MAPEL KIMIA BAB 2 REAKSI REDOKS DAN ELEKTROKIMIA

Elektrokimia merupakan cabang dari ilmu kimia yang secara khusus mempelajari hubungan listrik dan reaksi kimia. Proses-proses elektrokimia merupakan reaksi redoks (oksidasi-reduksi) di mana energi yang dihasilkan dari reaksi spontan dikonversi menjadi energi listrik atau di mana energi listrik digunakan untuk mendorong suatu reaksi nonspontan untuk terjadi.

Pada reaksi kimia, atom-atom hanya disusun ulang, tidak ada yang diciptakan ataupun dimusnahkan. Kemudian, ada beberapa hal khusus yang perlu diperhatikan. Elektron ditransfer dari satu substansi ke substansi lainnya, sehingga perlu diperhatikan dari mana dan ke mana elektron ditransfer serta besarnya muatan yang dibawa elektron-elektron tersebut. Jadi, ada 3 faktor yang perlu diperhatikan dalam menyetarakan persamaan reaksi redoks: (1) jumlah atom, (2) jumlah elektron yang ditransfer, dan (3) jumlah muatan reaktan dan produk.

Penyetaraan Reaksi Redoks

Ada 2 metode dalam menyetarakan persamaan reaksi redoks:

Metode Perubahan Biloks

1. Menentukan biloks masing-masing substansi dan mengidentifikasi atom/ion mana yang mengalami perubahan biloks.

2. Menuliskan jumlah elektron yang dilepaspada oksidasi dan jumlah elektron yang diterima pada reduksi berdasarkan jumlah perubahan biloks (bisa dibantu dengan menggambar garis antara atom/ion yang mengalami oksidasi dan yang mengalami reduksi)

3. Menghitung koefisien reaksi reaktan dengan bilangan bulat terkecil yang dapat menyetarakan jumlah elektron yang ditransfer selama oksidasi dan selama reduksi, lalu menyetarakan koefisien reaktan dan produk.

4. Menyetarakan atom O dengan H2O(l), lalu menyetarakan atom H dengan H+(aq)

Untuk reaksi redoks dalam larutan suasana basa:

5. Menambahkan OH−(aq) pada reaktan dan produk dengan jumlah sesuai dengan jumlah H+(aq)

6. Mengkombinasi H+(aq) dan OH−(aq) pada sisi yang sama membentuk H2O(l), dan menghilangkan jumlah H2O(l) yang sama pada kedua sisi

Metode Setengah-Reaksi (metode ion-elektron)

1. Membagi persamaan reaksi ke dalam 2 setengah-reaksi: oksidasi dan reduksi

2. Menyetarakan atom-atom selain H dan O pada masing-masing setengah-reaksi

3. Menyetarakan atom O dengan H2O(l), lalu menyetarakan atom H dengan H+(aq)

4. Menyetarakan muatan dengan elektron (e−)

5. Mengalikan koefisien masing-masing setengah-reaksi dengan bilangan bulat tertentu agar jumlah e−yang dilepas dalam setengah-reaksi oksidasi sama dengan jumlah e− yang diterima dalam setengah-reaksi reduksi

6. Menggabungkan kedua setengah-reaksi yang sudah setara tersebut menjadi satu persamaan reaksi, lalu menghilangkan jumlah spesi-spesi yang sama pada kedua sisi

Untuk reaksi redoks dalam larutan suasana basa:

7. Menambahkan OH−(aq) pada reaktan dan produk dengan jumlah sesuai dengan jumlah H+(aq)

8. Mengkombinasi H+(aq) dan OH−(aq) pada sisi yang sama membentuk H2O(l), dan menghilangkan jumlah H2O(l) yang sama pada kedua sisi

Contoh soal Reaksi Redoks

MnO4−(aq) + C2O42−(aq) → MnO2(s) + CO32−(aq)

Setarakan persamaan reaksi redoks dalam larutan suasana basa di atas dengan menggunakan metode:

(a) perubahan biloks

(b) setengah-reaksi

Elektrokimia merupakan cabang ilmu kimia yang membahas hubungan reaksi redoks dengan energi listrik. Keduanya berlangsung dalam sebuah alat yang disebut sel elektrokimia. 

Sel Elektrokimia terbagi menjadi dua jenis, yaitu sel volta/galvani dan sel elektrolisis. Sel volta mengubah energi kimia menjadi energi listrik sedangkan sel elektrolisis mengubah energi listrik menjadi energi kimia.

Tabel perbandingan sel volta dan sel elektrolisis

1. Sel Volta

 a. Reaksi Redoks Spontan

 Reaksi redoks spontan adalah reaksi redoks yang terjadi dengan sendirinya.

Misalnya apabila logam seng dicelupkan ke dalam larutan tembaga(II)sulfat, CuSO4. Lihat gambar berikut,

Logam seng (Zn) akan larut dengan melepas elektron membentuk ion seng (Zn2+), sementara ion tembaga (Cu2+) menangkap elektron dari logam seng dan membentuk endapan tembaga (Cu). Dalam reaksi tersebut terjadi transfer elektron secara langsung dari logam Zn ke ion Cu2+. Reaksi yang terjadi, 

Zn + Cu2+ → Zn2++ Cu

b. Rangkaian Sel Volta

Transfer elektron secara langsung pada reaksi di atas tidak akan menghasilkan arus listrik, oleh karena itu Luigi Galvani dan Alessandro Guiseppe Volta menemukan cara untuk menghasilkan arus listrik yaitu memisahkan logam Zn dan ion Cu2+ di wadah yang berbeda dalam suatu rangkaian yang disebut sel volta.

Rangkaian Sel volta

Pada rangkaian sel volta di atas, logam Zn dicelupkan ke dalam larutan ZnSO4 dan logam Cu dicelupkan ke dalam larutan CuSO4,  kemudian dihubungkan dengan voltmeter melalui kabel penghantar listrik, selanjutnya diberi jembatan garam.

Setelah rangkaian terhubung dengan baik, lempengan logam Zn teroksidasi menjadi ion Zn2+ yang akan larut di dalam larutan ZnSO4 dan melepas elektron. 

Zn → Zn2++ 2e

Elektron yang dilepas akan mengalir melalui kabel listrik melewati voltmeter menuju lempengan logam Cu. Kemudian elektron tersebut akan ditangkap oleh ion Cu2+ yang berasal dari larutan CuSO4 membentuk endapan Cu.

 Cu2+ + 2e → Cu 

 Elektron mengalir dari anoda ke katoda, sedangkan aliran arus sebaliknya mengalir dari katoda ke anoda.

Faktanya tanpa jembatan garam, elektron tidak mengalir. Hal ini karena terjadinya kelebihan muatan pada kedua wadah yang berisi larutan ZnSO4 dan larutan CuSO4. Di wadah pertama yang berisi larutan ZnSO4 akan mengalami kelebihan muatan positif karena terbentuk ion Zn2+ hasil reaksi oksidasi logam Zn, sementara di wadah kedua akan mengalami kelebihan muatan negatif SO42-karena ion Cu2+ jumlahnya berkurang.

Jembatan garam berfungsi untuk menetralkan kelebihan muatan pada kedua larutan tersebut, ion Na+ akan bergerak menuju larutan CuSO4 untuk menetralkan kelebihan muatan negatif dan ion SO42- bergerak menuju larutan ZnSO4  yang kelebihan muatan positif.

Lempengan logam Zn dan Cu disebut elektroda, elektroda tempat terjadinya reaksi oksidasi disebut anoda sedangkan elektroda tempat terjadinya reaksi reduksi disebut katoda. Logam yang bertindak sebagai anoda pada sel volta di atas adalah Zn, sementara Cu bertindak sebagai katoda. Anoda dalam sel volta disebut juga elektroda negatif dan katoda sebagai elektroda positif.

c. Diagram Sel Volta

Reaksi yang terjadi di kedua elektroda dapat dinyatakan dengan diagram sel atau notasi sel. Reaksi oksidasi ditulis di sebelah kiri sedangkan reduksi di sebelah kiri.

Sehigga reaksi sel volta di atas dapat ditulis :  Zn I Zn2+ II Cu2+I Cu

Dua garis sejajar menyatakan jembatan garam.

    a. Deret Volta
    

    b. Potensial Elektroda
    c. Potensial Sel

2. Sel Elektrolisis

a. Kaedah Elektrolisis

b. Aspek Kuantitatif Elektrolisis

        Hukum Faraday I

 Massa zat yang dihasilkan pada elektrolisis berbanding lurus dengan muatan listrik yang dialirkan.

        Hukum Faraday II

 Massa zat-zat yang dihasilkan oleh muatan listrik yang sama sebanding dengan berat ekivalen dari zat-zat tersebut.

Klik LINK Pembahasan Reaksi REDOKS

Klik LINK Pembahasan Sel Volta 

 

Klik LINK untuk mengerjakan PR 1

Klik LINK untuk mengerjakan QUIZ

Reaksi Redoks – Referensi
– Brown, Theodore L. et al. 2015. Chemistry: The Central Science (13th edition). New Jersey: Pearson Education, Inc.
– Chang, Raymond. 2010. Chemistry (10th edition). New York: McGraw Hill
– Gilbert, Thomas N. et al. 2012. Chemistry: The Science in Context (3rd edition). New York: W. W. Norton & Company, Inc.
– Jenkins, et al. 2003. Nelson Chemistry 12. Toronto: Thomson Nelson.
– Jespersen, Neil D., Brady, James E., & Hyslop, Allison. Chemistry: The Molecular Nature of Matter (6th edition). New Jersey: John Wiley & Sons, Inc.
– Petrucci, Ralph H. et al. 2011. General Chemistry: Principles and Modern Applications (10th edition). Toronto: Pearson Canada Inc.
– Purba, Michael. 2006. Kimia 3A untuk SMA Kelas XII. Jakarta: Erlangga.
– Silberberg, Martin S. 2009. Chemistry: The Molecular Nature of Matter and Change (5th edition). New York: McGraw Hill

MAPEL ENGLISH LM BAB 2 PAST TENSE VS PRESENT PERFECT TENSE

Objectives

Understand the distinctions between simple past and present perfect

Learn about it!

PAST TENSE

In the previous lesson, you have learned about simple past tense and past continuous tense. Now in this lesson, you will learn how to use those tenses in a conversation. However, before that let’s review what you have learned. Do you still remember when do we use simple past tense or past continuous tense in English?

Simple past tense is used to tell an event that happened in the past. For example: “My mother made a cake.” Past continuous is also used to tell an event that happened in the past, but it happened at a particular time. For instance: “I was doing my homework.” It means that I was doing my homework at a particular time in the past or in a specific time in the past I was doing my homework. That was how you say a statement in past tense.

Now, when you want to say the interrogative sentence, how do you use the tenses? How do you ask people using the past tense? In order to answer those questions, you need to listen to the following dialogue between Silva and Hera. They talk about their holiday experience.

Pay attention to how Silva asked Hera about her holiday. Silva asked Hera “what did you do during your holiday?, Did you go somewhere?, What did you do there?”

When asking someone about their past experience, you use ‘did’ and the verb is not in its past form, but it comes back to its present form. For instance, in the example above the verb is ‘do’ and ‘go’. How so?

Because the interrogative sentence (the sentence that you use to ask the question) in past tense requires the auxiliary verb ‘did’ in order to indicate past tense. So, if there is an auxiliary verb ‘did’, you do not have to use the past form of a verb, you use ‘did’ and present form of a verb instead. Here are other examples for you:

Where did you go?

What did you bring?

How did you get there?

Now let’s pay attention to the response said by Hera to Silva. In order to respond to Silva’s question, Hera said “Yes, actually I went to Bandung with my family.” Since Silva asked her using simple past tense, the answer must be in simple past tense as well.

To answer a question using past tense, the pattern is more or less the same with the pattern of simple past tense that we have already learn before. ‘Went’ is the past form of ‘go’. This sentence indicates that Hera told Silva about the event that happened in the past.

Silva: Did you go somewhere? (did + Verb1)

Hera: Yes, actually I went to Bandung with my family. (Verb2)

Now, listen again to another conversation.

In the dialogue, the teacher asks Lili about her holiday by asking “So, did you go somewhere on your last holiday?” This phrase can be used when you want to ask people about their holiday experience. You can also use “How was your holiday? / “Where did you go last holiday?” or else.

From the dialogue, we can also find out Lili’s response to the teacher’s question about her holiday. In order to response to the question about her holiday experience, Lili says “it was fun”. This sentence can be used to tell people about your experience. You can also use the following expressions as your choice.

It was great.

Not so bad/good.

It was wonderful/amazing/good.

Now, can you tell find out what type(s) of tense(s) are used in the dialogue? What are they?

Here is the transcript of the previous dialogue. Try to identify which sentences that use past tense.

PRESENT PERFECT TENSE

Miss Carol	:	OK everyone, this is your biggest challenge yet.
Jess	:	A day when we mustn't argue?
Miss Carol	:	No, no. The challenge will take longer than a day. A day and a night in fact. You're going to go camping.
Debbie	:	Camping!
Pete	:	Wicked! Are you coming too, Miss?
Miss Carol	:	No. You have to plan the trip yourselves, and go on it yourselves, too. You're going to be on your own.
Jess	:	Right, I've written out everything we need for the camping trip. I've listed the things you need for yourself and then some stuff for you to bring for all of us.
Debbie	:	Baked beans? I hate those.
Pete	:	Never mind, Debbie, I love them. You're bringing them for me!
Joel	:	There's loads of stuff here. Do we really need it all, Jess? I've got hardly any money at the moment.
Pete	:	OK, then, you can sell your guitar! No, seriously, you can borrow some money from me. But I want it back, though.
Jess	:	It's really important to be prepared. We don't want anything to go wrong.
Jess	:	Joel, Joel, hi! Have you got everything on your list yet?
Joel	:	Not yet. Give me a chance!
Jess	:	How are you getting on with the camping list, Debbie?
Debbie	:	Um, OK. What about you?
Jess	:	Oh, I've finished mine. All done!
Jess	:	Pete! Have you got everything on your list?
Pete	:	List? What list?
Jess	:	Pete!
Pete	:	No, only joking. Don't worry. I'm going to get everything together tonight.
Miss Carol	:	I'm going to take you all to the campsite and then leave you. You'll be on your own, OK?
Jess	:	Let's just check we've got everything before we go.
Joel	:	No. Not again, Jess.
Debbie	:	Eurgh, my feet are all wet.
Joel	:	Put your boots on.
Debbie	:	I didn't bring any.
Jess	:	hey were on your list.
Debbie	:	I forgot them, OK.
Jess	:	I said let's check everything, but no one wanted to.
Debbie	:	Oh be quiet, Jess. Ouch!
Jess	:	Oi! Help me get this back up. Can you guys light the fire?
Pete	:	No problem. Have you got the matches?
Joel	:	Jess, have you got the matches?
Jess	:	No but they were on someone's list.
Debbie	:	I didn't bring any.
Pete	:	I didn't either.
Joel	:	Nor me.
Jess	:	I don't believe it!
Pete	:	Oh come on, Jess. Calm down. I know things are going wrong, but we can still have fun.
Joel	:	Yeah, let's ask the farmer for some matches.
Pete	:	I'm on my way!
Pete	:	It's a good thing the farmer had some matches.
Debbie	:	And some milk!
Joel	:	And a tin opener! We're really bad campers, aren't we?
Jess	:	No, you're not.
Debbie	:	That's right, we're not. We're still here, aren't we?

“To be continue”  

Right, I've written out everything we need for the camping trip. I've listed the things you need for yourself and then some stuff for you to bring for all of us.

The above sentences (*red color) are use Present Perfect Tense

Present Perfect Tense is used to express an event that started in the past and the impact of the event is now continuing (or a long-running event that started in the past and is still going on). This tense is used to express actions completed recent past. In this tense, it is important whether the event occurred or not. ( or the result of the event is important, not the time of the event )

The Present Perfect Tense is formed by putting “to have  ( have or has )” before the past participle of the verb.

POSITIVE FORM (+) :  Subject ( I, YOU, WE, YOU, THEY ) + HAVE + V3 ( third form of main verb – past participle )

Subject ( HE, SHE, IT ) + HAS + V3 ( third form of main verb – past participle )

NEGATIVE FORM (-) : Subject ( I, YOU, WE, YOU, THEY ) + HAVE  + NOT + V3 ( third form of main verb – past participle )

Subject ( HE, SHE, IT ) + HAS + NOT + V3 ( third form of main verb – past participle )

QUESTION FORM (?) : HAVE  + Subject ( I, YOU, WE, YOU, THEY ) + V3 ( third form of main verb – past participle )

HAS + Subject ( HE, SHE, IT ) + V3 ( third form of main verb – past participle )

USE OF PRESENT PERFECT TENSE:

1. Present Perfect Tense is used to express an action that happened at an unspecified time in the past and the impact of the event is now continuing.

( Finished Actions —-Past Event & Present Result )

Examples:

She has lost her wallet ( She can’t find it )

We can’t reach Paul by phone. Have you seen him?

Tom isn’t at school. I think he has gone to the theater.

My sister has broken her arm. Her arm is still broken.

2. Present Perfect Tense is used to express an action that ended recently. We often use words like “just” or “recently” for the events taking place a very short time before now.

( Recent Completed Actions )

Examples:

I have just finished my project.

My mother has just cleaned the house.

We have recently eaten dinner.

3. Present Perfect Tense is used to talk about unfinished actions or states or habits that started in the past and continue to the present. In such sentences, it is emphasized that how long the action has continued. So we usually use ‘since’ or ‘for’ to express “how long”.  We often use stative verbs / non-continuous verbs / mixed verbs in such sentences).

( Unfinished Actions / Duration from Past until Now )

Examples:

The student has studied maths for three months.

I have lived in Paris for five years.

My son has been sick since Monday.

He has worked in Berlin since he graduated from the university.

She haven’t been calm since the accident.

Teachers have thought this subject for hours.

They have had many cars since I can remember.

4. Present Perfect Tense is used to express repeated actions in an unspecified time between the past and now. 

( Repeated or Multiple Actions at Different Times )

Examples:

We have watched that movie three times.

I have seen David several times.

She have had six exams so far this semester.

They have called him four times this month.

5. Present Perfect Tense is used when we talk about life experiences.

( Life Experiences )

Examples:

I have been to England in my life. 

Have you ever eaten Sushi before?

We have never tried to invent something new. 

6. Present Perfect Tense is used when we talk about accomplishments.

( Accomplishments )

Examples:

My father has spoken five languages.

Scientists have found a new way to get cancer cells to self-destruct.

Man has walked on the Moon. 

7. Present Perfect Tense is used when we talk about changes that has occurred over periods of time.

( Changes Over Time )

Examples:

Our english has improved a lot when we moved to England.

Her behavior and attitudes have changed a lot since you last saw her.

I have become more interested in medical issues.

Attention 1:

1. “Present Perfect Tense” is used with “an unfinished time word ( this week, today, etc. )“. The period of time is still continuing. 

Examples:

We haven’t called him this week. 

I have eaten lots of plumps today.

They have had several tests this month.

2.“Present Perfect Tense” is not used with “a finished time word ( yesterday, last week, etc. )“. In such sentences, we use “Simple Past Tense”

Examples:

I haven’t seen you yesterday. ( wrong )                         * I didn’t see you yesterday. ( correct )

We have finished the project last week. ( wrong )      *  We finished the project last week. ( correct )

He has bought a new house last year. ( wrong )          * They bought a new house last year. ( correct )

Attention 2:

We often use the present perfect with some time adverbials ( adverbs of time ). Some of them are mentioned here. 

1. “Since”  is used with a point ( previous point ) in time in the past.

Examples:

  Since last week / month / year / summer, etc.

  Since one o’clock / two o’clock / six o’clock, etc. 

  Since  Sunday, Monday, Friday, etc.

  Since April, June, January, etc. 

  Since 1950, 1999, 2017, etc

  Since they moved / we talked to him, etc.

2. “For” is used with a period of time in the past, present or future.

Examples:

  For one minute / ten minutes, etc.

  For  two hours / five hours, etc.

  For a day, four days, six days, etc.

  For three weeks, seven weeks, etc.

  For two months, three months, etc.

  For five years, ten years, etc.

  For hours, more than a week, ages, a couple of months, etc.

3. “Already” is used to emphasise that something was completed before something else happened. It is also used to show surprise about things that have happened or will have happened earlier than we expected. It usually comes between the auxiliary verb ( have or has ) and the main verb.

( We use “already” in affirmative ( positive ) sentences and questions but not usually in negative sentences )

Examples:

  She has already come.

  The plane has already landed.

  I have already done it.

  Have you already written to John?

  Has the train already left?

4. “Yet” is used to refer to a time which starts in the past and continues up to the present. It usually comes at the end of a sentence.

( We use “yet” mostly in negative sentences and questions but not usually in positive sentences )

Examples:

   It hasn’t stopped snowing yet.

   They haven’t opened it yet.

   Have they arrived yet?

   Has she driven her new car yet?

5. “Just” is used to refer to a short time before the moment of speaking.  It usually comes between the auxiliary verb ( have or has ) and the main verb.

(We use “just” in affirmative ( positive ) sentences and questions)

Examples:

   We have just decided to sell our car.

   The plane has just arrived.

   Have they just left?

6. “Ever” is used to refer to at any time. It usually comes between the auxiliary verb ( have or has ) and the main verb.

(We use “ever” mostly in questions)

Examples:

   Have you ever met her?

   Have you ever cooked cheesecake?

   Have you ever heard this sound?

7. “Never” is used to refer to not at any time. It usually comes between the auxiliary verb ( have or has ) and the main verb.

(We use “never” in negative sentences ( negative meaningful ))

Examples:

   I have never been to America.

   We have never heard anything so stupid.

   She has never flown in a plane before

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