Chesmistry U2M3 - Enthalpy Changes

Home

Get App

Create

Exothermic
- Increase temperature of surrounding
- Enthalpy of chemical system decreases as /\H is negative becuase the system has lost energy to surroundings
- Example is the combustion of hydrocarbon fuels
Endothermic
- Decreases the temperature of surroundings
- Increases the enthalpy of chemical systems as /\H is positive because the system has gained energy from surroundings
- Example is the decomposition of metal carbonates e.g. CaCO3
Enthalpy Profile Diagrams
Enthalpy Profile Diagrams
- Chemical reaction can be regarded as braking bonds followed by making bonds. Braking bonds needs energy, so energy must be put into the reaction
- The minimum energy needed to start a reaction is called activation energy
- After that bonds a re made which release energy
- When bonds are broken energy is taken in positive
- When bonds are made, energy is released, negative
- If the bonds made are stronger than the bonds broken then there is a overall decrease in enthalpy and the reaction is exothermic
- If the bonds broken are stronger than the bonds made there is an overall increase in enthalpy and the reaction in endothermic
Types of Enthalpy Change
- r=change of reaction
- c=chanhe of combustion
- f=change of formation
Standard Enthalpy change of formation definition
- Enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.
- When standard state=0
Standard enthalpy change of combustion

Enthalpy change when 1 mole of a substance (element or compound) reacts completely with oxygen under standard conditions
Standard enthalpy change of reaction

The enthalpy change that accompanies a reaction under standard conditions in the molar quantities expressed in a chemical equation, all reactants and products being in their standard states
How to calculate enthalpy changes
- Energy change in surroundings = m x c x T(difference in Temperature)
- m=mass
- c=4.2 (A constatnt)
50cm^3 of concentrated sulfuric acid 20.0mol dm^-3, was added to 950cm^3 of water at a temperature of 19Celcius. The solutionwas stirred gently until maximum temperature was reached at 28C. What is the enthalpy change for this reaction?
- STEP 1 : Write the equation. m x c x T=energy change
- STEP 2 : Write down values for symbols m, c and T. m=950+50=1000cm^3 c=4.2J T=28-19=9C
- STEP 3 : Do the calculation energy change = 1000 x 2.4 x 9= 37000J=37.8kJ
- It is an exothermic reaction so it has a negative sign -37.8kJ
- What is the enthalpy change per mole of sulfuric acid?
- STEP 4 : Calculate the amount in moles of Sulfuric acid
- amount(mol)=Vol(cm^3)/1000 x concentration
- 50/1000 x 20 = 1.00 mol
- STEP 5 : Divide the /\H value by moles =37.8/1 =37.8kJmol^-1
Hess's Law
- Hess' Law states that for any chemical change the enthalpy change is the same whatever the route taken
- /\H1=/\H2+/\H3
Calculating enthalpy changes indirectly using hess's Law

Author

oliviaobs

147761

Card Set

Chesmistry U2M3 - Enthalpy Changes

Description

U2M3 - Enthalpy Changes

Updated

2012-04-16T20:54:14Z

Show Answers