11 Energetics
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Is bond making exothermic or endothermic?
exothermic energy is released when bonds are formed
Is bond breaking endothermic or exothermic?
endothermic Energy must be added to break bonds
What does the quantity symbol H stand for? What is its unit?
molar enthalpy kJ / mol
What does the quantity Q stand for in Q = mc(change in T)? What is its unit?
Q stands for energy Unit : Joules (J)
What is the relationship between energy, mass, specific heat capacity and change in temperature?
Q = mc (Change in T)
What does a negative change in enthalpy value mean?
The reaction is exothermic
What does a positive change in enthalpy value mean?
The reaction is endothermic
During the complete combustion of propanol, 100 cm3 of water is heated and the temperature of the water rose from 23ºC to 43ºC. The specific heat capacity of water is 4.2 J/gºC. How is the energy released by the propanol be calculated?
• Find energy released using: Q =mc(change in T) Q = 100g x 4.2J/gºC x 20ºC
To determine the enthapy of reaction for the combustion of propanol, 100 cm3 of water is heated. The temperature started at 23ºC and rose to 46ºC. What other measurement/s must be made to calculate the enthalpy of reaction in kJ/mol?
Mass of spirit burner before AND after. • Find energy released using: Q =mc(change in T) 2: Find moles of propanol reacted using: mole of fuel burnt = mass / RFM of propanol • Calculate enthapy change using Enthalpy = Q / moles
To determine the enthapy of reaction for the complete combustion of propanol, 100 cm3 of water is heated. The temperature started at 23ºC and rose to 46ºC. State three things which must be done to ensure accurate results?
• Water must be stirred • Temperature must be read every minute after the spirt burner is blown out in order to record maximum temperature • Thermometer must not be touching the bottom of the beaker
Using the energy profile diagram below, determine if the reaction is exothermic or endothermic
exothermic change in enthalpy is negative
Using the energy profile diagram below, determine if the reaction is exothermic or endothermic?
Endothermic Enthalpy change is positive
Below is a diagram showing that more energy is released making bond in this reaction than energy supplied breaking bonds. Is this reaction exothermic or endothermic?
exothermic
Metal Displacement reactions of metals and metal salts are exothermic. Mg + CuSO4 –> MgSO4 + Cu How would the temperature change of the solution compare between magnesium in copper sulfate and zinc in copper sulfate?
The temperature rise would be greater for magnesium in copper sulfate than zinc in copper sulfate because magnesium and copper are futher apart in the reactivity series.
How does the addition of a catalyst alter the energy profile diagram?
A catalyst provides an alternative route with a lower activation energy.
Complete combustion of methane CH4 + 2O2 –> CO2 + 2H2O After calcuating the total energy required to break all the bonds in the reactants and the _total energy to make all the bond_s in all the products, how is the enthalpy change calculated? How do you know if the reaction is exothermic or endothermic?
negative value = exothermic positive value = endothermic
Complete combustion of methane CH4 + 2O2 –> CO2 + 2H2O What is the total bond enthalpy released when the bond in carbon dioxide are made?
2x 730kJ/mol = 1460 kJ/mol carbon dioxide has two C=O bonds!
Complete combustion of methane CH4 + 2O2 –> CO2 + 2H2O What is the total bond enthalpy required to break both oxygen molecules?
2x 502kJ/mol = 1010 kJ/mol oxygen has O=O bonds!
Complete combustion of methane CH4 + 2O2 –> CO2 + 2H2O What is the total bond enthalpy released when both water molecules are made?
4x 464 kJ/mol = 1856 kJ/mol H-O-H H-O-H 4 x O-H bonds are made!
Below is the calculation for the theroetical value for change in enthapy of the complete combustion of methane CH4 + 2O2 –> CO2 + 2H2O How and why is the experimental value different?
The experimental value will be less than -810 kJ/mol. Reasons: • Not all the energy released by the fuel goes to the water- some lost to the surroundings • Not all the full undergoes complete combustion - some reacts via incomplete combustion (flame was yellow not blue)
Calculate the enthalpy of reaction for hydrogen reacting with chlorine to produce hydrogen chloride
Bonds broken 432 + 239 = 671 kJ/mol Bonds made 2 x 427 = 854 kJ/mol Enthalpy of reaction 671 - 854 = - 183 kJ/mol EXOTHEMRMIC
Define an exothermic reaction
heat energy given out - surrounding warm up
Define endothermic reactions
heat is taken in - surrounding cool down
What are the key marking points when creating an energy profile diagram?
• y-axis labelled with enthalpy (or energy) • labelled reactants and products • bump to represent reaction path • activation energy arrow from reactants to top of bump • enthalpy change labelled from reactants to products - arrow up (endo) arrow down (exo)