mass of ethanol burnt (g) |
|---|---|---|---|---|---|---|
| 100 | 24.2 | 54.2 | 30.0 | 34.46 | 33.68 | 0.78 |
The amount of energy produced per gram of ethanol burnt can also be calculated:
The symbol ΔH is used to represent the change in heat (or enthalpy change) of a reaction.
ΔH is measured in kJ/mol (kilojoules per mole).
The change in heat (enthalpy change) can be represented on an energy level diagram. ΔH must also labelled.
In an exothermic reaction, the reactants have more energy than the products.
Energy is given out in the form of heat which warms the surroundings.
ΔH is given a negative sign, because the reactants are losing energy as heat, e.g ΔH = -211 kJ/mol.
In an endothermic reaction, the reactants have less energy than the products.
Energy is taken in which cools the surroundings.
ΔH is given a positive sign, because the reactants are gaining energy, e.g ΔH = +211 kJ/mol.
During chemical reactions, the bonds in the reactants must be broken, and new ones formed to make the products.
Breaking bonds need energy and therefore is described as endothermic.
Energy is released when new bonds are made and therefore is described as exothermic.
If bonds are both broken and made during chemical reactions, why can a reaction overall be describe as either exothermic or endothermic?
Example: hydrogen reacts with oxygen producing water. Overall energy is released and therefore the reaction is exothermic.
The reaction is exothermic because the energy needed to break the bonds is less than the energy released in making new bonds.
If a reaction is endothermic then the energy needed to break the bonds is more than the energy released in making new bonds.
Each type of chemical bond has a particular bond energy. The bond energy can vary slightly depending what compound the bond is in, therefore average bond energies are used to calculate the change in heat (enthalpy change, ΔH) of a reaction.
Example: dehydration of ethanol
Note: bond energy tables will always be given in the exam, e.g:
| Bond | Average bond energy in kJ/mol |
|---|---|
| H-C | 412 |
| C-C | 348 |
| O-H | 463 |
| C-O | 360 |
| C=C | 612 |
So the enthalpy change in this example can be calculated as follows:
| Breaking bonds | Making bonds | ||
|---|---|---|---|
| Bonds | Energy (kJ/mol) | Bonds | Energy (kJ/mol) |
| H-C x 5 | (412 x 5) = 2060 | C-H x 4 | (412 x 4) = 1648 |
| C-C | 348 | C=C | 612 |
| C-O | 360 | O-H x 2 | (463 x 2) = 926 |
| O-H | 463 | ||
| Energy needed to break all the bonds | 3231 | Energy released to make all the new bonds | 3186 |
Enthalpy change, ΔH = Energy needed to break all the bonds - Energy released to make all the new bonds ΔH = 3231 – 3186 = +45 kJ/mol (ΔH is positive so the reaction is endothermic) |
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Calorimetry allows for the measurement of the amount of energy transferred in a chemical reaction to be calculated.
EXPERIMENT1: Displacement, dissolving and neutralisation reactions
Example: magnesium displacing copper from copper(II) sulfate
Method:
| Initial temp. of solution (oC) | Maximium temp. of solution (oC) | Temperature rise (oC) |
|---|---|---|
| 24.2 | 56.7 | 32.5 |
Note: mass of 50 cm3 of solution is 50 g
EXPERIMENT2: Combustion reactions
To measure the amount of energy produced when a fuel is burnt, the fuel is burnt and the flame is used to heat up some water in a copper container
Example: ethanol is burnt in a small spirit burner
Method:
| Mass of water (g) | Initial temp of water (oC) | Maximum temp of water (oC) | Temperature rise (oC) | Initial mass of spirit burner + ethanol (g) | Final mass of spirit burner + ethanol (g) | Mass of ethanol burnt (g) |
|---|---|---|---|---|---|---|
| 100 | 24.2 | 54.2 | 30.0 | 34.46 | 33.68 | 0.78 |
The amount of energy produced per gram of ethanol burnt can also be calculated:
