Welcome to Acids and Alkalis!
In this chapter, we are exploring how chemicals interact when they meet. You will learn how to identify acids and alkalis, how they react with metals and rocks, and how we can use them to create useful "salts" (and no, we don't just mean the salt you put on your chips!). Understanding these reactions helps us explain everything from how your stomach digests food to how we make medicines. Don't worry if it seems like a lot to remember at first—we will break it down step-by-step!
1. The Reactions of Acids
Acids aren't just things found in science labs; they are all around us! However, in science, we focus on how they react with metals and carbonates.
Acids and Metals
When an acid reacts with a metal, it produces two things: a salt and hydrogen gas.
Acid + Metal \(\rightarrow\) Salt + Hydrogen
You only need to know how hydrochloric acid and sulfuric acid react with three specific metals: magnesium, zinc, and iron.
Example: Magnesium + Hydrochloric Acid \(\rightarrow\) Magnesium Chloride + Hydrogen
How do we know Hydrogen is there?
We use the Squeaky Pop Test!
1. Collect the gas in a test tube.
2. Hold a burning splint at the open end of the tube.
3. If you hear a "squeaky pop," hydrogen is present!
Acids and Metal Carbonates
Metal carbonates are compounds like limestone or marble. When an acid hits them, they fizz and dissolve.
Acid + Metal Carbonate \(\rightarrow\) Salt + Water + Carbon dioxide
How do we know Carbon Dioxide is there?
We use Limewater. If you bubble the gas through limewater and it turns cloudy (milky), you've found carbon dioxide!
HT ONLY: Redox Reactions
In Higher Tier, we describe the reaction between metals and acids as a redox reaction. This is because electrons are moving!
Oxidation is the loss of electrons.
Reduction is the gain of electrons.
(Memory Aid: OIL RIG - Oxidation Is Loss, Reduction Is Gain).
In these reactions, the metal atoms lose electrons (oxidation) to become positive ions, and the hydrogen ions from the acid gain those electrons (reduction) to become hydrogen gas.
Key Takeaway: Acids react with metals to make hydrogen, and with carbonates to make carbon dioxide. Always remember your gas tests!
2. Making Salts
A "salt" in chemistry is just a compound formed when the hydrogen in an acid is replaced by a metal ion. To make a salt, we perform a neutralisation reaction.
The Recipe for a Salt
There are three main ways to make a soluble salt:
1. Acid + Alkali \(\rightarrow\) Salt + Water (Alkalis are soluble bases like sodium hydroxide).
2. Acid + Base \(\rightarrow\) Salt + Water (Bases are often insoluble metal oxides or hydroxides).
3. Acid + Metal Carbonate \(\rightarrow\) Salt + Water + Carbon Dioxide
Naming the Salt
The name of the salt has two parts. The first part comes from the metal, and the "surname" comes from the acid:
- Hydrochloric acid makes chlorides.
- Sulfuric acid makes sulfates.
- Nitric acid makes nitrates.
Example: Copper Oxide + Sulfuric Acid \(\rightarrow\) Copper Sulfate + Water
Practical Steps: How to make a pure, dry salt
Don't worry, this is a classic required practical! Here is the process:
1. React: Add the solid (e.g., copper oxide) to the acid until no more dissolves. This means the acid is neutralised.
2. Filter: Use filter paper and a funnel to remove the excess (leftover) solid. You now have a pure salt solution.
3. Evaporate: Heat the solution gently in an evaporating dish over a water bath. Stop heating when crystals start to form.
4. Crystallise: Leave the rest of the water to evaporate slowly to get nice, large crystals.
Key Takeaway: To make a salt, neutralise an acid, filter the leftovers, and evaporate the water to leave the crystals behind.
3. Energy Changes in Reactions
Chemical reactions often involve a change in temperature. We can measure this by looking at the surroundings.
- Exothermic: Energy is transferred to the surroundings. The mixture gets hotter. Neutralisation is a great example of an exothermic reaction!
- Endothermic: Energy is taken in from the surroundings. The mixture gets colder. A common example is the reaction between citric acid and sodium hydrogen carbonate (found in some bath bombs!).
Key Takeaway: Most neutralisation reactions feel warm because they are exothermic.
4. The pH Scale and Neutralisation
The pH scale tells us how acidic or alkaline a solution is. It runs from 0 to 14.
- 0 to 6: Acidic (Red/Orange/Yellow)
- 7: Neutral (Green)
- 8 to 14: Alkaline (Blue/Purple)
What makes an Acid or Alkali?
It all comes down to ions (charged particles):
- Acids produce hydrogen ions \(H^+\) in water.
- Alkalis produce hydroxide ions \(OH^-\) in water.
The Ionic Equation for Neutralisation
When an acid and alkali react, the \(H^+\) and \(OH^-\) ions join together to form water (\(H_2O\)). Water is neutral! The general equation is:
\( H^+(aq) + OH^-(aq) \rightarrow H_2O(l) \)
Measuring pH
We can use Universal Indicator (which changes color) or a pH probe (which gives a digital reading). A pH probe is more accurate because it gives a precise numerical value rather than you having to guess the color!
Key Takeaway: pH 7 is neutral. Acids have \(H^+\) ions, alkalis have \(OH^-\) ions, and when they meet, they make neutral water.
5. Strong and Weak Acids (HT ONLY)
In Higher Tier, we look closer at how acids behave in water. It isn't just about how much acid you have (concentration), but how much of it "splits up" (ionisation).
Strong vs. Weak
- Strong Acids: These completely ionise in water. Every single molecule splits up to release \(H^+\) ions. Examples: Hydrochloric, Sulfuric, and Nitric acids.
- Weak Acids: These only partially ionise. Only a small fraction of the molecules split up. Examples: Ethanoic acid (vinegar), Citric acid, and Carbonic acid.
Concentration vs. Strength
Common Mistake: Many students think "strong" means the same as "concentrated." They are different!
- Strength is about what percentage of the acid molecules ionise.
- Concentration is about how much acid is dissolved in a certain volume of water (dilute vs. concentrated).
The pH Jump
For every decrease of 1 on the pH scale, the concentration of \(H^+\) ions increases by a factor of 10.
Example: A solution with pH 1 has 10 times more \(H^+\) ions than a solution with pH 2. A solution with pH 1 has 100 times more than pH 3!
Key Takeaway: Strong acids split up completely in water. A pH change of 1 means a 10x change in ion concentration.
Quick Review: Avoid These Traps!
1. Don't forget the gas tests! If it's a carbonate, you get \(CO_2\). If it's just a metal, you get \(H_2\).
2. Salt naming: Remember that Sulfuric acid always makes a "Sulfate." Don't write "Sulfuride"!
3. State Symbols: Use (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water). Acids are always (aq)!
4. Did you know? Your stomach contains hydrochloric acid with a pH of about 1 or 2. It’s strong enough to dissolve metal, but your stomach lining produces mucus to protect itself!