chemical energetics

Energy

Neither created nor destroyed , can be changed from one form to another , involve chemical change

Calorimeter

Used to measure the change in thermodynamic quantity thermochemical equations

Enthalpy (H)

Refers to energy content that is stored in a substance

• Smaller =more stable system

• Only possible to measure enthalpy change not enthalpy itself

enthalpy change (triangle H)

• amount of energy involved in a chemical reaction

• Represents the difference in energy content of the reactants and products

• Usually measured in kilojoules (kJ)

Formula of Enthalpy change

Total energy of products - total energy of reactants

Exothermic reaction

A reaction in which heat is lost to the surroundings causing an increase in temperature of the surroundings

Release heat + rise in temp of the surroundings = exothermic

Negative sign

Means lose heat , always for exothermic reactions

Temperature change for exothermic reactions

Temperature of the reaction mixture increases until the highest temperature is reached , and then decreases to room temperature

Energy level

• Reactants (more energy , less stable ) —(Enthalpy change , energy loss to surroundings , near is released to the surroundings and temperature of surroundings rises)→products have less energy , more stable

• Diff in energy levels of the products and the reactants = the amount of energy released during the reaction

Exothermic reaction example

Physical = condensing steam to water (solid to liquid ) + freezing (liquid to solid )

Chemical = combustion of fuels , respiration and neutralisation

Combustion of fuels example

C + O2 = CO2

2H2 + O2 =2H2O

CH4 +2O2 = CO2 +2H2O

Respiration chemical equation

C6H12O6 + 6O2 = 6CO2 + 6H2O

Neutralisation between acid and alkali formula eg

NaOH + HCL =NaCl +H2O

Endothermic reaction

A reaction in which heat is absorbed from the surroundings causing a decrease in the temperature in the surroundings

(Heat gain from surroundings , temperature of surroundings drop)

Temperature change in endothermic reaction

Temperature of the reaction mixture decreases until the lowest temperature is reached and then increases to room temperature

Endothermic energy level reaction

• Products have a higher total energy , less stable than the reactants . Heat is absorbed from the surroundings and temperature of the surroundings drops

• Difference in the energy levels of the products and the reactants is equal to the amount of energy absorbed during the reaction ( a,ways positive)

Positive sign

Only for endothermic means that energy is gained/ absorbed from the surroundings

Endothermic process

Physical : evaporation (liquid to gas), melting (solid to liquid )

Chemical reactions : photosynthesis , decomposition

Bond breaking

Endothermic

Bond forming

Exothermic

When bonds are broken

Energy is absorbed into the surroundings

When bonds are formed

Energy is released to the surroundings (exo)

Overall enthalphy change formula

Total energy absorbed for bond breaking - total energy released from bond forming

In an exothermic reaction

More energy is released in forming bonds than absorbed in breaking bonds (overall gives out energy)

In an endothermic reaction

More energy is absorbed in breaking bonds than released in forming bonds (more energy taken in than given out)

Bond enthalphy of an X-Y bond

Average amount of heat absorbed to break one mol of that bind in gaseous state

The amount of energy absorbed to break a chemical bond is the same as

A hit. Fk energy released when chemical bond is formed

Stronger bond

More energy is absorbed to break that bond and thus the higher its bond energy (KJmol)

To calculate Enthalpy change of a reaction

Total energy absorbed for bond breaking (endo, reactant)- total energy released from bond forming (exo,product)