Hur vet man om en reaktion är exoterm eller endoterm?

Summary

This educational video script delves into the distinction between exothermic and endothermic reactions, explaining that it's not always possible to determine this from the reaction formula alone. It introduces the SIV-rule for diluting concentrated acids safely and uses it as an example of an exothermic process. The script then outlines general rules: the formation of bonds is always exothermic, while bond breaking is endothermic. It also touches on spontaneous redox reactions being exothermic, contrasting them with non-spontaneous ones that are endothermic. The script further explains that combustion is typically exothermic, while photosynthesis is endothermic. It concludes by emphasizing that a reaction with ΔH < 0 is exothermic and ΔH > 0 is endothermic, providing a clear guideline for students to determine the nature of chemical reactions.

Takeaways

• 🔍 Students often ask how to determine if a reaction is exothermic or endothermic, but it's not always possible to tell just by looking at the chemical equation.
• 📚 Every aspiring chemist must learn the SIV-rule: when diluting a concentrated acid, especially sulfuric acid, always add the acid to water, not the other way around.
• ♨️ The reason for the SIV-rule is that a lot of heat is generated when sulfuric acid dissolves in water, which is an example of an exothermic process.
• 🔥 Exothermic processes can be identified through experience, such as feeling heat, which indicates that energy is being released.
• 📏 Chemists have conducted numerous measurements to derive general rules for determining if a process is exothermic or endothermic.
• 💥 A fundamental rule is that the formation of bonds is always an exothermic process, with no exceptions.
• 🔗 Conversely, the breaking of bonds is always an endothermic process, requiring energy input.
• ❄️ An example of bond formation is the transition of water from liquid to solid (ice), where hydrogen bonds form between water molecules, releasing energy.
• 🌡 Despite the release of energy, the surroundings become cold when water freezes, highlighting the difference between the process and the environment.
• 🔋 Spontaneous redox reactions, such as those in galvanic cells, are also exothermic processes.
• 🔄 The reverse of a spontaneous redox reaction, which requires energy input, is endothermic, like in electrolytic cells.
• 🔥 Combustion reactions, including cellular respiration, are a type of spontaneous redox reaction and are therefore exothermic.
• 🌱 Photosynthesis, the reverse of respiration, converting carbon dioxide and water into glucose, is an endothermic process.
• ⚠️ For reactions not covered by general rules, look for clues such as ΔH < 0 indicating an exothermic reaction, and ΔH > 0 indicating an endothermic reaction.
• ✅ Remembering that the formation of bonds is exothermic and the breaking of bonds is endothermic is crucial for understanding chemical processes.

Q & A

How can you determine if a chemical reaction is exothermic or endothermic just by looking at the reaction formula?

You cannot determine if a reaction is exothermic or endothermic just by looking at the reaction formula. It requires experience or measurements to ascertain.

What is the SIV-rule mentioned in the script and why is it important?

The SIV-rule states that when diluting a concentrated acid, primarily sulfuric acid, you should pour the acid into water, not the other way around. This is important because dissolving sulfuric acid in water releases a significant amount of heat, which can be dangerous if done incorrectly.

Why is the process of dissolving sulfuric acid in water considered an exothermic process?

The process is considered exothermic because it releases heat. This is known from experience or by feeling the temperature rise when the acid is mixed with water.

How do chemists determine whether a process is exothermic or endothermic?

Chemists determine if a process is exothermic or endothermic by conducting measurements. They have also developed general rules based on extensive measurements.

What is the first general rule mentioned for identifying exothermic processes?

The first rule is that when bonds are formed, it is always an exothermic process. There are no exceptions to this rule.

What happens when bonds are broken in a chemical reaction?

When bonds are broken, it is always an endothermic process, meaning energy is absorbed.

Can you give an example of an exothermic process involving bond formation?

An example is the formation of ice from liquid water. Hydrogen bonds form between water molecules, releasing energy in the process.

Why does the temperature not drop below 0°C when water freezes?

When water freezes, hydrogen bonds form between molecules, releasing a small amount of heat. This prevents the temperature of the freezing water from dropping below 0°C.

What is the relationship between spontaneous redox reactions and exothermic processes?

Spontaneous redox reactions, such as those in galvanic cells, are exothermic. They release energy as they proceed.

How can you identify non-spontaneous redox reactions and their thermodynamic nature?

Non-spontaneous redox reactions, like those in electrolytic cells, require energy input and are therefore endothermic.

What is the thermodynamic nature of combustion reactions and why?

Combustion reactions are exothermic because they release energy, which is also a characteristic of spontaneous redox processes.

What is photosynthesis and how does its thermodynamic nature differ from combustion?

Photosynthesis is the process where carbon dioxide and water are converted into glucose, and it is endothermic. Unlike combustion, it requires energy input to proceed.

How can you determine if a reaction is exothermic or endothermic from the reaction's enthalpy change (ΔH)?

If ΔH is less than 0 (ΔH < 0), the reaction is exothermic. If ΔH is greater than 0 (ΔH > 0), the reaction is endothermic.

What does it mean when energy is included to the right of the reaction arrow in a chemical equation?

It means that energy is released during the reaction, indicating that the process is exothermic.

How can you summarize the key points to determine if a process is exothermic or endothermic?

You should remember that bond formation is always exothermic, bond breaking is endothermic, spontaneous redox reactions are exothermic, non-spontaneous redox reactions are endothermic, combustion is exothermic, and photosynthesis is endothermic. Also, a negative enthalpy change (ΔH < 0) indicates an exothermic process, while a positive enthalpy change (ΔH > 0) indicates an endothermic process.

Outlines

00:00

🔍 Understanding Exothermic and Endothermic Reactions

This paragraph discusses how to determine if a chemical reaction is exothermic or endothermic, which can't be discerned solely from the reaction formula. The instructor introduces the SIV-rule, emphasizing the importance of adding concentrated acid to water to prevent heat generation. The explanation continues with the concept that exothermic processes release heat, as experienced when sulfuric acid dissolves in water. The instructor then shares empirical knowledge that the formation of bonds is always exothermic, while bond breaking is endothermic. The paragraph provides examples such as the formation of hydrogen bonds when water freezes and the spontaneous redox reactions in galvanic cells. It also contrasts exothermic processes like combustion with endothermic ones like photosynthesis. The paragraph concludes with advice on how to approach unknown reactions, using enthalpy change (ΔH) as an indicator, where ΔH < 0 signifies an exothermic reaction and ΔH > 0 indicates an endothermic reaction.

05:03

📘 Recapitulation of Exothermic and Endothermic Processes

The second paragraph serves as a recapitulation of the concepts discussed in the first. It reinforces the idea that the formation of bonds is an exothermic process and bond breaking is endothermic. It also reiterates that spontaneous redox reactions, such as those in galvanic cells, are exothermic, while non-spontaneous redox reactions, like in electrolytic cells, are endothermic. The paragraph ends with a reminder of the significance of enthalpy change (ΔH) in determining the nature of a process, with ΔH < 0 indicating an exothermic process and ΔH > 0 an endothermic one. This summary aims to solidify the student's understanding of how to identify exothermic and endothermic reactions.

Keywords

💡Exothermic reaction

An exothermic reaction is a chemical reaction that releases energy, usually in the form of heat, to its surroundings. In the context of the video, the theme revolves around identifying whether a reaction is exothermic or endothermic. The script uses the example of dissolving sulfuric acid in water, which is an exothermic process because it releases heat. This is a key concept because it helps to explain how to determine the nature of a chemical reaction based on energy changes.

💡Endothermic reaction

An endothermic reaction is one that absorbs energy from its surroundings, typically in the form of heat. The video script contrasts exothermic reactions with endothermic ones, using the example of photosynthesis, which is an endothermic process because it requires the absorption of energy to convert carbon dioxide and water into glucose. This concept is crucial for understanding the energy dynamics in chemical reactions and is a central theme in the video.

💡SIV-rule

The SIV-rule, mentioned in the script, stands for 'Sulfuric acid In VAtter' and is a safety guideline for chemists when diluting concentrated acids, particularly sulfuric acid. The rule emphasizes that the acid should be added to water, not the other way around, to prevent a violent exothermic reaction that could occur if water is added to the acid. This rule is a practical example of how understanding exothermic processes can be applied in laboratory safety.

💡Bond formation

Bond formation refers to the process where two or more atoms come together to form a chemical bond. The video script states that whenever bonds are formed, it is always an exothermic process because energy is released. This is a fundamental concept in chemistry, as it explains why certain reactions release energy and is used in the script to illustrate the general rule for identifying exothermic reactions.

💡Bond breaking

Bond breaking is the process where a chemical bond is separated into individual atoms, which requires energy input. The script explains that bond breaking is always an endothermic process because it absorbs energy. This concept is essential for understanding why some reactions require an input of energy and helps in identifying endothermic reactions.

💡Spontaneous redox reactions

Spontaneous redox reactions are chemical reactions that occur without the need for an external energy source because they release energy. In the video, the script mentions that spontaneous redox reactions, such as those in galvanic cells, are exothermic. This is an important concept because it helps to distinguish between reactions that naturally occur and those that require an energy input to proceed.

💡Non-spontaneous redox reactions

Non-spontaneous redox reactions are those that do not occur naturally and require an external energy source to take place. The script contrasts these with spontaneous redox reactions, stating that non-spontaneous reactions, such as those in electrolytic cells, are endothermic. Understanding this concept is key to knowing which reactions will proceed without additional energy and which will not.

💡Combustion

Combustion is a chemical reaction that occurs between a fuel and an oxidizing agent, producing heat and light in an exothermic process. The video script uses the example of cellular respiration, where glucose is combusted with oxygen to form carbon dioxide and water, releasing energy. This concept is important in the video's narrative as it exemplifies a type of exothermic reaction.

💡Photosynthesis

Photosynthesis is a biological process by which plants, algae, and some bacteria convert carbon dioxide and water into glucose using light energy. The script mentions photosynthesis as an endothermic process because it requires the absorption of energy from sunlight. This is a key concept in the video as it provides an example of a non-spontaneous process that is the reverse of combustion.

💡ΔH (enthalpy change)

ΔH, or the change in enthalpy, is a measure of the heat absorbed or released during a chemical reaction. In the script, it is mentioned that if ΔH is less than zero (ΔH < 0), the reaction is exothermic, and if ΔH is greater than zero (ΔH > 0), the reaction is endothermic. This concept is central to the video's theme as it provides a quantitative way to determine whether a reaction is exothermic or endothermic.

Highlights

Students often ask how to determine if a reaction is exothermic or endothermic, but it cannot be known just by looking at the chemical equation.

Every aspiring chemist must learn the SIV rule for diluting concentrated acids, especially sulfuric acid, by pouring the acid into water, not the other way around.

Diluting sulfuric acid in water releases a lot of heat, which is an example of an exothermic process.

Chemists know a process is exothermic through experience, such as feeling heat, which indicates energy release.

To determine if a process is exothermic or endothermic, one must measure it, as chemists have done extensively.

A general rule is that the formation of bonds is always an exothermic process, with no exceptions.

Conversely, the breaking of bonds is always an endothermic process.

An example of bond formation releasing energy is water freezing, where hydrogen bonds form between water molecules.

Despite the cold environment, the formation of hydrogen bonds during freezing releases heat, preventing the temperature from dropping below 0°C.

Spontaneous redox reactions, such as in galvanic cells, are exothermic processes.

Non-spontaneous redox reactions, like in electrolytic cells, are endothermic.

Combustion reactions, including cellular respiration, are a type of spontaneous redox reaction and are therefore exothermic.

Photosynthesis, the reverse of cellular respiration, converting carbon dioxide and water into glucose, is an endothermic process.

For reactions not covered by general rules, one can refer to the enthalpy change (ΔH) indicated in the problem or chemical equation.

If ΔH < 0, the reaction is exothermic; if ΔH > 0, it is endothermic.

Energy included in the chemical equation to the right of the reaction arrow indicates energy release, suggesting an exothermic reaction.

A summary table of exothermic and endothermic processes is provided, emphasizing the importance of understanding bond formation and breaking, redox reactions, and combustion.

Transcripts

Här är en fråga som jag ofta får av mina elever: Hur ser man på en reaktionsformel om reaktionen är exoterm eller endoterm?

Problemet är tyvärr att det kan man inte! Men hur kan man då veta om en reaktion är exoterm eller endoterm?

Häng med, så ska jag förklara!

Men vi ska börja med en regel som varje blivande kemist måste lära sig: SIV-regeln.

Den säger att när man ska späda en koncentrerad syra, främst svavelsyra, så måste man hälla syran i vattnet, och inte tvärtom.

Anledningen till detta är att det utvecklas en massa värme när svavelsyran löses i vattnet.

Det här är alltså ett exempel på en exoterm process.

Men hur vet vi att detta är en exoterm process?

Jo, vi vet det av erfarenhet! Någon (eller vi själva) har känt att det blivit varmt, och då vet vi att det är en exoterm process.

Man kan alltså bara ta reda på om en process är exoterm eller endoterm genom att mäta på den.

Men nu har ju kemister gjort en himla massa mätningar, och ur alla dessa mätningar kan man vaska fram några tumregler.

Den första är här att när bindningar uppstår, så är det alltid, alltid fråga om en exoterm process.

Det finns inga undantag från den regeln.

Och då gäller också det omvända, det vill säga att när en bindning bryts, så är det alltid en endoterm process.

Ett exempel på när bindningar uppstår är det här, när vatten går från flytande form till fast form, alltså is.

Då uppstår det vätebindningar mellan vattenmolekylerna. Därför avges det energi, och därför är detta en exoterm process.

Men, när vatten fryser, då blir det ju kallt, tänker kanske du?

Nej, det blir inte kallt, utan det är kallt runtomkring, och det är en viktig skillnad!

När vätebindningarna uppstår mellan vattenmolekylerna, så frigörs det lite värme.

Det gör att vatten som håller på att frysa till is aldrig blir kallare än 0°C.

Det här förklarar jag i min videogenomgång om fasövergångar.

Nästa tumregel, det är att spontana redoxreaktioner också är exoterma processer.

I min videogenomgång om galvaniska element pratar jag om den elektrokemiska spänningsserien.

Med hjälp av den kan man se att till exempel zink är ett starkare reduktionsmedel än koppar,

och därför kommer en bit zink som man stoppat ner i en bägare med kopparjoner lösta i vatten

att reducera kopparjonerna till metalliskt koppar.

Ett exempel på en spontan redoxreaktion är alltså när fast zink får reagera med kopparjoner.

Då oxideras zinkmetallen till zinkjoner, samtidigt som kopparjonerna reduceras till fast koppar.

Den omvända reaktionen är dock inte exoterm,

alltså när zinkjoner reagerar med koppar så att det bildas fast zink och kopparjoner.

För att få den reaktionen att ske måste man hela tiden tillföra energi i form av elektrisk energi i en slags elektrolysreaktion.

Den reaktionen är alltså endoterm.

Förbränningar måste du också lära dig att det är en slags exoterma processer.

(Och faktum är ju dessutom att förbränningar är en slags spontana redoxprocesser!)

Som exempel på det tar jag här cellandningen, där glukos förbränns med hjälp av syre och bildar koldioxid och vatten.

Och den omvända reaktionen,

nämligen fotosyntesen, då koldioxid och vatten omvandlas till glukos, är då förstås istället endoterm.

Hur ska man sedan göra med övriga reaktioner? Ja, såhär kan det till exempel se ut på ett prov,

om man får frågan att avgöra om reaktionen när natriumhydroxid löses i vatten

till natriumjoner och hydroxidjoner är exoterm eller endoterm.

Hur ska man veta det?

Ja, nu kan det vara så att din lärare tycker att just den här måste du lära dig utantill,

men ofta står det också med som i det här fallet att ΔH < 0.

Då måste du komma ihåg att när ΔH < 0 så betyder det att reaktionen är exoterm.

(Vilket då också innebär att om ΔH > 0, så är reaktionen istället endoterm.)

Eller också kan det se ut såhär, att man har tagit med energin i reaktionsformeln.

I det här fallet står energin till höger om reaktionspilen, vilket betyder att det frigörs energi i reaktionen.

Vi tar och sammanfattar det här nu, och skriver en tabell med exoterma och endoterma processer i tabellhuvudet.

Det första som du absolut måste lära dig här, det är att när bindningar uppstår, så är det alltid en exoterm process –

– och när de bryts handlar det istället om en endoterm process.

Sedan har vi då att spontana redoxreaktioner, som till exempel i galvaniska element, är exoterma.

Då vet du också att icke-spontana redoxreaktioner, som till exempel i elektrolysceller, istället är endoterma.

Förbränningar, som till exempel cellandningen, är en typ av spontana redoxreaktioner, och är därför exoterma.

Och fotosyntesen måste du då lära dig att det är ett annat exempel på en icke-spontan redoxreaktion,

som alltså är endoterm.

Slutligen här måste du komma ihåg att om man anger att ΔH < 0 så handlar det om en exoterm process,

och om ΔH > 0 så är det en endoterm process.

Så med detta hoppas jag nu att du har blivit lite säkrare på att avgöra om en process är exoterm eller endoterm!