Heat in Chemical Reactions

10.1 Chemical Reactions that Involve Heat

1. Heat: Energy (symbol - q) that is transferred from one object to another due to a difference in temperature. Measured in Joules (symbol - J)

2. Thermochemistry: The study of heat changes in a chemical reaction.

3. Types of Chemical Reactions

a. Exothermic Reactions: release heat into their surroundings. Heat is a product of the reaction and temperature increases. This occurs during bond formation.

Combustion reactions are exothermic: burning propaneC3H8 + 5O2 → 3CO2 + 4H2O + 2043 kJ

Exothermic Reaction

surroundings

Exothermic Reaction

b. Endothermic Reactions: Heat is absorbed by the reactants and stored in the chemical bonds of the products. Heat acts as a reactant and temperature decreases. This occurs during bond breaking.

Electrolysis of water requires electrical energy.

2H2O + 572kJ → 2H2 + O2Endothermic Reaction

surroundings

Exothermic/Endothermic

Reactions

10.2 Heat and Enthalpy Changes

1. Enthalpy: The heat content of a system at constant pressure (symbol is H ).

2. Enthalpy Change: The heat absorbed or released during a reaction (symbol is ΔH ).

3. Enthalpy Diagrams: #1 #2

#1 #2

a. Which has a higher enthalpy? Products or Reactants R P

b. Was heat absorbed or released? R A

c. Is this an endothermic or exothermic reaction?

ExoEnd

o

d. Is ΔH for this reaction positive or negative?

- +

e. Would the ΔH be on the left or right side of the yield sign? R L

f. Is the reverse reaction exothermic or endothermic?

Endo

Exo

g. Rewrite each equation with the heat term

in the reaction as a reactant or product:

#1) C3H8 + 5O2 → 3CO2 + 4H2O + 2043 kJ

#2) C + H2O + 113kJ → CO + H2

= “change in” H = Hproducts ─ Hreactants

H Hproducts Hreactants

Exothermic Reaction - < >Endotherm

ic Reaction + > <

4. When reactions take place at standard temperature and pressure, q = H.

5. Standard Enthalpy Change (H): Enthalpy change that occurs when reactants in their standard states (most stable form) change to products in their standard states. STP Standard Temperature and Pressure are: 0C and 1 atm.The H is listed after the equation. If the H is positive the reaction is endothermic and heat was absorbed. If the H is negative, the reaction is exothermic and heat was released.

6. The amount of heat absorbed or released in a reaction depends upon the number of moles of reactants.

Heat• Temperature is not the same as heat.

• Temperature is a measure of the average kinetic energy of the particles in an object.

• A temperature change is a result of a energy transfer.

• Julius Sumner Miller - Physics - Heat & Temperature

• Temperature vs. Heat Animation

7. Enthalpy Changes in Stoichiometry Problems:

Ex) How much heat will be released if 5.0 g of H2O2 decomposes?

2H2O2 → 2H2O + O2 ΔH = -190 kJ

2 22 2

2 2 2 2

1 mol H O -190 kJ5.0 g of H O = -14 kJ or 14 kJ released

34 g of H O 2 mol H O

Ex) How much heat is transferred when you eat a 10. g Jolly rancher which is made of glucose (C6H12O6)? It reacts in your body with oxygen according to the following equation. If 4.184kJ = 1 Cal, how many Calories are in the Jolly Rancher?

C6H12O6 + 6O2 → 6CO2 + 6H2O ΔH = -2803 kJ

6 12 66 12 6

6 12 6 6 12 6

1 mol C H O -2803 kJ 1 Cal10.0 g of C H O = -160 kJ = 38 Cal

180 g of C H O 1 mol C H O 4.184 kJ

C6H12O6 + 6O2 → 6CO2 + 6H2O + 2803 kJ

10.3 Hess’s Law - (1802-1850)

1. The enthalpy change for a reaction is the sum of the enthalpy changes for a series of reactions that adds up to the overall reaction.

2. This is also called the Law of Heat of Summation (Σ)

3. This allows you to determine the enthalpy change for a reaction by indirect means when a direct method cannot be done.

4. Steps for using Hess’s Law

1. Identify the compounds2. Locate the compounds on the Heat of

Reaction Table.3. Write the reaction from the table so the

compound is a reactant or product.4. Write appropriate ΔH for each ‘sub

equation.”• If needed, multiply equation and enthalpy change value.

(coefficients)• If you reverse the equation, change the sign of the

enthalpy change.

5. Add the equations to arrive at the desired net (original) equation.

6. Add ΔH (enthalpy changes) of each “sub equation.”

10.3 Calorimetry:1. The Kinetic Theory states that heat

results from the motion & vibration of particles.

2. Heat: The transfer of kinetic energy from a hotter object to a colder object. Heat is dependent on composition and amount.

3. Temperature is a measure of how hot or cold something is; specifically it is the measure of the average kinetic (avg. speed of particles) energy of the particles in an object. It is independent of amount.

4. Calorimetry is the study of heat flow and measurement.

5. Calorimetry experiments determine the heats of reactions (enthalpy changes) by making accurate measurements of temperature changes produced in a calorimeter.

6. A Calorimeter is an insulated device used to measure heat absorbed or released in a chemical or physical change.

7. Heat Capacity: The amount of heat needed to raise the temperature of an object by 1°C.

8. Specific Heat (Cp): The amount of heat needed to raise 1 g of a substance by 1C.

Formula for specific heat:

m=mass (substance) T=change in temperature of the

substance (Tf─Ti)

Specific Heat of Water = 4.184 J/g ºC = 1 calorie or .001Calorie (food)

p

qC =

mΔT

9. Measuring Specific Heat of a Metal:

Ex #1) What is the specific heat of a nickel if the temperature of a 32.2 g sample of nickel is increased by 3.5ºC when 50. J of heat is added.

p

qC =

mΔT 50.J

32.2g 3.5 C

0.44 J/g C

Ex #2) How much heat is released when a 26.2 g sample of aluminum (Cp = 0.897 J/gºC) increases in temperature from 25.3ºC to 65.9ºC?

pq = mC ΔT

o oq = .897 J/g C 26.2g 40.6 C

q = 954 J

10. Measuring Heat (q) of a Substance Dissolved in Water:

You can rearrange this formula to determine the heat released or absorbed by the surroundings (solution) as the substance dissolves based on this assumption:

q reaction = -q surroundings

q = 1) Calculate qfor the surroundings (solution) and

determine qrxn.2) Calculate the moles of solute dissolved in the water.3) Calculate H =

heat (q)

mol

pmC ΔT ΔT = Tf - Ti

Ex) When a 4.25 g sample of solid NH4NO3 dissolves in 60.0 g of H2O in a calorimeter,the temperature drops from 21.0 ºC to 16.9 ºC. Calculate H. Rewrite the thermochemical equation with the heat term as a reactant or product.

NH4NO3(s) → NH4+

(aq) + NO3-(aq)

H = ?

1. qsur=mCpΔT = (64.25g)(4.184J/g°C)(-4.1°C)

qsur=-1100J

qrxn = -qsur = +1100J

2.

3.

4 34 3

4 3

1 mol NH NO4.25 g NH NO = 0.0531 mol

80 g NH NO

+q +1100JΔH = = = 21,000 J/mol = +21 kJ/mol

mol .0531 mol

+ 21 kJ

11. Foods as Fuels:A. Carbohydrates typically have high

enthalpies; however, the products of their combustion, CO2 and H2O, have low enthalpies.

B. Therefore, the combustion of carbohydrates and fats, is exothermic.

C. Sugars and Starches break down to glucose, which reacts with O2 in a combustion reaction.

D. Nutritional information on food labels can be gathered using a calorimeter.