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Chemistry - Electrochemistry

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    Study Guide Chemistry – Electrochemistry 1. Electrochemical Cells 1.1 What Is an Electrochemical Cell? An electrochemical cell (also called a voltaic cell ) is a device that uses a chemical reaction to produce electricity . Because reactions happen more easily in liquids than in solids or gases, most electrochemical cells use a liquid solution . This liquid is called an electrolyte . 1.2 Electrolytes and Conductivity An electrolyte is a solution that contains ions and can conduct electricity . • Pure distilled water conducts electricity very poorly. • When substances dissolve and form ions, conductivity increases. Types of Electrolytes • Strong electrolytes : Substances (like many acids, bases, and salts) that ionize almost completely in water. • Weak electrolytes : Substances that ionize only slightly. The more ions present, the better the solution conducts electricity. 1.3 Structure of a Simple Electrochemical Cell A simple electrochemical cell can be made using two test tubes connected by a third tube , shaped like an “H”. • The connecting tube allows ions to move , which is necessary to complete the circuit. • Each test tube contains a different solution and metal electrode .

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    Study Guide What Goes in Each Side? • Left test tube : o Zinc metal strip o Zinc sulfate solution (ZnSO ₄ ) • Right test tube : o Copper metal strip o Copper(II) sulfate solution (CuSO ₄ ) The two metal strips are connected to a voltmeter using wires. Figure 1. A V oltaic c ell. 1.4 How Electricity Is Produced When the circuit is complete: • The voltmeter shows about 1.10 volts . • Electrons move through the wire from zinc to copper. • This movement of electrons is what we call electric current . 1.5 Oxidation at the Zinc Electrode (Anode) At the zinc electrode , zinc atoms lose electrons . This process is called oxidation . Oxidation Reaction:

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    Study Guide • Zinc metal becomes zinc ions. • Electrons are released into the wire. The electrode where oxidation occurs is called the anode . • The anode attracts negative ions , called anions . • In this cell, the anode is positive . 1.6 Reduction at the Copper Electrode (Cathode) At the copper electrode , copper ions gain electrons . This process is called reduction . Reduction Reaction: • Copper ions turn into solid copper metal. • Copper metal builds up on the electrode. The electrode where reduction occurs is called the cathode . • In this cell, the cathode is negative . 1.7 Why the Charges Make Sense • Electrons move from anode to cathode . • Since electrons are negative: o The anode becomes positive o The cathode becomes negative This is specific to voltaic (electrochemical) cells . 1.8 Overall Reaction of the Cell When we add the oxidation and reduction reactions together, we get the overall redox reaction : Zn(s) + Cu² ⁺ (aq) → Cu(s) + Zn² ⁺ (aq) This reaction explains how chemical energy is converted into electrical energy in a zinc – copper electrochemical cell.

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    Study Guide Summary • OIL RIG helps remember redox: o Oxidation Is Loss (of electrons) o Reduction Is Gain (of electrons) 2. Quiz: Electrochemical Cells 1. Question Another name for a spontaneous electrochemical cell is Answer Choices • salt bridge • volt meter • voltaic cell Correct Answer voltaic cell Why This Is Correct A spontaneous electrochemical cell generates electrical energy from a chemical reaction on its own. Such cells are called voltaic cells (or galvanic cells). A salt bridge is only a component of the cell, and a voltmeter is a measuring device. 2. Question A voltaic cell uses a chemical reaction to produce

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    Study Guide Answer Choices • pressure • change in temperature • electricity Correct Answer electricity Why This Is Correct In a voltaic cell, chemical energy from a redox reaction is converted directly into electrical energy, which flows through an external circuit. 3. Question The ions in a solution which conduct electricity are known as Answer Choices • colloids • electrolytes • electrodes Correct Answer electrolytes Why This Is Correct Electrolytes dissociate into ions in solution, allowing the solution to conduct electricity. Colloids do not fully ionize, and electrodes are solid conductors, not ions in solution. 4. Question Reduction occurs at the

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    Study Guide Answer Choices • anion • cathode • salt bridge Correct Answer cathode Why This Is Correct Reduction (gain of electrons) always occurs at the cathode in electrochemical cells. This is summarized by the mnemonic “RED CAT” (Reduction at Cathode). 5. Question In a voltaic cell, oxidation occurs at the __________ and is the __________ electrode. Answer Choices • cathode, positive • anode, positive • anode, negative Correct Answer anode, negative Why This Is Correct Oxidation (loss of electrons) occurs at the anode . In a voltaic cell, the anode is negative because it supplies electrons to the circuit. 6. Question The positive electrode in a voltaic cell is known as a(n)

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    Study Guide Answer Choices • anode • cathode • antinode Correct Answer cathode Why This Is Correct In a voltaic cell, electrons flow toward the cathode, making it the positive electrode . An antinode is unrelated to electrochemistry. 7. Question Compounds that have a high degree of ionization in solution are known as Answer Choices • weak electrolytes • nonconductors • strong electrolytes Correct Answer strong electrolytes Why This Is Correct Strong electrolytes ionize almost completely in solution, producing many ions that conduct electricity efficiently.

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    Study Guide Use the following balanced ionic equation to answer questions 8 – 10: Equation 8. Question Which species is oxidized? Answer Choices • MnO ₄⁻ • Fe • Mn² ⁺ Correct Answer Fe Why This Is Correct Iron (Fe) changes from Fe (0) to Fe² ⁺ , meaning it loses electrons. Loss of electrons is oxidation. 9. Question Which species is the oxidizing agent? Answer Choices • MnO ₄⁻ • Fe • Mn² ⁺

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    Study Guide Correct Answer MnO ₄⁻ Why This Is Correct The oxidizing agent is the substance that gets reduced . MnO ₄⁻ is reduced to Mn² ⁺ , so it causes oxidation of iron. 10. Question How many electrons are transferred in the balanced redox reaction? Answer Choices • 2 • 5 • 10 Correct Answer 10 Why This Is Correct Each Fe atom loses 2 electrons , and there are 5 Fe atoms involved: 5 × 2 = 10 electrons transferred . 3. Electrode Potential 3.1 What Is Electrode Potential? Electrode potential is a measure of how strongly an electrode can gain or lose electrons. It is measured in volts (V) . In an electrochemical cell, electricity is produced because chemical reactions occur at two electrodes . The total voltage of the cell depends on the substances used at these electrodes.

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    Study Guide 3 .2 Electromotive Force (EMF) The voltage produced by a cell is called EMF (electromotive force) . EMF is simply another name for electrical potential . For any electrochemical cell: EMF of the cell = EMF of oxidation + EMF of reduction This means we add the voltage from the oxidation reaction and the voltage from the reduction reaction to find the total cell voltage. 3.3 The Standard Hydrogen Electrode (SHE) To compare different electrodes, scientists needed a reference point. They chose the standard hydrogen electrode (SHE) . Conditions for the standard hydrogen electrode: • Hydrogen gas at 1 atmosphere • A platinum electrode • A solution with 1 M H ⁺ ions • Temperature of 25°C The standard hydrogen electrode is assigned a potential of 0.00 V . All other electrode potentials are measured relative to this value .

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