Acids with Metals

Reaction of acids with metals

When acids react with metals, they produce a salt and hydrogen gas. This is a typical acid-metal reaction.

The general word equation is:

acid + metal 12 salt + hydrogen

During the reaction, you will observe bubbles of hydrogen gas forming on the metal
9s surface. This fizzing is a key sign that the reaction is taking place.

For example, when hydrochloric acid reacts with magnesium, the equation is:

magnesium + hydrochloric acid 12 magnesium chloride + hydrogen

In symbol form:

$$\mathrm{Mg} + 2\mathrm{HCl} \rightarrow \mathrm{MgCl}_2 + \mathrm{H}_2$$

The hydrogen gas produced can be tested by holding a lit splint near the mouth of the test tube; it will make a 'pop' sound.

For instance, if zinc reacts with hydrochloric acid, zinc chloride and hydrogen gas are formed:

$$\mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2$$

Example: When zinc reacts with hydrochloric acid, the salt formed is zinc chloride because hydrochloric acid contains chloride ions, so the salt is a chloride.

Types of metals reacting with acids

Not all metals react with acids in the same way. The speed and vigour of the reaction depend on the metal
9s reactivity.

More reactive metals, such as magnesium and zinc, react vigorously with acids, producing lots of hydrogen bubbles quickly.

Less reactive metals, like iron, react more slowly, producing hydrogen at a slower rate.

Some metals, such as copper, do not react with dilute acids under normal conditions and so no hydrogen gas is produced.

The reactivity series helps predict whether a metal will react with an acid:

• Metals above hydrogen in the reactivity series will react with acids to produce hydrogen gas.
• Metals below hydrogen will not react with acids.

For example, zinc reacts with hydrochloric acid, but copper does not:

$$\mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2$$

Copper + hydrochloric acid 12 no reaction

Formation of salts

The salt formed in the reaction depends on the acid and the metal used.

Salts are named by combining the metal name with the acid
9s second part:

• Hydrochloric acid forms chloride salts (e.g., zinc chloride)
• Sulfuric acid forms sulfate salts (e.g., magnesium sulfate)
• Nitric acid forms nitrate salts (e.g., copper nitrate)

For example, when zinc reacts with hydrochloric acid, the salt formed is zinc chloride:

$$\mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2$$

When magnesium reacts with sulfuric acid, the salt formed is magnesium sulfate:

$$\mathrm{Mg} + \mathrm{H_2SO_4} \rightarrow \mathrm{MgSO_4} + \mathrm{H}_2$$

Safety and practical considerations

When working with acids and metals, safety is very important:

• Always wear safety goggles to protect your eyes from acid splashes.
• Handle acids carefully to avoid spills and skin contact.
• Use a fume cupboard or work in a well-ventilated area if possible.

Collecting hydrogen gas safely:

• Hydrogen is highly flammable, so keep it away from flames or sparks.
• When testing for hydrogen, use a small amount and hold a lit splint carefully at the mouth of the test tube.

Observing reaction rates:

• Faster reactions produce hydrogen bubbles more quickly.
• Temperature, concentration, and surface area affect reaction rates but are covered in other topics. Briefly, higher temperature, higher concentration, and greater surface area increase the rate of reaction.

For example, when magnesium ribbon is added to dilute hydrochloric acid, bubbles of hydrogen gas form rapidly, the magnesium gradually disappears, and the solution may become warm due to the exothermic reaction.