Heat in chemical reactions

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