HSC Chemistry: Measure the enthalpy of Neutralisation| Module 6

Summary

This script discusses the principles of calorimetry, focusing on measuring the enthalpy change of reactions. It explains the setup of a typical calorimeter experiment, introduces the concept of specific heat capacity, and demonstrates how to calculate enthalpy change using the formula q = mcΔT. The video also covers molar enthalpy of neutralization, highlighting the standard value of -57 kJ/mol for strong acid-strong base reactions and noting that weaker acids or bases result in less energy release. Practical aspects, such as choosing the right base for neutralizing a sulfuric acid spill, are also addressed, emphasizing safety and the importance of minimizing heat loss in experiments.

Takeaways

• 🔍 Calorimetry is the process of determining the enthalpy change of a reaction by measuring the heat absorbed or released.
• 🧪 A calorimeter is an instrument used to measure the enthalpy change in a system, often involving a setup with an insulated cup and a thermometer.
• 🌡️ The temperature change during a reaction is a key measurement, which can be used to calculate the enthalpy change when combined with the mass of the substance and its specific heat capacity.
• ⚖️ The specific heat capacity (c) is the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius or Kelvin, typically 4.18 J/g·K for water.
• 🔄 The enthalpy change (ΔH) is calculated using the formula ΔH = -q/n, where q is the heat exchanged and n is the number of moles of product formed.
• 💧 Molar enthalpy of neutralization is the heat change when one mole of water is formed in a neutralization reaction, typically around -57 kJ/mol for strong acid-strong base reactions.
• 🌡️ Lab conditions for measuring molar enthalpy of neutralization are standardized at 25 degrees Celsius and 100 kilopascals to ensure consistency.
• 🧩 In a neutralization reaction, all reactions go to completion regardless of the strength of the acid or base, but the enthalpy change can vary with the presence of weak acids or bases.
• 🚨 In chemical spills, using a weak base like sodium carbonate can be safer than a strong base due to less heat release and its ability to absorb the acid.
• 📝 The importance of significant figures in recording measurements and calculations cannot be overstated, as incorrect rounding can lead to significant errors in results.
• 🔍 The main source of error in calorimetry experiments is heat loss to the surroundings, which can be mitigated by using better insulation or heat shields.

Q & A

What is calorimetry?

Calorimetry is the process of determining the enthalpy of a reaction by measuring the heat absorbed or released during the reaction.

What is a calorimeter?

A calorimeter is an instrument used to measure the enthalpy change of a reaction by observing the temperature change of a known amount of substance.

What are the typical components of a calorimeter setup?

A typical calorimeter setup includes an insulated cup containing a known mass of water and a thermometer to measure the temperature change during a reaction.

How is the enthalpy change of a reaction calculated?

The enthalpy change of a reaction is calculated using the formula ΔH = -q/n, where q is the heat absorbed or released by the system and n is the number of moles of product formed.

What is q in the context of calorimetry?

In calorimetry, q represents the heat absorbed or released by the substance, which can be calculated using the formula q = mcΔT, where m is the mass of the substance, c is the specific heat capacity, and ΔT is the temperature change.

What is the specific heat capacity and why is it important in calorimetry?

The specific heat capacity is the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius or one Kelvin. It is important in calorimetry because it is used to calculate the heat absorbed or released by the substance during a reaction.

What is the molar enthalpy of neutralization?

The molar enthalpy of neutralization is the enthalpy change in a neutralization reaction when exactly one mole of water is formed.

Why is the molar enthalpy of neutralization typically negative?

The molar enthalpy of neutralization is typically negative because neutralization reactions are exothermic, meaning they release heat.

What is the typical temperature and pressure for laboratory settings when measuring enthalpy changes?

The typical temperature and pressure for laboratory settings when measuring enthalpy changes are 25 degrees Celsius and 100 kilopascals, respectively.

Why would a weak base be preferred over a strong base when neutralizing a chemical spill, such as sulfuric acid?

A weak base would be preferred because it can absorb the liquid of the acid, limit the spread of the acid, and release less heat during the reaction, reducing the risk of additional dangers in a classroom setting.

What is the main source of error in a calorimetry experiment?

The main source of error in a calorimetry experiment is heat loss to the surroundings, which can result in a lower measured q value than in theory.

How can the experiment be improved to prevent heat loss?

The experiment can be improved by using double insulation, such as double-walled styrofoam cups, to minimize heat loss to the surroundings.

Outlines

00:00

🔍 Introduction to Calorimetry and Enthalpy Measurement

This paragraph introduces the concept of calorimetry, which is the science of measuring the enthalpy change of a reaction. It explains that a calorimeter is an instrument used for this purpose. The typical setup for calorimetry in an educational setting is described, involving an insulated cup with water and a thermometer to measure temperature changes due to chemical reactions. The goal is to measure the enthalpy change by observing the heat absorbed or released, changing the water's temperature. Key equations are introduced, such as enthalpy change being equal to the negative heat quantity (q), and the formula for heat (q = mcΔT), where m is mass, c is the specific heat capacity, and ΔT is the temperature change.

05:01

🧪 Understanding Calorimeter Setup and Neutralization Reactions

The paragraph delves into a typical calorimeter setup, including the use of a thermometer and an insulated cup filled with water to measure temperature changes during a reaction. It discusses the concept of molar enthalpy of neutralization, which is the enthalpy change when one mole of water is formed in a neutralization reaction. The standard conditions for such measurements are room temperature and pressure. The paragraph also touches on the practical considerations of choosing a weak base over a strong base for neutralizing chemical spills to control heat release and limit damage.

10:02

📚 Calorimetry Calculations and Experimental Procedure

This section focuses on the practical application of calorimetry calculations, starting with determining the number of moles of reactants using given concentrations and volumes. It emphasizes the importance of identifying the limiting reagent to understand how much of the reactants are consumed in the reaction. The goal is to find the number of moles of water produced, which is crucial for calculating the enthalpy change using the formula ΔH = -q/n. The paragraph also discusses the significance of accurate measurements and the impact of rounding on the final results.

15:02

🔢 Analyzing Calorimetry Data and Error Sources

The paragraph discusses the process of analyzing data from a calorimetry experiment, including calculating the heat absorbed by the water (q), the mass of the water, and the final temperature change. It highlights the importance of maintaining significant figures in scientific reporting and the potential discrepancies between theoretical and experimental values. The main source of error in such experiments is identified as heat loss to the surroundings, which can be mitigated by using double insulation or other heat shields.

20:04

🛠 Improving Calorimetry Experiments and Reflecting on Accuracy

In this paragraph, the discussion revolves around improving the accuracy of calorimetry experiments by addressing heat loss issues. Suggestions include using double insulation or reflective materials to minimize heat loss. The importance of using a sufficient amount of reagents to ensure a noticeable temperature change is emphasized, as it is crucial for accurate readings. The paragraph concludes with a reminder of the challenges in achieving precise measurements and the inherent limitations of experimental setups.

Keywords

💡Calorimetry

Calorimetry is the scientific technique used to measure the heat of chemical reactions or physical changes. It is central to the video's theme as it discusses how to determine the enthalpy of a reaction. The script mentions calorimetry as the process of measuring enthalpy changes using a calorimeter, which is directly related to the main educational message of the video.

💡Enthalpy

Enthalpy is a thermodynamic property that represents the total heat content within a system. In the context of the video, enthalpy is the key quantity being measured during calorimetry experiments. The script explains that the goal of calorimetry is to measure enthalpy changes, particularly during chemical reactions.

💡Calorimeter

A calorimeter is an instrument used to measure the enthalpy of a reaction by quantifying the heat absorbed or released. The video describes a typical calorimeter setup, which includes an insulated cup with a thermometer to measure temperature changes in water or another substance during a reaction.

💡Temperature Change

Temperature change refers to the difference in temperature before and after a chemical reaction or physical process. In the script, temperature change is measured using a thermometer in the calorimeter to determine the heat absorbed or released, which is essential for calculating the enthalpy change.

💡Specific Heat Capacity (c)

Specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius or one Kelvin. The video emphasizes its importance in the formula q = mcΔT, where 'c' is used to calculate the heat absorbed or released by the water in the calorimeter.

💡Molar Enthalpy of Neutralization

Molar enthalpy of neutralization is the heat change when one mole of water is formed in an acid-base neutralization reaction. The video discusses this concept as a typical example of enthalpy change measurement, often occurring at room temperature and pressure.

💡Limiting Reagent

The limiting reagent in a chemical reaction is the reactant that is completely consumed and determines the amount of product formed. The script explains the importance of identifying the limiting reagent to calculate the number of moles of product, which is necessary for finding the enthalpy change.

💡Exothermic Reaction

An exothermic reaction is a chemical process that releases energy, usually in the form of heat. The video mentions that the heat from an exothermic reaction is absorbed by the water in the calorimeter, which is crucial for understanding how to measure enthalpy changes.

💡Heat Loss

Heat loss refers to the energy that escapes from the system into the surroundings during an experiment. The video script discusses heat loss as a source of error in calorimetry, as it can lead to an underestimation of the actual enthalpy change.

💡Significant Figures

Significant figures are the digits in a number that carry meaningful information about its precision. The video emphasizes the importance of using the correct number of significant figures in reporting experimental results, as it affects the accuracy and reliability of the data.

Highlights

Calorimetry is defined as the process of determining the enthalpy of a reaction.

A calorimeter is an instrument used to measure enthalpy changes during reactions.

Typical calorimetry setup includes a thermometer in an insulated cup with water or another substance to measure temperature changes.

The enthalpy change of a reaction can be calculated using the mass of water, specific heat capacity, and temperature change.

The formula for calculating the heat (q) absorbed or released by water is q = mcΔT, where m is mass, c is specific heat capacity, and ΔT is temperature change.

The specific heat capacity (c) of water is 4.18 joules per gram per kelvin, a value important for accurate calorimetry calculations.

Molar enthalpy of neutralization is the energy change when one mole of water is formed in a neutralization reaction.

Standard conditions for laboratory settings are typically 25 degrees Celsius and 100 kilopascals.

The typical molar enthalpy of neutralization for strong acids and bases is around -57 kilojoules per mole.

Weak acids or bases result in a lower enthalpy change compared to strong acids or bases due to differences in bond energies.

In a chemical spill, a weak base like sodium carbonate is preferred over a strong base to control heat release and limit acid spread.

All neutralization reactions go to completion regardless of the strength of the acid or base involved.

The importance of rounding to the correct number of significant figures in chemistry calculations is emphasized for exam accuracy.

Heat loss to the surroundings is a primary source of error in calorimetry experiments.

Strategies to reduce heat loss include using double insulation and reflective materials like aluminum to retain heat within the system.

The significance of using a noticeable temperature change for accurate readings on a thermometer is discussed.

The transcript includes a step-by-step walkthrough of a calorimetry problem, illustrating the process of finding the limiting reagent and calculating the final temperature.

Transcripts

joe tell me about calorimetry

uh calorimetry is determining the

enthalpy of a reaction

good the goal is to measure

enthalpy triangle h reset tell me what a

calorimeter is

[Music]

yeah um

so a calorimeter is an instrument that's

used to measure enthalpy right that's

literally what it is it's the instrument

that we use in calorimetry

to actually measure the enthalpy

now gerald what was a typical setup for

the color emitter that you might have

done in year 11

um

so you would have a thermometer

sticking inside an insulated

cup and there would be water or some

other substance which you could measure

the temperature change of um during the

reaction good exactly right so the

perfect setup is right here

right so we have a volume of water that

is going to be absorbing any heat change

that occurs due to the reaction so

imagine we drop substance a and b

settles right to the center of the

solution and we have a chemical reaction

occurring

and that reaction is going to absorb

heat or it's going to release heat and

thus change the temperature of the water

does everyone agree

now we can measure the temperature

change using the thermometer that's

being stuck in right here

and we can use that

and the mass of the water to determine

the enthalpy change of reaction

okay so what are the two equations we

need to measure enthalpy change so

enthalpy change

equals to negative q on it you might

have seen that before

there is a form of a queue

that what is q equal to

um

q is equal to

seen mcat have you heard of q equals to

anything okay

yeah i've just looked back to that do it

so q refers to the energy

that is absorbed or released by the

water right so the energy

change

of

the body of water

so we need the mass of water does

everyone see that

we need the mass of water here it could

be 200 grams 100 grams etc so this is

the mass of water in grams

what is c gerald do you want to define

what c is

um that's the specific heat capacity of

the

substance

good what does that mean

um

it's the amount of energy required per

mole to change the temperature of that

substance by one kelvin

is it per mole

um

the unit is joules

per gram per kelvin

right so it's the amount of energy in

joules required to raise the temperature

of one gram of that substance by one

degree celsius or one degree kelvin so

it's joules

per gram times kelvin does that make

sense

and for water the value is 4.18 you

should know that it's in your data sheet

but i would commit it to memory to save

time

okay and finally t is a temperature

change

now

let's go up to the earlier reaction

now this reaction here is everything to

do the body of water i want you to

remember that q equals m cat is all

about the water don't run with me

whereas the n value here do you see this

n here

it has everything to do the actual

reaction that is going on inside of the

water

n is the number of moles of product you

produce

okay that's what n is

and so you can use this reaction to thus

find the energy change per mole of

substance produced

does that make sense that is how we

determine enthalpy changes

good now

who's learnt molar enthalpy of

neutralization at school

okay well we'll describe it so i'm

actually not gonna show you the

definition you're gonna intuitively

derive it

okay

molar enthalpy of neutralization but

what did we say enthalpy was gerald

that's the heat i mean g within the

system good right so if we're looking at

enthalpy of neutralization we're

measuring

the energy

of neutralization

and it's molar so what does that mean

it's per mole

and what is the one product you looked

at the net ionic equation of

neutralization what is the one product

that's always fall

water good so the molar enthalpy of

neutralization is the enthalpy change

in a neutralization reaction

when exactly one mole of water is formed

now

in general lab settings what would the

pressure and temperature

because you've got to standardize this

right you could get a different molar

enthalpy if you're doing it in the

antarctic or whether you're doing it in

you know uh uluru so

what's the typical temperature and

pressure in a

lap

25 degrees celsius and 100 kilopascals

exactly room temperature pressure

conditions so that is the literal

word-for-word definition have a look at

this the energy in kilograms per mole

liberated per mole of water

in rtp room temperature and pressure

now what did i say that number typically

was

when we did our net ionic equation

question it's typically negative

57

okay so typically entropy of

neutralization is negative 57 kilojoules

per mole

and that is because all acid-base

reactions have the same net ionic

equation now i want to stress this

this negative 57 happens when you have a

strong acid strong base

so if you have a strong acid weak base

you will liberate less energy than this

okay and that is to do with the bonds in

that weak base so remember if you ever

have a weak acid or weak base it's going

to lower this enthalpy change

everyone with me

so let me give you a thought experiment

this is how they test you in an exam

question if a student spilled

concentrated sulfuric acid

in the classroom

what would be an appropriate base

to neutralize that spill

and why

pick any base you have freedom of choice

would you pick a strong base like sodium

hydroxide or would you pick a weak base

like sodium carbonate

strong place

if you pick a strong base okay and what

about you gerald

oh weak base

okay

now

jared why would you pick a weak base

um because you ideally want to decrease

the um

actually

actually wait can i change my answer

are you sure you want to

explain your original thought you

wouldn't

originally it was um because you would

want to um probably decrease damage to

surroundings by decreasing um change in

l2p or the heat which is produced so you

would use the weak base you're

completely right actually right so you

need to understand this even with a

strong acid strong base or a weak acid

strong base so imagine imagine we had

hcl which is a strong acid and it

combines with a weak base like sodium

carbonate

the reaction will always go to

completion

you never form an equilibrium i want you

to remember this all neutralizations go

to completion

so we don't care about how strong the

acid or base is what we care about now

is the enthalpy i told you it's highly

exothermic so if we were to put sodium

hydroxide

on the hydrochloric acid

that would cause extreme amounts of heat

release in fact you could get boiling of

the solution

right and that would be a whole other

additional danger to the classroom

setting does that make sense the other

reason we actually prefer to use sodium

carbonate is because this comes as a

solid it looks like sand

right you've seen baking soda before

right the sodium carbonate has a similar

solid composition so it can actually

absorb the liquid of the acid as well

and thus limit the spread of the acid

physically

so that's why we prefer to use a weak

base that is a solid

over a strong base in chemical spills

this question has come up before so

that's why it's higher to understand

that

all right are we ready to start doing

some exam questions

all right so i'm going to throw you in

the deep end here is your

here's your first question

joe how about you uh i've got an answer

but um

i'm a bit suspect about the way i would

do that so yeah okay what's your answer

what's the final time

i got 25 points

25.75 degrees celsius

okay

okay

how many sig figs is in the question

though

um

three

is it three what's the lowest six you

see

um minus fifty seven point two

what about point one

potassium hydrogen oh right

make sure your answer is always to the

correct sig figs because there'll always

be one question in the hsc that they've

marked sig figs on and if you don't do

it right for that one question you'll

lose a mark

one mark is bigger than hsc one mark

drops you down thousands of

ranks and you're um you can go from a

state rank to a mid band six in one

single mark so that's why it's so

important

okay

let's start going through it so we'll

start with you gerald what's the first

thing i should do

um first thing i did was calculate the

number of moles of potassium

equation right

so what is the equation

uh koh plus hcl

and that gives you h2o plus kcl good

all right

balanced or not balanced all right

that's balance so okay so now tell me

what data do we have

um so

you have the

um volume of volume and concentration of

potassium hydroxide good i want you to

say c and v give it to me mathematically

so c of k koh is is equal to 0.1 molar

okay and what about v of koh and that is

equal to 27.9 milliliters

okay done

that's c and v what about uh

hcl um c of hcl is equal to 0.11 molar

okay and what's v of hcl and v of hcl is

equal to

31.25

milliliters

okay what else do we have

what are the data

and then we also have the initial

temperature

okay and let's say i'm stuck now what

should i do

um

as in like what's the next step yeah

uh so i calculated number of moles of

potassium hydroxide and hcl

good sure that works out what you could

also do is draw a diagram if you'd like

and then like i said frame the question

what is it actually asking it's asking

for tf

equals to question mark does that make

sense

so if we want to find tf we've got ti

we need to find change in t so the true

question is what is the change in

temperature if you find change in t

you've got your answer

right that's what i mean by framing the

question good so

number of moles of koh what did you get

so tell me for n of ko h n of hcl what

did you get

for nfk of h i got

0.00279 volts

and um for hcl i got

point zero zero three four three seven

five volts

joe do you get the same

uh yeah yeah that's right okay so why

did we find the number of moles of each

reason what are we trying to do

now uh we're trying to find which one is

the

uh limiting limiting reagent to find out

how much of it is are actually used up

good okay all right why don't we worry

about the limiting reagent gerald what's

the end goal of all this limiting

reagent work because we want to find out

number of moles of h2o produce very good

right because that's our n value if you

remember the equation triangle h

equals negative q and n well you've got

triangle h did you all see that so i'll

write that as a triangle h equals

negative 57.2 kilojoules per mole so

you've got triangle h if we can use

limiting axis to find n we can find q

and we've got all the other data so we

can thus find change in t

so the approach is actually quite

straightforward so now it's just about

following through so good so limiting

reagent would be

koh right

reset how do you find the number of

moles of water

uh use the stoichiometry so that you

know that then the number moles of

h2o is also the number of moles of kvh

which is

a let's call this as b so yeah that's

going to be equal to a moles

very good

so now we can use triangle h equals

negative q on n and we can find q

q equals to triangle h

times n or negative triangle h

so this is going to be negative of

negative 57.2 kilojoules remember it's

always exothermic per mole

times

a moles now do you see i'm writing units

here i don't usually do that but the

reason i'm doing that

is because triangle h is always in

kilojoules per ml whereas when you use q

you must convert it to joules per mole

okay that is why units is very important

so always be cautious of that it's a

huge silly mistake students make going

from joules to kilojoules so

what's our answer for q

tell me the answer in kilojoules and

joules gerald

um so in kilojoules i got minus 0.159588

so minus zero point

one five

nine five

eight eight

okay let's do that as should shouldn't

it be plus here because it should be

it's minus or minus so it should be plus

remember q is the energy absorbed by the

water if it's an exothermic reaction

think about it where did all that heat

go to the water from the system so it

has to be plus and mathematically it

makes sense because it's an exothermic

reaction

and q is negative

negative of an exothermic or negative

value is positive does that make sense

good so that's our kilojoules but let's

convert it to joules so that's going to

be is it

159.58 joules

uh yeah

good let's store that as d all right

okay so now we know q equals to m cap

what's our mass value

what's a mass of water

rheostat

is it

27.9 plus 31.25 which is exactly right

it's all just water that's what the

solution is so what is that

59.15 millions

good

so convert that to liters and what's

this is the heat capacity of water 4.18

so what's our final answer

um

so

okay

okay

uh yeah

what did we get

i got 25.6 degrees celsius i mean 0.6

that's the change

okay what was the full number

what was the full number was it exactly

0.6 what's the full number oh full

number was

well it was long it was like 0.6454

[Music]

yeah okay good so round that off to x

good and the final thing we can say is

well

well we know that change in t is equal

to t f minus t i we're trying to find tf

so tf

is change in t plus ti so the answer is

equals to 25.645

you said 25.7 is that because you

rounded it but it wouldn't round up

oh

um

i think it's six four five i think the

last time i'd done this question it was

point six four five double check you're

working yeah yeah i think i might have

rounded a bit too early somewhere yeah

never around in chemistry until the very

end you want to make this

practice never ever always store it and

at the very end you then round it

okay because they can mark you down even

if you're off by the decimal points so

good so then this will round off

to final temperature being 30 degrees

because it's to one sig fig

um if i still got 30 degrees well like

yeah if i still got 30 degrees celsius

like the um to signal never configures

even if my um

like

unrounded answer was like slightly off

would that be a problem

it really depends on your marker see

that's the thing you're i think we have

to realize that schools are very

non-systematic they do whatever they

like right so what would happen in that

case is

if you set your exam and you did that

and if there was a very small standard

deviation in the cohort then they would

make that part of the criteria and you

would lose the mark okay so that's

typically what happens

in the hsc

they try to be lenient so they may give

it to you but certain questions it's

very strict it all depends what was made

as a marketing criteria

so i would say to be safe you want to

have a perfect answer rendered off at

the very end okay

good

all right good job homework is going to

be

you should have access to this booklet i

will double check your access if you

don't have access throughout mid next

week do message me okay and i will uh

i'll follow through on that for us

so

page 20

21

22 are all additional practice questions

of the same type so you can do that in

your own time okay

if you can do those questions it's very

easy and i guess the one thing we'll

talk about

discrepancy between theoretical and lab

values so reset what would the main

discrepancy be here

what would result in a discrepancy

between the true value and our

calculated value for enthalpy change

what is the main source of error in this

experiment

come on there's a reason we use double

double styrofoam cups

uh energy released into the surroundings

exactly right and that results in a

lower q value

than in theory

right because

yeah sorry my parents are here and they

just need me to open them so i'll just

okay no worries okay so um yeah so

that's what i'm mentioning the main

issue here is heat loss to the

surroundings

heat loss okay gerald how would you

improve this experiment to prevent the

heat loss

double insulated so what else yeah

um

use less

reactants such that the change in

temperature isn't great such that so you

won't lose as much heat to the

environment

uh change in temperature is not so great

that you will not

but see the the when we talk about

accuracy it's about percentage error

right so the percentage area will still

be there even if the temperature change

is larger i get where you're coming from

um

i would say if you used more reagent you

cause more heat to change

i think that's good because if you think

about this if you use a very tiny amount

of reagent what if your thermometer

can't even pick up the temperature

change accurately that's the limiting

factor right if the temperature change

is 0.0002 degrees celsius your

thermometer won't even pick up a

temperature change and thus you'll be

limited by the accuracy of your

instruments so i would say you'd want a

noticeable temperature change so you can

actually pick it up accurately on a

thermometer

but good thought good thought but um i

would say one of the main things is heat

shield so you could use

certain structures like aluminium and

that can refract reflect heat back into

the system

but uh again it's a bit wishy-washy it

doesn't work too well

good let's move on