Energy – the ability to do work or produce Potential energy


Endothermic system __________ heat Heat flows ______ the system Temperature goes ___________ Exothermic



Download 124.62 Kb.
Page2/4
Date13.08.2021
Size124.62 Kb.
#93735
1   2   3   4
Endothermic

system __________ heat

Heat flows ______ the system

Temperature goes ___________



Exothermic

Heat flows _________ the system and _________the surroundings

Temperature goes _______

Only look at heat (q) to determine if the system is endo or exo


Enthalpy

EnthaIpy (H) is a state function equal to E +pV

DH = DE + D (pV), with constant pressure,

DH = DE + p DV is equal to the expression for q at­ ____________. Therefore, at constant pressure DH = qp. This means that the change in enthalpy is equal to the heat flow. DH = H final - H initial = qp

Enthalpy of reaction: DH = ___________________

Enthalpy

When Δ H is negative, heat is ___________the system and the reaction is ____________.

When Δ H is positive, heat is ______ to the system and the reaction is ___________.

Enthalpy


When 1 mole of methane is burned at constant pressure, 890 kJ of energy is released as heat. Calculate the ΔH for a process in which a 5.8 g sample of methane is burned at constant pressure.

Enthalpy


Specific Heat

Specific Heat (c) – the amount of _____________ to raise the temperature of 1 g of a substance 1 °C

The units for specific heat are ______

The specific heat of water (in a liquid form is _________)

All substances have a particular specific heat

Specific heat equation

q = _____

q = heat gained or lost (J)

m = mass (grams) note that this is different than the energy calculations

c = specific heat (J/g°C)

DT = change in temperature (°C) = Tf -Ti
Specific heat calculations

How much heat is required to raise 250 g of water from 22°C to 98°C?

Specific heat calculations

A piece of metal with a mass of 4.68 g absorbs 256 J of heat when its temperature is increased by 182°C. What is its specific heat?

Specific heat calculations

60.0 J of heat are applied to a 5.00 g sample of calcium (c = 0.647J/g°C). If the final temperature is 51.1°C, calculate the original temperature.

Calorimetry

A calorimeter is an insulated device used for measuring the ____________________________________ during a chemical or physical process.

Calorimetry

When using calorimetry, you are usually trying to determine the ________ of an unknown metal by finding its specific heat

The heat lost from the metal will be _______ y the water

q metal = - q water

Calorimetry

q metal = - q water

(m metal)(c metal)(DT metal) = - (m water)(c water)(DT water)

Calorimetry Examples

A 58.0 g sample of a metal at 100.0 °C is placed in a calorimeter containing 60.0 g of water at 18.0 °C. The temperature of that water increases to 22.0 °C. Calculate the specific heat of the metal.

Calorimetry Examples


Calorimetry Examples

A piece of metal with a mass of 4.68 g at 135°C is placed in a calorimeter with 25.0 g of water at 20.0 °C. The temperature rises to 35.0 °C. What is the specific heat of the metal?

Calorimetry Examples
More calorimetry

3.25 g Mg is placed into 125 mL of HCl. The initial temperature of the calorimeter is 18.5°C & the final temperature is 26.6°C. If the heat capacity of the calorimeter is 4.86 J/g°C, calculate the enthalpy of the reaction.

More calorimetry

Enthalpy will be in KJ/mol


Bomb Calorimetry

For constant volume, (bomb calorimeter)

-P D V =0, so D E = q + w, but w = 0, So DE = qv

qv = _____

c= heat capacity, the energy required to change the temperature of the bomb 1°C


Thermochemistry 2

Hess’s Law

Heat of Formation

Heat of Combustion

Bond Enthalpy

Hess’s Law

Hess's Law -the Heat of Reaction, Enthalpy (ΔH) of a reaction is the sum of the _____________ for each step of the reaction _______ .

Hess’s Law (cont’d)

ΔHfº for Step #1

+ ΔHfº for Step #2

+ ΔHfº for Step #3

ΔH for the reaction

Look at the example below.

Try this one.

Answer:

ΔHo=?

= (-790 kJ) - (-196 kJ)

= -594 kJ

for 1 mole of S02,

ΔHo= 1/2 ( -594 kJ) = -297 kJ


Heats of Formation

Heat of Formation (Hf) -amount of energy needed to form _________________.

Standard Heat of Formation DHf°- Heat of Formation at _______________.To determine the amount of energy needed to decompose a reactant, ______________________, The more negative the ΔHfº, the more stable the compound.

Sign changes (if any) for reactions.

For the reaction A + B →C, A and B are__________- change the sign for ΔHfº.

C is _______ -keep the sign for ΔHfº.

ΔHfº for any free element (H2, 02, Zn, S, AI etc.) = _____

Hess’s Law

Want:

C(graphite) à C(diamond) D H = ?

Given:

C(graphite) + O2 à CO2 D H = -394 KJ



C (diamond) + O2 à CO2 D H = -396 KJ

Hess’s Law


Want:

2B(s) + 3H2 (g) à B2 O3 (g) D H = ?

Given:

2B (s) + 3/2 O2 (g) à B2O3 (s) DH = -1273 KJ



B2H6 (g) + 3O2 (g) à B2O3 (s)+ 3H2O (g) DH = -2035 KJ

H2 (g) + ½ O2 (g) à H2O (l) DH = -286 KJ

H2O(l) à H2O(g) DH = 44KJ

Standard Enthalpy of Formation (DHf)

What is the DHf for CuCl2 (s)?

Look up in Appendix 4 (pg A20)

What is the DHf for 2 moles of FeCl3 (s)?

Remember, these are heats of FORMATION which means that the compound is being produced. If it on the reactant side, you must flip the sign!

Standard Enthalpy of Formation (DHc)



Download 124.62 Kb.

Share with your friends:
1   2   3   4




The database is protected by copyright ©essaydocs.org 2022
send message

    Main page