heat in chemical reactions how do we measure heat?
TRANSCRIPT
HEAT IN CHEMICAL REACTIONS
How Do We Measure Heat?
What is Heat?
What will cause more discomfort?bucket of boiling water ordrop of boiling water
They are at the same temperature but the amount of stored heat energy is different
Temperature
Hotness or coldness Proportional to average kinetic energy Intensive property (does not depend on
amount) Put a thermometer in a hot oven
The fast oven particles bounce off the thermometer The thermometers particles speed up The oven particles slow down Heat is transferred!
Energy
The capacity to do work or produce heat
Energy transfer is measured Extensive property – depends on
amount units are joules, calories or BTU’s
joules in science (metric unit) BTU’s for air conditioning, heaters etc Calories in some science like food 1 cal = 4.184 j 1 Calorie = 1 Kcal
Heat transfer (flow)
Depends onThe capacity of substance to absorb heat The mass The change in temperatureWhy are pots made of metal and handles
made of wood or plastic?
Specific heatSpecific heat
amount of energy required to raise the temperature of 1 gram of that substance 1 oC
Specific heat = Specific heat = heat (j or cal)heat (j or cal)
mass (g) x change in temp.(C)
Water has a specific heat of 4.184 J/(g Water has a specific heat of 4.184 J/(g ooC)C) Copper has a specific heat of 0.387 J/(g Copper has a specific heat of 0.387 J/(g ooC)C)
Specific heat problems
1. Calculate the Specific heat of Fe ifheat= 145 Jmass= 89.3 g
Δ temperature= 13.2 oC2. Calculate the heat of water if
Specific heat= 4.18 J/g Cmass= 43.6 gΔ temperature= 8.3 o C
Lab: Specific heat
Purpose: To determine the specific heat of Copper and an unknown metal
Measuring heat transfer
Calorimeter – well insulated container that minimizes
the amount of heat transferred to the surroundings
Their Calorimeter
Our calorimeter
Heat transfer
Use water in calorimeter to absorb or release heat .
We can measure mass of water in calorimeter measure the change in temperature of the water in
the calorimeter. know the specific heat of water. Heat(q)H2O = massH2O∆TH2OSpHeat(S)H2O
qsur = -qsystem
qH2O = -qreaction
Can’t measure heat directlyBut can see measure the effectsof energy being released or absorbed by the reaction. Water surrounding the rxnchamber will increase or
decrease in temperature. Can measure this temperature change.
Coffee Cup Calorimeter
M = mass of water in cup
SWater =4.184 j/ g C
TAfter – Tbefore = Tchange
Tf - Ti = T (for the water)
Heat water= mw x Sw x Tw
calculate heat collected by calorimeter
Thermo
Calorimeter Basics
Heat absorbed = - Heat releasedHeat water = heat metal
mwater x Swater x Twate = -[mmetal x Smetal x Tmetal] Final temp. of metal= final temp. of the water
Determine the specific heat of an unknown metal with a mass of 23.8g which is heated to 100.0°C and dropped into a calorimeter containing 50.0g of water
at 24.0°C. The final temperature is 32.5°C.
H2O: mass = 50.0g
T1= 24.0°C, T2= 32.5°C
Sp Heat(S)= 4.184.J/g°C
Metal: mass= 23.8g
T1= 100.0C, T2= 32.5C
Sp Heat (S) = ?
Calorimeter
Thermohot metal is added to 100.0 g of 25.6 oC water. TheFinal temp is 35.9 oC, How much heat is transferred? q = 100.0g x 4.184 J x ( 35.9 – 25.6 C)
g C q = 4300 J or 4.30 x 103 J or 4.30 kJ
Specific Heat LabFind the Specific Heat of a metal
Mass metal Heat metal to 100 °C Place water in calorimeter
Know mass water Know Initial temp
Put hot metal in cold water What happens to temp of water? What happens to temp of metal?
When does the temp stop changing?
Specific Heat LabFind the Specific Heat of a metal
Heat into water = -heat out of metalqwater = -qmetal
mwater x Swater x Twater = -[mmetal x Smetal x Tmetal]
Temperature
What happens to the particles of water when I put them into the freezer? The slow moving air particles hit the water The fast water molecules move slower The slow air particles move faster Heat is transferred!
Energy Change
Exothermic – energy (heat) given off in a reaction
Condensing, freezingEndothermic – energy (heat) absorbed in a reaction
Vaporizing, melting
Changes of State
How much energy does it take to heat 10.0 g ice from –20.0 to 100.0o C steam Specific Heat ice 2.07 j/gC Heat of fusion 334 j/g Heat of evaporation 2260 j/g Specific heat water 4.184 j/gC
Changes of State
It takes energy to change from one state to another
Heat of fusion energy to change a solid into a liquid Is this process endo or exothermic?
Heat of vaporization energy to change a liquid to a gas Is this process endo or exothermic?
Cooling Curve (p.481)
Liquid
Solid
Gas Heat of vaporization
Heat of fusion
Heat or Energy Problems
heat = mass x specific heat x temp change
q (∆H) = m x S x T
joules = g x J x C° g C°
Energy from -20 to 0 q = m x S x T = 10.0g x 2.07j x 20oC
goC
Energy from solid to liquid q = 10.0g x 6.7 j/g
Energy from 0 – 100 q = m x S x T
= 10.0g x 4.18j x 100oC goC
Energy from liquid to steam q = 10.0 x 41.0 j/g
Heat of reaction
A calorimeter is filled with 75.0g of water at an initial temperature of 19.5°C. A 0.50mole sample of solid NaOH is added and the temperature increases to 26.7°C. What is the enthalpy change per mole of NaOH for this solution process.
NaOH(s) Na+(aq) + OH-
(aq)
We can write expression for heat for both the metal and water
Heat(q) H2O = massH2O ∆T H2O Sp Heat(Cp) H2O
Heat( q )metal = mass metal ∆T metal Sp Heat( Cp )metal
Heat( q) H2O =Heat( q)metal
massH2O ∆T H2O Sp Heat(Cp) H2O = massmetal ∆Tmetal Sp HeatCpmetal
Make chart with data
H2O Metal
mT1
T2
∆TSp Heat(Cp)
50.0g 23.8g
24.0°C 100.0°C
32.5°C 32.5°C
8.5°C 67.5°C
4.184J/g°C x
massH2O ∆TH2O Sp Heat (Cp)H2O = massmetal∆Tmetal Sp
Heat( Cp)metal
(50.0g)(8.5°C)(4.184J/g°C)= (23.8g)(67.5°C)x
x= 1.1 J/g°C
1. Make chart & put data in chart H2O NaOHm 75.0g (0.050 mol)T1 19.8 °CT2 26.7 °C∆T 6.9 °CCp 4.184 J/g °C
Heat(q)H2O = mass ∆T Sp Heat(Cp) = (75.0g)(6.9°C)(4.184J/g°C) = 2200J Heat(q)H2O = Heat(q)NaOH therefore
Heat(q)NaOH= 2200J2200J = x x= 44000J 0.050 mol 1 mol ∆H= -44000J/mol NaOH
Calculation of enthalpy changes2H2O2 2H2O + O2 Δ H=-190 kJ
What is the heat released if 2.3 g H2O2
react?
Measuring Heat Transfer
“All” heat went into water Know heat into water All came from your experiment. Heat water = -heat experiment
Minus sign important!
Thermo
Heat Problem
How much does it take to heat 5.0 g of aluminum (Cp =0.879 j/g C) from 22 C to 100. C?
q = m x Cp x T q = 5.0g x ( 0.879 j/g C) x (100. – 22) q = 340 or 3.4 x 102 j are required