1. Enthalpy and Calorimetry
Continuation of topic #33

2. Enthalpy of Reactions
Enthalpy is the heat content of a system at constant pressure. Enthalpy of reaction: The change in enthalpy for a reaction. Same as heat of reaction H = Hfinal – Hinitial H = Hproducts – Hreactants since we will be working all reactions under constant pressure we can say, q= Hrxn

3. General Reaction Scheme
Reaction Coordinate
Energy
Reactants
Products Ea ΔH

4. Exothermic Reaction – “hot pack”
Reaction Coordinate
Energy
Reactants
Products Ea ΔH

5. Endothermic Reaction – “cold pack”
Energy
Products
Reactants ΔH
Reaction Coordinate

6. Ea = Activation Energy
The “hump” represents a hurdle that must be overcome to go from reactants to products. This is called the activation energy (Ea) Catalyst –a susbtance that speeds up the reaction by lowering the activation energy but is not itself consumed in the reaction.

7. Standard Enthalpies of Formation
The standard enthalpy of formation of a substance, denoted DHfo, is the enthalpy change for the formation of one mole of a substance in its standard state from its component elements in their standard state.
Note that the standard enthalpy of formation for a pure element in its standard state is zero. 4

9. Standard Enthalpies of Formation
The law of summation of heats of formation states that the enthalpy of a reaction is equal to the total formation energy of the products minus that of the reactants.
S is the mathematical symbol meaning “the sum of”. 4

10. A Problem to Consider
You record the values of DHfo under the formulas in the equation, multiplying them by the coefficients in the equation.
You can calculate DHo by subtracting the values for the reactants from the values for the products. 4

12. How is the heat of sublimation, Hsub, the enthalpy
change for the reaction:
H2O(s)  H2O(g)
related to Hfis and Hvap?

13. What is Calorimetry?
Calorimetry – study of heat flow and heat measurement.
Calorimetry experiments determines the heat/ enthalpy changes of reactions by making accurate measurements of temperature changes in a calorimeter.

14. Heat capacity & Specific Heat
Calorie – The amount of heat or energy needed to raise the temperature of 1g of water by 1 °C
Heat capacity – the amount of heat needed to raise the temperature of the object by 1°C. (depends on mass and composition of the object).
Specific heat (C) – Heat capacity of 1g of a substance.

15. Calorimeter
A calorimeter is an insulated device used for measuring the amount of heat absorbed or released in a chemical reaction or physical process.

16. Foam Cup Calorimeter

17. Soda Can Calorimeter

18. Exit Ticket
A student conducted an experiment to determine whether the formation of magnesium oxide, MgO, is an exothermic or endothermic reaction. The student monitored the temperature inside a calorimeter while the reaction was taking place. The table below shows the temperature data collected. Time (minutes) Temperature (°C) 0 (initial) 24.4 °C °C °C °C °C Based on the experimental data, the student concluded that the formation of MgO is an exothermic reaction. 1. Is the student's conclusion correct? 2. Use the experimental data and what you know about exothermic and endothermic reactions to justify your answer.