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Thermochemistry

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Thermochemistry

- 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?

- Describe how calorimeters are used to measure heat flow.
- Constructthermochemical equations.
- Solve for enthalpy changes in chemical reactions by using heats of reaction.

- 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”

- 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

- Includes the ΔH as either a product or a reactant
CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ

H = Hproducts – Hreactants

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

566.8kJ given off

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

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 ----> 1CΔH = -100 kJ
- 1/2 A + 1B ----> 1/2 CΔH = -50 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?

- If 10. 3 grams of CH4 are burned completely, how much heat will be produced?

Convert moles to desired unit

Convert to moles

Start with known value

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.

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

CaO + H2O

Enthalpy (H)

ΔH = -65.2 kJ

Ca(OH)2