exothermic and endothermic reactions. chemical reactions usually involve a temperature change (heat...

Exothermic and endothermic reactions

• Chemical Reactions usually involve a temperature change

(heat is given out or taken in)

Law of conservation of energy

• Energy cannot be created or destroyed, but only changed from one form into another

Exothermic Reactions

• Examples include:– Burning reactions including the

combustion of fuels.– Detonation of explosives.– Reaction of acids with metals.

Thermit reaction

Magnesium reacting with acid

Exothermic reactions increase in temperature.

• Magnesium + Hydrochloric acid

Gets hot

25o C 45o C

magnesium

Hydrochloricacid

Heatenergygivenout


Reactants convert chemical energy to heat energy.

The temperature rises.

25o C45o C


45o C

• Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature.

25o C


Ene

rgy

/ kJ

)

Progress of reaction (time)

Energy Level Diagram for an Exothermic Reaction

reactants

Reactants have more chemical energy.

Some of this is lost as heat which spreads out into the room.

productsProducts now have less chemical energy than reactants.

Exothermic reactions give out energy. There is a temperature rise

and H is negative.

Exothermic Reaction - Definition

products

Ene

rgy

/ kJ

)

Progress of reaction

reactants

H is negative

Heat changes also happen when substances change state.

An exothermic reaction

• When hydrocarbons burn in oxygen they produce carbon dioxide and water vapour.

• The reaction also involves the loss of heat so it is an exothermic reaction.

Endothermic reaction

• An endothermic reaction is when heat is taken in in a reaction.

Endothermic Reactions

• Endothermic chemical reactions are relatively rare.

• A few reactions that give off gases are highly endothermic - get very cold.

Cools

Heatenergytakenin as the mixture returns back to room temp.

Starts 25°C Cools to 5°C

Ammoniumnitrate

Water

Endothermic reactions cause a decrease in temperature.

Returns to 25°C


25o C

• The cold reaction products start to gain heat from the surroundings and eventually return to room temperature.

5o C The reactants gain energy.25o C

This comes from the substances used in the reaction and the reaction gets cold.

Eventually heat is absorbed from the surroundings and the mixture returns to room temperature.

Overall the chemicals have gained energy.


Endothermic reactions take in energy. There is a temperature

drop and H is positive.

Endothermic Reaction Definition

H=+

products

Ene

rgy

/ kJ


reactants

Heat of reaction

• The heat of reaction is the heat change when the number of moles of reactants indicated in the balanced equation react completely.

For an exothermic reaction:

the heat of reaction is always negative e.g ∆H = -34kJ

For an endothermic reaction:

the heat of reaction is always positive e.g ∆H = +34kJ

Chemical hazard notes

Concentrated hydrochloric acid : Very corrosive to eyes and skin, and its vapour is very irritating to lungs.

Sodium hydroxide : Caustic, harmful to skin and especially to eyes. Always wear eye protection.

Measuring the heat of reaction of hydrochloric acid and sodium hydroxide

A polystyrene cup is used as it is an insulator of heat – it doesn’t let the heat escape ( has negligible heat capacity)

The equation for the reaction is

HCl + NaOH → NaCl + H2O

Other precautions to ensure an accurate result!

1. Make sure both solutions are at the same temperature before you start!

2. Wash the thermometer and dry it before switching solutions.

3. Stir the mixture slowly and make sure none of the mixture is splashed out of the cup.

Results

Temperature of HCl solution before mixing = oC Temperature of NaOH solution before mixing = oC Highest temperature reached after mixing = oC Temperature rise = oC Number of moles of acid used = Number of moles of base used =

Calculations –

What is the heat change in the reaction carried our:

Heat change = mc ∆ T

Mass in kilogramsSpecific heat capacity

Temperature rise

Q260. Calculations for the heat of reaction

• 261. Calculate the heat of reaction (using the formula ΔH = mcΔT) for the reaction between and nitric acid sodium hydroxide from the following experimental results:

• Volume of nitric acid = 100 cm3 of 1.0 M• Volume of sodium hydroxide = 100 cm3 of 1.0 M• Initial temperature of the solutions = 17.5 oC• Final temperature of the solutions = 24.4 oC• Specific heat capacity of the mixture = 4080 J Kg—1oC—1

Heat change=

(.2kg) x (4080Jkg-1K-1) x 6.9 oC

Heat change = 5630J

1. What is the heat change in the reaction:


Mass in kilograms Specific heat capacity

Temperature rise

2. How many moles of Nitric acid were reacted?:

• 100cm3 of a 1M solution of HNO3 was reacted

1 x 100 = 0.1 moles1000

Number of moles of HNO3 reacted = 0.1moles

Question: What is the heat of reaction?

Balanced equation:HNO3 + NaOH NaNO3 + H2O1 1 1 1

0.1 moles of HNO3 = 5630J of heat1 mole of HNO3 = 5630 X10 = 56300

The heat of reaction = - 56300J (or – 56.3kJ)The negative sign is because the reaction is

exothermic ( heat is given out – the temperature went up!)


• A student carried out an experiment to measure the heat of reaction (neutralisation) of nitric acid by sodium hydroxide in a container made of plastic of negligible heat capacity. She used 100 cm3 of 1.0 M nitric acid and 100 cm3 of 1.0 M sodium hydroxide. The initial temperature of the solutions was 15.6 oC and the final temperature of the solution was 22.4 oC.

Given that specific heat capacity of the solution is 4080 J Kg—1 K—1, calculate the heat of reaction.

• (Assume that the density of the solution is 1 g cm—3)

Heat change=

(.2kg) x (4080Jkg-1K-1) x 6.8 oC

Heat change = 5548.8J




Temperature rise

2. How many moles of Nitric acid were reacted?:

• 100cm3 of a 1M solution of HNO3 was reacted

• 1000cm3 of solution = 1 mole in it.• 100cm3 of solution = x moles

(1) = x10

Number of moles of HNO3 reacted = 0.1moles


Balanced equation:HNO3 + NaOH NaNO3 + H2O1 1 1 1

0.1 moles of HNO3 = 5548J of heat1 mole of HNO3 = 5548 x 10 = 55480kJ

The heat of reaction = -55480kJ (or – 55.48kJ)The negative sign is because the reaction is



• A student mixed 250 cm3 of 0.5 M HCl with an equal volume of 0.5 M NaOH in a plastic container. The original temperature of both solutions was 14.8 oC and the final temperature was 18.2 oC. Calculate the heat of reaction (neutralisation) of hydrochloric acid and sodium hydroxide.

• Assume that the density of the final solution is 1 g cm—3 and its specific heat capacity is 4060 J Kg—1 K—1.

Heat change=

(.5kg) x (4060Jkg-1K-1) x 3.4 oC

Heat change = 6902J




Temperature rise

2. How many moles of Hydrochloric acid were reacted?:

• 250cm3 of a 0.5M solution of HCl was reacted

1000cm3 of solution = 0.5 mole in it.250cm3 of solution = (0.5/ 1000) x 250 = 0.125moles

Number of moles of HCl reacted = 0.125moles


Balanced equation:HCl + NaOH NaCl + H2O1 1 1 1

0.125 moles of HCl = 6902J of heat1 mole of HNO3 = 6902 x 8 = 55216

The heat of reaction = -55216J (or – 55.216kJ)The negative sign is because the reaction is


Q266

(d) In the experiment 50 cm3 of 1 M hydrochloric acid (HCl) and 50 cm3 of 1M sodium hydroxide (NaOH) were mixed. The temperature rise was recorded as 6.8 K. Assuming the densities and heat capacities of both solutions are the same as that of water, calculate the heat produced by the reaction.

• [Density of water is 1g /cm3 specific heat capacity of water is 4.2 kJ kg―1 K—1.]

(e) How many moles of hydrochloric acid were used in the experiment?

Calculate the heat of reaction (ΔH) when 1 mole of each solution is used.

Heat change=

(.1kg) x (4.2kJkg-1K-1) x 6.8 K





Temperature rise

Q266





How many moles of Hydrochloric acid were reacted?:

• 50cm3 of a 1M solution of HCl was reacted

• 1000cm3 of solution = 1 mole in it.• 50cm3 of solution = x moles

(50)(1) = x(1000)

(1) x (50) = x1000


Q266







0.05 moles of HCl = 2.856kJ of heat1 mole of HNO3 = 2.856 /20 = 57.12 J of heat

The heat of reaction = -57.12kJ The negative sign is because the reaction is


• Q267• (f) Calculate the number of moles of acid neutralised in this

experiment.• In an experiment to measure the heat of reaction for the reaction

between sodium hydroxide with hydrochloric acid, a student added 50 cm3 of 1.0 M HCl solution to the same volume of 1.0 M NaOH solution in a polystyrene foam cup.Taking the total heat capacity of the reaction mixture used in this experiment as 420 J K–1, calculate the heat released in the experiment if a temperature rise of 6.7 ºC was recorded.

• Hence calculate the heat of reaction for NaOH + HCl → NaCl + H2O

How many moles of Hydrochloric acid were reacted?:

• 50cm3 of a 1M solution of HCl was reacted

• 1000cm3 of solution = 1 mole in it.• 50cm3 of solution = (( 1/1000) x 50) = 0.05 moles


• Q267• (f) Calculate the number of moles of acid neutralised in this

experiment.• In an experiment to measure the heat of reaction for the reaction

between sodium hydroxide with hydrochloric acid, a student added 50 cm3 of 1.0 M HCl solution to the same volume of 1.0 M NaOH solution in a polystyrene foam cup.Taking the total heat capacity of the reaction mixture used in this experiment as 420 J K–1, calculate the heat released in the experiment if a temperature rise of 6.7 ºC was recorded.

• Hence calculate the heat of reaction for NaOH + HCl → NaCl + H2O

Heat change=

(.1kg) x (420Jkg-1K-1) x 6.7 oC





Temperature rise



0.05 moles of HCl = 281.4J of heat1 mole of HCl = (281.4 x 20) = 5628 J of heat

The heat of reaction = -5628J ( - 5.628kJ)The negative sign is because the reaction is


Bond energy

Breaking chemical bonds

• Breaking chemical bonds requires energy – is an endothermic process.

Heat taken in

Energy needed to overcome the

bonding between the atoms

En

erg

y in

ch

emic

als

Energy needed

Activation Energy.

– Before new bonds can be formed we need to break some existing chemical bonds.

– This requires an energy input –known as the activation energy (Eactivation energy (Eaa or E Eactact))

– Activation energy is the energy input into a reaction to Activation energy is the energy input into a reaction to allow chemical bonds to be broken.allow chemical bonds to be broken.

Making chemical bonds

• Energy will be given out in an exothermic process when bonds are formed.

Heat given out

Energy given out as bonds form between

atoms

En

erg

y in

ch

emic

als

Energy given out

Bond energy

This is the energy needed to break 1 mole of covalent bonds

or

The energy released when 1 mole of covalent bonds are made

Changes to chemical bonds

• Again some existing bonds are broken (endothermic)

Energy taken in as old bonds break

• And new bonds are formed (exothermic)

En

erg

y in

ch

emic

als

reactants

products

Energy given out as new bonds form

H

Overall exothermic – in this case

• The formation of nitrogen (IV) oxide (formula NO2) from reaction of nitrogen with oxygen in car engines has a H value of +33.2kJ per mol of nitrogen oxide.

1. Write a word equation for the reaction.2. Write a chemical equation for the reaction.3. Is H positive or negative?4. Is the reaction exothermic or endothermic?5. Draw an simple energy diagram for the reaction

(showing bond breaking and forming.) 6. Which involves the biggest energy change: bond

breaking or bond forming?

Activity

1. Nitrogen + oxygen nitrogen(IV)oxide2. N2 + 2O2 2NO2.3. H positive (+33.2kJ/mol).4. The reaction is endothermic.5. Energy diagram 6. Bond breaking involves the biggest energy change.

Answer

Ene

rgy

/ kJ

)


reactants

products

H= -

Activation Energy and Exothermic Reactions

Ea= +Activation energy

Ene

rgy

/ kJ

)


reactants

products

H=+

Ea= +Activation energy

Activation Energy and Endothermic Reactions

Copy the energy diagram and use it to help you explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps.

Ene

rgy

/ kJ

)


Petrol +

oxygen

Carbon dioxide + water

ActivationEnergy

Activity

The reaction is exothermic but requires the Activation energy to be provided before the reaction can get underway.This is necessary to break some of the bonds in the oxygen or petrol before new bonds can start forming.

Ene

rgy

/ kJ

)


Petrol +

oxygen

Carbon dioxide + water

ActivationEnergy

Answer

• This is an exothermic reaction

Bond Forming

BondBreaking


En

erg

y in

ch

emic

als

OO

OO

H

CH

HH

O OOO

C H H H H

O C OO

O

H H

H H

H

Burning Methane

Heat of combustion

• The heat change that occurs when 1 mole of substance is burned in excess oxygen

Uses of heat generated from burning substances

Transportation

Generating electricity

A bomb calorimeter

• Use : measuring the energy contents of fuels or foods

Kilogram Calorific value Amount of heat generated when 1 kilogram

of fuel when it is completely burned.

Fuels Gross calorific value/ MJ kg−1

Ethanol 30

General purpose coal (5–10% water) 32–42

Peat (20% water) 16

Diesel fuel 46

Gas oil 46

Heavy fuel oil 43

Kerosine 47

Petrol 44.8–46.9

Wood (15% water) 16

Natural gas `54

Hydrogen 141.9

Heat of formation • The heat change when 1 mole of a

substance is formed from its elements in their standard states

C(S) + 2H2(g) = CH4(g) ∆H = -74.9kJmol-1

H2(g) + S(S) +2O2(g)= H2SO4(g) ∆H = -811kJmol-1

O2(g)= O2(g) ∆H = 0kJmol-1 The heat of formation of any element is zero

Hess’s Law• The heat change for a

reaction is the same whether it takes place in one step or in a series of steps

• This rule can be used for calculations using Hess’s Law

∆Hr = ∑ ∆Hf products - ∑ ∆Hf reactants

An exampleWhat is the heat of reaction of

SO2 + 1/2O2 SO3?

The heat formation of SO3 and SO2 are -395 kJmol-1 and -297kJmol-1 respectively


∑ ∆Hf products = 1 ( -395) = - 395 kJmol-1

∑ ∆Hf reactants = 1 ( - 297 ) + 1/2 (0) = -297∆Hr = ∑ ∆Hp - ∑ ∆Hr

∆Hr = (-395) – (-297) = - 98 kJmol-1 heat of reaction for the given equation

Q270. Using Hess’s Law• The combustion of liquid benzene is described by the following equation

2C6H6 + 15O2 12CO2+ 6H20

• Given that the heats of formation of carbon dioxide gas, liquid water and liquid benzene are -394,-286 and 49kJmol-1 respectively, calculate the heat of combustion of liquid benzene

• ∑ ∆Hf products = 12 ( -394) + 6 (-286) = -6444kJmol-1

• ∑ ∆Hf reactants = 2 ( 49) + 14 (0) = 98

• ∆Hr = ∑ ∆Hp - ∑ ∆Hr

• ∆Hr = -6444 – = - 6542kJmol-1 heat of reaction for the given equation ( for 2 moles of benzene)

• Heat of combustion is heat change when 1 mole of a substance completely reacts in excess oxygen so heat of combustion of benzene is ( - 6542/ 2) = - 3271kJmol -1

∆Hr = ∑ ∆Hp - ∑ ∆Hr ∆Hr = ∑ ∆Hf products - ∑ ∆Hf reactants

Q271. Using Hess’s Law• The combustion of butane is described by the following equation

2C4H10 + 13O2 8CO2+ 10H20

• Given that the heats of formation of butane, carbon dioxide gas, liquid water are -125,-394 and -286kJmol -1 respectively, calculate the heat of combustion of butane

• ∑ ∆Hf products = 8( -394) + 10 (-286) = -6012kJmol-1

• ∑ ∆Hf reactants = 2 ( -125) + 13 (0) = -250kJmol-1

• ∆Hr = ∑ ∆Hp - ∑ ∆Hr

• ∆Hr = - 6012 – (-250) = - 5762kJmol-1 heat of reaction for the given equation ( for 2 moles of butane)

• Heat of combustion is heat change when 1 mole of a substance completely reacts in excess oxygen so heat of combustion of butane is (5762 / 2) = - 2881kJmol -1


Q272. Using Hess’s Law• The combustion of cyclohexane is described by the following equation

C6H12 + 9O2 6CO2+ 6H20

• Given that the heats of formation of cyclohexane, carbon dioxide gas, liquid water are -156, -394,-286 kJmol -1 respectively, calculate the heat of combustion of cyclohexane


• ∑ ∆Hf reactants = 1( -156) + 9(0) = -156 kJmol-1

• ∆Hr = ∑ ∆Hp - ∑ ∆Hr

• ∆Hr = -4080 – (-156) = -3924 kJmol-1 heat of reaction for the given equation ( for 1 mole of cyclohexane)

• Heat of combustion is heat change when 1 mole of a substance completely reacts in excess oxygen so heat of combustion of cyclohexane is - 3924 kJmol-1


Q273. Using Hess’s LawPropane may be used in gas cyclinders for cooking appliances

Propane burns according to the following equatioon:

C3H8 + 5O2 3CO2+ 4H20

(i) Given that the heats of formation of propane, carbon dioxide gas, liquid water are -104, -394,-286 kJmol -1 respectively, calculate the heat of combustion of propane

• ∑ ∆Hp = 3 ( -394) + 4 (-286) = -2326 kJmol-1

• ∑ ∆Hr = 1( -104) + 5(0) = -104

• ∆Hr = ∑ ∆Hp - ∑ ∆Hr

• ∆Hr = - 2326 – (-104) = - 2222kJmol-1 heat of reaction for the given equation ( for 1 mole of propane)

• Heat of combustion is heat change when 1 mole of a substance completely reacts in excess oxygen so heat of combustion of propane is - 2222kJmol-1


• If 500kJ of energy are needed to boil a kettle of water what mass of propane gas must be burned to generate this mass of heat? Give your answer to the nearest gram

x moles of propane = 500kJ of heat released when combusted

1 mole of propane = 2222Kj of heat released when combusted

500 x1 = x

2222

0.225022502 = x

0.225 = number of moles of propane needed to release 500 kJ of heat when combusted

Find number of moles needed Find the number of grams needed

0.225 moles of propane , how many grams?

(44)(0.225) = 9.901

Answer :9.901 g of propane would generate 5000Kj of heat

• If 500kJ of energy are needed to boil a kettle of water what mass of propane gas must be burned to generate this mass of heat? Give your answer to the nearest gram

Find number of moles needed Find the number of grams needed

X RMM

Q274. Using Hess’s LawI. Write a balanced equation for the combustion of ethanol C2H5OH.

II. Given that the heats of formation of ethanol, carbon dioxide and water are -278, -394,-286 kJmol-1 respectively, calculate the heat of combustion of ethanol.

i. C2H5OH + 3O2 2CO2+ 3H20

• ii)• ∑ ∆Hf products = 2( -394) + 3(-286) = -1646kJmol-1

• ∑ ∆Hf reactants = 1( -278) + 3(0) = -278 kJmol-1

• ∆Hr = ∑ ∆Hf products - ∑ ∆Hf reactants

• ∆Hr = -1646 – (-278) = -1368 kJmol-1 heat of reaction for the given equation ( for 1 mole of ethanol)

• Heat of combustion is heat change when 1 mole of a substance completely reacts in excess oxygen so heat of combustion of ethanol is - 1368 kJmol-1


Q275. Using Hess’s LawI. Write a balanced equation for the combustion of methanol CH3OH.

II. Given that the heats of formation of ethanol, carbon dioxide and water are -239, -394,-286 kJmol-1 respectively, calculate the heat of combustion of methanol.

i. CH3OH + 1½O2 1CO2+ 2H20

• ii)• ∑ ∆Hf products = 1( -394) + 2(-286) = -966 kJmol-1

• ∑ ∆Hf reactants = 1( -239) + 1.5(0) = -239 kJmol-1


• ∆Hr = -966 – (-239) = - 727 kJmol-1 heat of reaction for the given equation ( for 1 mole of methanol)

• Heat of combustion is heat change when 1 mole of a substance completely reacts in excess oxygen so heat of combustion of methanol is - 727 kJmol -1


Q276b) Using Hess’s Law• The combustion of methane is described by the following balanced equation

CH4 + 2O2 CO2+ 2H20 ∆H = -890.4kJmol-1

• Given that the heats of formation of carbon dioxide gas, liquid water are -394,-286 kJmol -1 respectively, calculate the heat of formation of methane

• ∆Hr = -890.4kJmol-1


• ∑ ∆Hf reactants = 1( x ) + 2(0) = x kJmol-1


• -890.4kJmol-1 = -966kJmol-1 – (x kJmol-1 )

• - 75.6kJmol-1 = x kJmol-1 • Heat of formation of methane is - 75.6kJmol-1


Let x = heat of formation of methane

Q277) Using Hess’s Law• The combustion of ethyne is described by the following balanced equation

C2H2 + 2½O2 2CO2+ H20 ∆H = -1299kJmol-1

• Given that the heats of formation of carbon dioxide gas, liquid water are -394,-286 kJmol -1 respectively, calculate the heat of formation of ethyne

• ∆Hr = -1299kJmol-1

• ∑ ∆Hf products = 2( -394) + 1(-286) = -1074kJmol-1

• ∑ ∆Hf reactants = 1( x ) + 2½ (0) = x kJmol-1


• -1299kJmol-1 = -1074kJmol-1 – (x kJmol-1 )

• - 225kJmol-1 =- x kJmol-1 • Heat of formation of ethyne is 225kJmol-1


Let x = heat of formation of ethyne

Q278) Using Hess’s Law• The combustion of propane is described by the following balanced equation

C3H8 + 5O2 3CO2+ 4H20 ∆H = -2222kJmol-1

• Given that the heats of formation of carbon dioxide gas, liquid water are -394,-286 kJmol -1 respectively, calculate the heat of formation of propane

• ∆Hr = -2222kJmol-1

• ∑ ∆Hf products = 3( -394) + 4(-286) = -2326kJmol-1

• ∑ ∆Hf reactants = 1( x ) + 5 (0) = x kJmol-1



• - 104kJmol-1 = x kJmol-1 • Heat of formation of propane is - 104kJmol-1


Let x = heat of formation of propane

Q279) Using Hess’s Law• Write a balanced equation for the combustion of ethanol

i. C2H5OH + 3O2 2CO2+ 3H20 ∆H = -1300kJmol-1

• Given that the heats of formation of carbon dioxide gas, liquid water are -394,

-286 kJmol-1 respectively, calculate the heat of formation of ethanol

• ∆Hr = -1300kJmol-1


• ∑ ∆Hf reactants = 1( x ) + 3(0) = x kJmol-1



• - 346kJmol-1 = x kJmol-1 • Heat of formation of ethanol is - 346kJmol-1


Let x = heat of formation of ethanol

exothermic and endothermic reactions. chemical reactions usually involve a temperature change (heat...

Documents

heat of reaction

energy energy

c endothermic reactions

reaction of acids

cools heat energy

o c exothermic reactions

kj slide

cold reaction products