3. PHYSICAL CHEMISTRY
A – Energetics:
3.1 – know that chemical reactions in which heat energy is given out are described as exothermic, and those
in which heat energy is taken in are described as endothermic
o The changes in heat content can be determined and measured with a thermometer
o During a chemical reaction, energy is exchanged between the system and surroundings
o The temperature of the system may rise or fall
o System – the reactants and products shown in the chemical equation
o Surroundings – energy else in the universe which is not the sytem (test tubes, beakers, hands)
Exothermic heat exits from the system
o Energy is transferred to the surroundings, which causes the surrounding temperature to rise
o Heat is given out to the surroundings
o Examples: combustion, explosion, burning, oxidation reactions
o Negative sign of energy change
Endothermic heat enters from the surroundings
o Energy is taken from the surroundings
o Surroundings: decrease in temp = rate of reaction increases
o Heat is taken in from the surroundings system has gained chemical energy
o The system has greater energy than before
o Positive sign of energy change
o Examples: thermal decomposition, melting ice, evaporation, cooking, alkane cracking, photosynthesis
3.2 – describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving
and neutralisation
o Calorimetry – technique used to measure the energy change during a reaction
o A polystyrene cup is used is an insulator reduces heat loss
o Can also reduce heat loss by using cotton wool to insulate the cup or add a lid
o Neutralisation + combustion = exothermic
o Dissolving + displacement = exothermic/endothermic
Dissolving, displacement and neutralisation:
o Mix the reactants together (make sure all the reactants begin at the same temperature)
o Measure the temperature afterwards
o Example: magnesium + copper (II) sulfate magnesium sulfate + copper
Combustion:
1. Measure the amount of energy produced when a fuel is burnt
2. Place some ethanol in a spirit burner
3. Measure initial mass of spirit burner + ethanol
4. Put 50 cm3 of water in a copper can and record the temperature
5. Place the copper can over the spirit burner
6. Stir the water continuous until the temp rises by 30 degrees
7. Turn the burner off but leave the can until the maximum temp of the
water is reached – record this
8. Weigh the spirit burner and remaining ethanol
9. Then calculate the enthalpy change
, 3. PHYSICAL CHEMISTRY
3.3 – calculate the heat energy change from a measured temperature change
o Q=m× c × Δ T
o Heat energy transferred ( J )=mass of solution being heated ( g ) × specific heat capacity
×temperature change of the liquid (̊ C )
o Specific heat capacity of water = 4.18 J/g/̊ C assume the same for aqueous solutions
o Mass of solution: ignore solids 1 cm3 or 1ml = 1g
3.4 – calculate the molar enthalpy change (ΔH) from the heat energy, Q
Q
o ΔH = Errors if theoretical value is less
moles
than calculated value:
energy change (kJ )
o Enthalpy change (kJ /mol)=
moles of substance which reacted (mol) o Heat/energy loss
o The energy change (Q) has to be in kJ, not J (as calculated in 3.3) divide o Incomplete combustion
by 1000
o Last step: add sign (minus sign = exothermic, plus sign = endothermic)
3.5 – draw and explain energy level diagrams to represent exothermic and endothermic reactions
Exothermic – ΔH = negative: Endothermic – ΔH = positive:
3.6
Heat is released Heat is absorbed
– know that bond-breaking is an endothermic process and that bond-making is an exothermic process
o Energy is needed to break bonds which is absorbed from the reaction surroundings endothermic
o Energy is released to form bonds back into the surroundings exothermic
o Whether a reaction is overall endo/exothermic depends on the difference between the sum of the bonds broken
and bonds made
o EXOTHERMIC STATEMENT: The energy released in making the product bonds is greater than the energy
required to break the reactant bonds
o ENDOTHERMIC STATEMENT: The energy required to break the reactant bonds is greater than the energy
released making bonds in the product
3.7 – use bond energies to calculate the enthalpy change during a chemical reaction
o Bond energy – measured in kJ/mol
Bond energies are always given in the
o Energy change ( ΔH ¿ = energy taken in – energy given out
test
o Find difference between bonds broken and the bonds formed
o If answer is positive number endothermic (vice versa)
3.8 – practical: investigate temperature changes accompanying some of the following types of change: salts
dissolving in water, neutralisation reactions, displacement reactions and combustion reactions
A – Energetics:
3.1 – know that chemical reactions in which heat energy is given out are described as exothermic, and those
in which heat energy is taken in are described as endothermic
o The changes in heat content can be determined and measured with a thermometer
o During a chemical reaction, energy is exchanged between the system and surroundings
o The temperature of the system may rise or fall
o System – the reactants and products shown in the chemical equation
o Surroundings – energy else in the universe which is not the sytem (test tubes, beakers, hands)
Exothermic heat exits from the system
o Energy is transferred to the surroundings, which causes the surrounding temperature to rise
o Heat is given out to the surroundings
o Examples: combustion, explosion, burning, oxidation reactions
o Negative sign of energy change
Endothermic heat enters from the surroundings
o Energy is taken from the surroundings
o Surroundings: decrease in temp = rate of reaction increases
o Heat is taken in from the surroundings system has gained chemical energy
o The system has greater energy than before
o Positive sign of energy change
o Examples: thermal decomposition, melting ice, evaporation, cooking, alkane cracking, photosynthesis
3.2 – describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving
and neutralisation
o Calorimetry – technique used to measure the energy change during a reaction
o A polystyrene cup is used is an insulator reduces heat loss
o Can also reduce heat loss by using cotton wool to insulate the cup or add a lid
o Neutralisation + combustion = exothermic
o Dissolving + displacement = exothermic/endothermic
Dissolving, displacement and neutralisation:
o Mix the reactants together (make sure all the reactants begin at the same temperature)
o Measure the temperature afterwards
o Example: magnesium + copper (II) sulfate magnesium sulfate + copper
Combustion:
1. Measure the amount of energy produced when a fuel is burnt
2. Place some ethanol in a spirit burner
3. Measure initial mass of spirit burner + ethanol
4. Put 50 cm3 of water in a copper can and record the temperature
5. Place the copper can over the spirit burner
6. Stir the water continuous until the temp rises by 30 degrees
7. Turn the burner off but leave the can until the maximum temp of the
water is reached – record this
8. Weigh the spirit burner and remaining ethanol
9. Then calculate the enthalpy change
, 3. PHYSICAL CHEMISTRY
3.3 – calculate the heat energy change from a measured temperature change
o Q=m× c × Δ T
o Heat energy transferred ( J )=mass of solution being heated ( g ) × specific heat capacity
×temperature change of the liquid (̊ C )
o Specific heat capacity of water = 4.18 J/g/̊ C assume the same for aqueous solutions
o Mass of solution: ignore solids 1 cm3 or 1ml = 1g
3.4 – calculate the molar enthalpy change (ΔH) from the heat energy, Q
Q
o ΔH = Errors if theoretical value is less
moles
than calculated value:
energy change (kJ )
o Enthalpy change (kJ /mol)=
moles of substance which reacted (mol) o Heat/energy loss
o The energy change (Q) has to be in kJ, not J (as calculated in 3.3) divide o Incomplete combustion
by 1000
o Last step: add sign (minus sign = exothermic, plus sign = endothermic)
3.5 – draw and explain energy level diagrams to represent exothermic and endothermic reactions
Exothermic – ΔH = negative: Endothermic – ΔH = positive:
3.6
Heat is released Heat is absorbed
– know that bond-breaking is an endothermic process and that bond-making is an exothermic process
o Energy is needed to break bonds which is absorbed from the reaction surroundings endothermic
o Energy is released to form bonds back into the surroundings exothermic
o Whether a reaction is overall endo/exothermic depends on the difference between the sum of the bonds broken
and bonds made
o EXOTHERMIC STATEMENT: The energy released in making the product bonds is greater than the energy
required to break the reactant bonds
o ENDOTHERMIC STATEMENT: The energy required to break the reactant bonds is greater than the energy
released making bonds in the product
3.7 – use bond energies to calculate the enthalpy change during a chemical reaction
o Bond energy – measured in kJ/mol
Bond energies are always given in the
o Energy change ( ΔH ¿ = energy taken in – energy given out
test
o Find difference between bonds broken and the bonds formed
o If answer is positive number endothermic (vice versa)
3.8 – practical: investigate temperature changes accompanying some of the following types of change: salts
dissolving in water, neutralisation reactions, displacement reactions and combustion reactions
