The Reactivity Series
4. Reactions with Acids
Learning outcomes
- I can describe how metals react with acids.
- I can identify the products of metal-acid reactions.
- I can test for hydrogen gas produced during reactions.
- I can compare reaction rates for different metals.
- I can use observations to rank metals by reactivity.
Reactions with Acids
Many metals react with dilute acids to produce a salt and hydrogen gas.
The general reaction is:
metal + acid → salt + hydrogen
However, metals do not all react at the same rate. Some produce hydrogen very rapidly, others react slowly, and some metals show no reaction with common dilute acids.
These differences provide useful experimental evidence for comparing metals and constructing the reactivity series.
The General Metal-Acid Reaction
When a suitable metal reacts with an acid:
metal + acid → salt + hydrogen
For example:
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Symbol equation:
Mg + 2HCl → MgCl₂ + H₂
Two important products are formed:
- a salt
- hydrogen gas
The exact salt depends on both the metal and the acid being used.
What Is an Acid?
An acid is a substance that produces H⁺ ions in aqueous solution.
Common laboratory acids include:
- hydrochloric acid, HCl
- sulfuric acid, H₂SO₄
When a reactive metal is placed in a suitable dilute acid, metal atoms can lose electrons and hydrogen ions can gain those electrons.
This eventually produces hydrogen gas:
H₂
Recognizing a Metal-Acid Reaction
Suppose a piece of magnesium is placed into dilute hydrochloric acid.
You may observe:
- bubbles forming
- fizzing
- magnesium gradually disappearing
- the test tube becoming warmer
- hydrogen gas being produced
These observations indicate that a chemical reaction is taking place.
The bubbling is caused by hydrogen gas, not boiling.
Magnesium and Hydrochloric Acid
Magnesium reacts readily with dilute hydrochloric acid.
Word equation:
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Balanced symbol equation:
Mg + 2HCl → MgCl₂ + H₂
Magnesium is relatively high in the reactivity series, so the reaction is usually easy to observe.
What Happens to the Magnesium?
During the reaction, magnesium atoms form magnesium ions.
At the particle level:
Mg → Mg²⁺ + 2e⁻
The magnesium atoms lose electrons.
Therefore, magnesium is oxidized.
The magnesium ions become part of the magnesium chloride solution.
This explains why the visible piece of magnesium gradually becomes smaller and may eventually disappear.
What Happens to the Hydrogen Ions?
Hydrogen ions from the acid gain electrons:
2H⁺ + 2e⁻ → H₂
Hydrogen ions are reduced.
Therefore, the overall metal-acid reaction involves:
- oxidation of the metal
- reduction of hydrogen ions
This means metal-acid reactions are examples of redox reactions.
The Ionic Equation
For many reactions between metals and dilute acids, the essential chemical change can be represented as:
metal + hydrogen ions → metal ions + hydrogen
For magnesium:
Mg + 2H⁺ → Mg²⁺ + H₂
This equation helps explain why different acids can show similar reactions with reactive metals: the hydrogen ions are directly involved in producing hydrogen gas.
Testing for Hydrogen Gas
Hydrogen can be identified using the squeaky pop test.
A small sample of the gas is collected and tested using an appropriate flame under controlled laboratory conditions.
Hydrogen produces a characteristic:
squeaky pop
The sound occurs because hydrogen rapidly reacts with oxygen.
Word equation:
hydrogen + oxygen → water
Balanced symbol equation:
2H₂ + O₂ → 2H₂O
Why Test the Gas?
Bubbles alone do not prove that hydrogen has been produced.
Many chemical reactions can produce gases.
A chemical test provides additional evidence about the identity of the gas.
For metal-acid reactions:
observation: bubbles are produced
test: gas gives the characteristic pop
conclusion: hydrogen is present
This distinction between observation and conclusion is important in experimental science.
Metals Do Not All React Equally
Consider four metals:
- magnesium
- zinc
- iron
- copper
If similar pieces are placed into identical samples of dilute hydrochloric acid, their reactions differ significantly.
A typical pattern is:
magnesium > zinc > iron > copper
in reactivity.
Magnesium reacts relatively rapidly.
Zinc reacts less rapidly.
Iron reacts more slowly.
Copper shows no reaction with dilute hydrochloric acid under normal classroom conditions.
Comparing Magnesium and Zinc
Suppose equal masses of magnesium and zinc are added to equal volumes of hydrochloric acid at the same concentration and temperature.
Magnesium produces hydrogen more rapidly.
Zinc also reacts, but usually more slowly under comparable conditions.
Therefore, the observations support:
magnesium > zinc
in the reactivity series.
Zinc and Hydrochloric Acid
Word equation:
zinc + hydrochloric acid → zinc chloride + hydrogen
Balanced symbol equation:
Zn + 2HCl → ZnCl₂ + H₂
Typical observations include:
- bubbling
- gradual disappearance of zinc
- production of hydrogen
- temperature increase
Iron and Hydrochloric Acid
Iron also reacts with dilute hydrochloric acid.
Word equation:
iron + hydrochloric acid → iron(II) chloride + hydrogen
Balanced symbol equation:
Fe + 2HCl → FeCl₂ + H₂
Iron normally reacts more slowly than magnesium and zinc under similar conditions.
This provides evidence for the order:
Mg > Zn > Fe
Copper and Hydrochloric Acid
Copper is below hydrogen in the reactivity series.
Copper does not normally react with dilute hydrochloric acid to produce hydrogen.
Therefore:
copper + dilute hydrochloric acid → no reaction
You would expect:
- no significant fizzing
- no hydrogen production
- little visible change to the copper
This gives us an important rule.
Hydrogen in the Reactivity Series
Hydrogen is included in the reactivity series even though it is not a metal.
It acts as a useful reference point.
A simplified section is:
Magnesium
Aluminium
Zinc
Iron
Hydrogen
Copper
Silver
Gold
Metals above hydrogen can generally displace hydrogen from suitable dilute acids.
Metals below hydrogen generally cannot.
Therefore:
metal above hydrogen + suitable dilute acid → salt + hydrogen
For metals below hydrogen:
usually no hydrogen-producing reaction with common dilute non-oxidizing acids
Predicting Reactions Using the Reactivity Series
Suppose you are asked whether zinc reacts with dilute hydrochloric acid.
Locate zinc:
zinc is above hydrogen
Therefore:
reaction occurs
Products:
zinc chloride + hydrogen
Now consider copper:
copper is below hydrogen
Therefore:
no hydrogen-producing reaction with dilute hydrochloric acid
The reactivity series lets us make predictions before carrying out the experiment.
Reactions with Sulfuric Acid
Reactive metals can also react with dilute sulfuric acid.
The general pattern is still:
metal + acid → salt + hydrogen
But sulfuric acid produces sulfate salts.
For example:
magnesium + sulfuric acid → magnesium sulfate + hydrogen
Symbol equation:
Mg + H₂SO₄ → MgSO₄ + H₂
Zinc and Sulfuric Acid
Word equation:
zinc + sulfuric acid → zinc sulfate + hydrogen
Balanced symbol equation:
Zn + H₂SO₄ → ZnSO₄ + H₂
Again, hydrogen gas is released.
The acid determines the second part of the salt's name:
hydrochloric acid → chloride
sulfuric acid → sulfate
Predicting the Salt
When a metal reacts with an acid, identify:
- the metal
- the acid
The metal provides the first part of the salt name.
The acid determines the second part.
| Acid | Salt Type |
|---|---|
| Hydrochloric acid | Chloride |
| Sulfuric acid | Sulfate |
For example:
magnesium + hydrochloric acid → magnesium chloride + hydrogen
magnesium + sulfuric acid → magnesium sulfate + hydrogen
A Note About Nitric Acid
The simple rule:
metal + acid → salt + hydrogen
works well for many introductory reactions involving suitable metals and dilute hydrochloric or dilute sulfuric acid.
Nitric acid behaves differently because it is an oxidizing acid.
Metal reactions with nitric acid do not generally follow the simple hydrogen-producing pattern.
For this topic, predictions should therefore be based mainly on the common dilute-acid reactions being studied.
Reaction Rate
Reaction rate describes how quickly reactants are converted into products.
For a metal-acid reaction, rate can be investigated by measuring:
- bubbles produced over time
- volume of hydrogen produced over time
- loss of metal mass over time
- loss of total mass as hydrogen escapes
- time required for a fixed amount of metal to react
More quantitative measurements generally provide stronger evidence than simply describing a reaction as "fast" or "slow."
Measuring Hydrogen Volume
A useful experiment involves collecting hydrogen in a gas syringe.
As the metal reacts with acid:
metal + acid → salt + hydrogen
hydrogen enters the syringe.
The volume can be recorded at regular time intervals.
For example:
| Time (s) | Hydrogen Volume (cm³) |
|---|---|
| 0 | 0 |
| 10 | 18 |
| 20 | 31 |
| 30 | 40 |
| 40 | 46 |
| 50 | 49 |
| 60 | 50 |
The reaction is fastest near the beginning.
Later, the rate decreases.
Eventually, the hydrogen volume stops increasing because the reaction has finished.
Interpreting a Reaction-Rate Graph
A graph can be plotted with:
time on the horizontal axis
and:
hydrogen volume on the vertical axis
A steeper slope means hydrogen is being produced more rapidly.
Therefore:
steeper graph → faster reaction
When the graph becomes horizontal:
no additional hydrogen is being produced
The reaction has effectively stopped.
Comparing Metals Using Rate Graphs
Suppose equal amounts of magnesium, zinc and iron react separately with identical acid samples.
If the magnesium graph is steepest, zinc is intermediate and iron is shallowest, the evidence suggests:
magnesium reacts fastest
zinc reacts more slowly
iron reacts slowest
Under controlled conditions, this supports the reactivity order:
Mg > Zn > Fe
The initial slope is particularly useful for comparing reaction rates.
Same Final Volume but Different Rates
An important idea is that two metals can produce the same total amount of hydrogen but at different rates.
Suppose equal mole amounts of two metals both eventually produce 50 cm³ of hydrogen.
Metal A reaches 50 cm³ in 30 seconds.
Metal B reaches 50 cm³ in 100 seconds.
The final amount of gas is the same.
But:
Metal A reacted faster.
This distinction between rate and amount produced is extremely important.
Why Does the Reaction Slow Down?
At the beginning of a reaction, there are many reactant particles available.
As the reaction proceeds:
- acid is consumed
- metal may be consumed
- reactant concentrations can decrease
As a result, successful collisions become less frequent.
The reaction rate decreases.
Eventually, one of the reactants may be completely used up.
The reaction then stops.
Collision Theory
Collision theory helps explain reaction rates.
For a chemical reaction to occur, particles must:
- collide
- collide with sufficient energy
- have an appropriate arrangement for the reaction
A faster reaction involves a greater frequency of successful collisions.
Factors that affect collision frequency and energy therefore affect reaction rate.
Factors Affecting Metal-Acid Reaction Rate
The observed reaction rate depends on more than just the identity of the metal.
Important factors include:
- metal reactivity
- acid concentration
- temperature
- metal surface area
- amount of reactants
Therefore, these factors must be controlled when comparing the reactivities of different metals.
Acid Concentration
Increasing acid concentration means there are more acid particles in a given volume.
This usually increases the frequency of collisions with the metal surface.
Therefore:
higher acid concentration → generally faster reaction
If magnesium is tested with 0.5 mol/L acid while zinc is tested with 2.0 mol/L acid, the comparison would not fairly test their relative metal reactivities.
Temperature
At higher temperatures, particles move faster.
They:
- collide more frequently
- collide with greater average kinetic energy
A larger proportion of collisions have enough energy to react.
Therefore:
higher temperature → faster reaction
Temperature must therefore be controlled when comparing metals.
Surface Area
A metal powder exposes more surface to the acid than a large lump of the same mass.
Therefore:
greater surface area → faster reaction
For example, powdered zinc generally reacts faster than a large piece of zinc of equal mass.
This does not mean powdered zinc is chemically "more reactive" in the reactivity-series sense.
It simply reacts faster because more particles are exposed to the acid.
Reactivity vs Reaction Rate
These two ideas are related but different.
Reactivity describes the chemical tendency of a substance to react.
Reaction rate describes how quickly a particular reaction occurs under particular conditions.
Magnesium is intrinsically more reactive than zinc.
But poorly designed experimental conditions could make zinc appear to react faster—for example, if finely powdered zinc were compared with a large, oxide-coated piece of magnesium.
Therefore, experimental comparisons must be carefully controlled.
Designing a Fair Experiment
Suppose you want to rank magnesium, zinc and iron by reacting them with hydrochloric acid.
Keep constant:
- mass or amount of metal
- comparable surface area
- acid volume
- acid concentration
- acid temperature
- apparatus
- gas collection method
- measurement intervals
Change only:
type of metal
This is the independent variable.
A useful dependent variable might be:
volume of hydrogen produced in 30 seconds
or:
initial rate of hydrogen production
Example Investigation
Suppose 0.10 g samples of three metals are reacted separately under controlled conditions.
After 30 seconds:
| Metal | Hydrogen Produced |
|---|---|
| Magnesium | 42 cm³ |
| Zinc | 23 cm³ |
| Iron | 8 cm³ |
If the comparison is fair, these observations provide evidence that:
magnesium > zinc > iron
in reactivity.
Repeating the experiment would increase confidence in the results.
Repeats and Reliability
One trial may contain random error.
Scientists therefore repeat measurements.
Suppose magnesium produces:
Trial 1: 41 cm³
Trial 2: 43 cm³
Trial 3: 42 cm³
Mean:
mean = (41 + 43 + 42) / 3
mean = 42 cm³
Repeated measurements help identify unusual results and provide a more reliable estimate.
Using Mass Loss
Another way to measure reaction rate is to place the reaction container on a balance.
As hydrogen gas escapes:
mass decreases
The faster the mass decreases, the faster hydrogen is leaving the system.
A graph of mass against time can therefore provide evidence about reaction rate.
However, care is required because the gas must be allowed to escape while preventing liquid from being lost.
Energy Changes
Many metal-acid reactions are exothermic.
Energy is transferred to the surroundings, so the reaction mixture may become warmer.
For example, magnesium reacting with hydrochloric acid can produce a noticeable temperature increase.
This provides another observation that a chemical reaction is occurring.
From Observations to a Reactivity Ranking
Suppose four metals are tested under identical conditions.
Metal A
Extremely rapid fizzing.
Metal B
Steady fizzing.
Metal C
Slow fizzing.
Metal D
No visible reaction.
A reasonable ranking is:
A > B > C > D
But scientists should ideally collect quantitative data and use additional reactions before drawing a final conclusion.
Connecting Acid Reactions to the Reactivity Series
The reactivity series allows us to predict metal-acid reactions.
A simplified section is:
more reactive
Magnesium
Aluminium
Zinc
Iron
Hydrogen
Copper
Silver
Gold
less reactive
Therefore:
Mg + acid → reaction
Zn + acid → reaction
Fe + acid → reaction
but:
Cu + dilute HCl → no hydrogen-producing reaction
The position of hydrogen acts as the dividing reference point.
Using Several Types of Evidence
Reactions with acids are useful for ranking many metals, but they cannot provide the complete reactivity series by themselves.
Very reactive metals may react too dangerously with acids for ordinary classroom comparison.
Metals below hydrogen may all show no hydrogen-producing reaction, making them difficult to rank against one another using this test alone.
Scientists can combine evidence from:
- reactions with water
- reactions with acids
- reactions with oxygen
- displacement reactions
Together, these experiments provide a stronger reactivity ranking.
Real-World Connections
Metal-acid reactions are important beyond the classroom.
Acid Cleaning
Acids can react with certain metals, so industrial cleaning processes must choose materials carefully.
Corrosion
Acidic environments can accelerate the deterioration of some metals.
Material Selection
Engineers must consider whether metals will contact acidic substances.
Hydrogen Production
Metal-acid reactions provide a convenient laboratory method for producing small amounts of hydrogen.
Chemical Analysis
Reaction behaviour can provide evidence about the identity or reactivity of an unknown metal.
Common Mistakes
Saying the Gas Is Oxygen
The gas produced in the typical metal-acid reaction is:
hydrogen
Forgetting the Salt
The products are:
salt + hydrogen
not simply hydrogen.
Thinking Every Metal Reacts with Dilute Acid
Metals below hydrogen generally do not release hydrogen from common dilute non-oxidizing acids.
Assuming More Bubbles Always Means More Reactive
More bubbles per second can indicate a faster reaction, but concentration, temperature and surface area must be controlled.
Confusing Rate with Total Gas Produced
A faster reaction does not necessarily produce a greater final amount of gas.
Saying Bubbles Prove Hydrogen Is Present
Bubbles show that gas is produced.
A suitable gas test provides evidence that the gas is hydrogen.
Forgetting How the Salt Is Named
Hydrochloric acid produces chlorides.
Sulfuric acid produces sulfates.
Confusing Reactivity with Reaction Rate
Reactivity is a chemical property.
Reaction rate depends on both the substances and the conditions.
Forgetting Hydrogen in the Reactivity Series
Hydrogen provides an important reference for predicting whether a metal will release hydrogen from suitable dilute acids.
Applying the Simple Rule to Every Acid
Nitric acid does not generally follow the simple metal + acid → salt + hydrogen pattern.
Key Terms
Acid — A substance that produces H⁺ ions in aqueous solution.
Metal — An element that typically forms positive ions by losing electrons.
Salt — An ionic compound produced in many acid reactions.
Hydrogen gas — H₂, the gas commonly produced when suitable metals react with dilute acids.
Hydrochloric acid — HCl, an acid that forms chloride salts.
Sulfuric acid — H₂SO₄, an acid that forms sulfate salts.
Metal chloride — A chloride salt containing a metal ion.
Metal sulfate — A sulfate salt containing a metal ion.
Reactivity — The tendency of a substance to undergo chemical reactions.
Reactivity series — A ranking of metals according to relative chemical reactivity.
Reaction rate — How quickly reactants are converted into products.
Hydrogen test — A laboratory identification test in which a small hydrogen sample produces a characteristic pop when appropriately ignited.
Oxidation — Loss of electrons.
Reduction — Gain of electrons.
Redox reaction — A reaction involving oxidation and reduction.
Hydrogen ion — H⁺, an ion involved in acidic solutions.
Metal ion — A positively charged ion formed when a metal atom loses electrons.
Collision theory — A model explaining reaction rates in terms of particle collisions.
Surface area — The amount of material exposed to another reactant.
Concentration — The amount of dissolved substance within a given volume of solution.
Independent variable — The variable deliberately changed in an investigation.
Dependent variable — The variable measured in response to the change.
Control variable — A factor kept constant to ensure a fair comparison.
Gas syringe — Apparatus used to collect and measure gas volume.
Exothermic reaction — A reaction that transfers energy to the surroundings.
Quantitative data — Numerical measurements collected during an investigation.
Reliability — The extent to which repeated measurements produce consistent results.
Mean — The sum of measured values divided by the number of values.
Key Takeaways
- Many metals react with dilute acids.
- The general reaction is:
metal + acid → salt + hydrogen
- Hydrochloric acid produces chloride salts.
- Sulfuric acid produces sulfate salts.
- Hydrogen gas is represented by H₂.
- Hydrogen can be identified using the characteristic squeaky pop test under appropriate laboratory conditions.
- Bubbles alone do not prove that hydrogen is present.
- More reactive metals generally react more vigorously with suitable dilute acids.
- Magnesium reacts more rapidly than zinc under comparable conditions.
- Zinc generally reacts more rapidly than iron.
- Copper does not normally release hydrogen from dilute hydrochloric acid.
- Hydrogen is included in the reactivity series as a reference point.
- Metals above hydrogen generally displace hydrogen from suitable dilute acids.
- Metals below hydrogen generally do not.
- During a metal-acid reaction, the metal loses electrons.
- The metal is therefore oxidized.
- Hydrogen ions gain electrons.
- Hydrogen ions are therefore reduced.
- Metal-acid reactions are redox reactions.
- Reaction rate can be measured using hydrogen volume over time.
- A steeper gas-volume graph represents a faster reaction.
- A horizontal section of the graph indicates that gas production has stopped.
- Reaction rate and total amount of product are different ideas.
- Acid concentration affects reaction rate.
- Temperature affects reaction rate.
- Metal surface area affects reaction rate.
- These factors must be controlled when comparing different metals.
- Quantitative measurements provide stronger evidence than descriptions such as "fast" and "slow."
- Repeated trials improve the reliability of experimental results.
- Reactions with acids can be used to help rank metals by reactivity.
- Evidence from acid reactions can be combined with water, oxygen and displacement reactions to construct a more complete reactivity series.
The most important patterns are:
metal + suitable dilute acid → salt + hydrogen
and:
more reactive metal → generally faster reaction under the same conditions
and:
above hydrogen → generally reacts with suitable dilute acid to release H₂
Check Your Understanding
1. State the general word equation for a metal reacting with a suitable dilute acid.
2. What gas is produced?
3. How can hydrogen gas be identified?
4. Why are bubbles alone not enough to prove that hydrogen has been produced?
5. Write the word equation for magnesium reacting with hydrochloric acid.
6. Write the balanced symbol equation for magnesium reacting with hydrochloric acid.
7. What salt forms when zinc reacts with hydrochloric acid?
8. Write the balanced equation for this reaction.
9. What salt forms when magnesium reacts with sulfuric acid?
10. Write the balanced equation for magnesium reacting with sulfuric acid.
11. Describe three observations you might make when magnesium reacts with hydrochloric acid.
12. What happens to magnesium atoms in terms of electrons?
13. What happens to hydrogen ions?
14. Explain why the reaction is a redox reaction.
15. Which would normally react faster with the same dilute hydrochloric acid: magnesium or zinc?
16. Which would normally react faster: zinc or iron?
17. Predict whether copper will produce hydrogen with dilute hydrochloric acid.
18. Explain your answer using the reactivity series.
19. Why is hydrogen included in the reactivity series?
20. Explain how a gas syringe can be used to measure reaction rate.
21. What does a steep slope on a hydrogen-volume graph indicate?
22. What does a horizontal section indicate?
23. Explain why increasing acid concentration usually increases reaction rate.
24. Explain why increasing temperature usually increases reaction rate.
25. Explain why powdered metal usually reacts faster than a large piece of the same mass.
26. Explain the difference between reactivity and reaction rate.
27. Identify four variables that should be controlled when comparing different metals with acid.
28. Why should an experiment be repeated?
29. Explain how metal-acid reactions can be used to rank metals by reactivity.
30. Challenge: Four unknown metals, A, B, C and D, are reacted separately with identical samples of dilute hydrochloric acid.
After 30 seconds:
A produces 48 cm³ H₂
B produces 27 cm³ H₂
C produces 9 cm³ H₂
D produces 0 cm³ H₂
a. Suggest a reactivity order from the evidence.
b. Which metal appears most reactive?
c. Which appears least reactive in this test?
d. Can you conclude that D is completely unreactive? Explain.
e. What additional experiment could help investigate D?
f. Identify the independent variable.
g. Identify a suitable dependent variable.
h. Give four control variables.
i. Explain why surface area must be controlled.
j. Explain why acid concentration must be controlled.
k. Sketch the expected relative shapes of hydrogen-volume-against-time curves for A, B and C.
l. Explain what the slopes of those curves represent.
m. If A and B eventually produce the same final volume of hydrogen, does that mean they reacted at the same rate? Explain.
n. Describe how you would obtain more reliable results.
o. Explain how these observations could be combined with water, oxygen or displacement experiments to construct a stronger reactivity series.