Thermochemistry

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# Thermochemistry - PowerPoint PPT Presentation

Thermochemistry . DO NOW. The specific heat of ethanol is 2.44 J/ g°C . How many kilojoules of energy are required to heat 50.0 g of ethanol from -20.0°C to 68°C?. Objective . Describe how calorimeters are used to measure heat flow . Construct thermochemical equations .

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### Thermochemistry

DO NOW
• The specific heat of ethanol is 2.44 J/g°C. How many kilojoules of energy are required to heat 50.0 g of ethanol from -20.0°C to 68°C?
Objective
• Describe how calorimeters are used to measure heat flow.
• Constructthermochemical equations.
• Solve for enthalpy changes in chemical reactions by using heats of reaction.
Calorimetry
• Calorimetry - the measurement of the heat into or out of a system for chemical and physical processes.
• Based on the fact that the heat released = the heat absorbed
• The device used to measure the absorption or release of heat in chemical or physical processes is called a “Calorimeter”
Enthalpy
• Heat absorbed or released by a reaction at constant pressure

H = qq = H = m x C x T

Note: We cannot calculate the actual value of enthalpy, only the change in enthalpy

Thermochemical equation
• Includes the ΔH as either a product or a reactant

CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ

Heat of Reaction

H = Hproducts – Hreactants

Exothermic Reaction

2CO(g) + O2(g)  2CO2(g) H = -566.8 kJ

Or

2CO(g) + O2(g)  2CO2(g) + 566.8 kJ

Negative sign means energy is released

Energy

Change is down

ΔH is <0

= Exothermic (heat is given off)

Reactants

®

Products

2CO + O2

Energy

2CO2

Reactants

®

Products

2CO(g) + O2(g) → 2CO2(g) + 566.8 kJ

566.8kJ given off

Endothermic Reaction

2CO2(g)  2CO(g) + O2(g) H = +566.8 kJ

Or

2CO2(g) + 566.8 kJ  2CO(g) + O2(g)

Positive sign means energy is absorbed

Energy

Change is up

ΔH is > 0

= Endothermic (heat is absorbed)

Reactants

®

Products

2CO(g) + O2(g)

Energy

2CO2(g)

Reactants

®

Products

CaCO3→ CaO + CO2

2CO2(g) + 566.8 kJ →2CO(g) + O2(g)

566.8kJ absorbed

Chemistry Happens in

MOLES

• An equation that includes energy is called a thermochemical equation
• CH4 + 2O2® CO2 + 2H2O + 802.2 kJ
• 1 mole of CH4 releases 802.2 kJ of energy.
• When you make 802.2 kJ you also make 2 moles of water

The magnitude of ΔH is directly proportional to the amount of reactants or products.

• 1A + 2 B ----> 1C ΔH = -100 kJ
• 1/2 A + 1B ----> 1/2 C ΔH = -50 kJ
CH4 + 2O2® CO2 + 2H2O; ΔH = -802.2 kJ

Rewrite chemical equation as a thermochemical equation.

Exothermic or endothermic reaction?

If 3 moles of O2 react with excess CH4 how much heat will be produced?

1CH4(g) + 2 O2(g)® CO2(g) + 2 H2O(l) + 802.2 kJ
• If 10. 3 grams of CH4 are burned completely, how much heat will be produced?

Convert moles to desired unit

Convert to moles

1 mol CH4

802.2 kJ

10. 3 g CH4

16.05 g CH4

1 mol CH4

= 514 kJ

Ratio from balanced equation

ΔH = -514 kJ, which means the heat is released for the reaction of 10.3 grams CH4

• Based upon the thermochemical equation given, calculate the heat associated with the decomposition of 1.15 g of NO.
• Calculate the mass of ethane, C2H6, which must be burned to produce 100 kJ of heat.
Heat of Reaction

CaO(s) + H2O(l)  Ca(OH)2(s) ΔH = -65.2 kJ

CaO + H2O

Enthalpy (H)

ΔH = -65.2 kJ

Ca(OH)2