Flashcards · 5070 · Topic 6

Chemical reactions flashcards

28 cards on Topic 6, subtopics 6.1 to 6.4. Pick a subtopic or study them all.

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6.1 Physical and chemical changes

Physical change: No new substance is formed, and the change is usually easy to reverse (for example, melting ice).
Chemical change: A new substance is formed, and the change is usually hard to reverse (for example, burning).
Signs that a chemical change has happened: A new substance forms — often shown by a colour change, a gas given off (effervescence), a precipitate, or a temperature change.

6.2 Rate of reaction

Four ways to increase the rate of a reaction: Raise the temperature; increase the concentration (or the pressure of a gas); increase the surface area (use smaller pieces); add a catalyst.
Catalyst: A substance that increases the rate of a reaction and is unchanged in mass and chemically at the end of the reaction.
Two ways to follow the rate of a reaction that gives off a gas: Measure the volume of gas with a gas syringe at regular times, or measure the loss in mass on a balance.
Collision theory: why does raising the temperature increase the rate?: Particles move faster, so collisions are more frequent, and a greater proportion of collisions have energy equal to or greater than the activation energy.
Collision theory: why does increasing concentration increase the rate?: There are more particles per unit volume, so collisions are more frequent.
Collision theory: why does increasing surface area increase the rate?: More particles are exposed, so collisions are more frequent.
How does a catalyst increase the rate?: It provides an alternative pathway with a lower activation energy.

6.3 Reversible reactions and equilibrium

What does the symbol ⇌ mean?: The reaction is reversible: it can go in both directions.
Heating hydrated copper(II) sulfate, then adding water: Heating: blue hydrated crystals → white anhydrous powder. Adding water: white → blue again.
Colours of hydrated and anhydrous cobalt(II) chloride: Hydrated: pink. Anhydrous: blue.
Features of a dynamic equilibrium: The forward and reverse reactions happen at the same rate, so the concentrations stay constant. It needs a closed system.
Effect of increasing pressure on an equilibrium involving gases: The position of equilibrium shifts towards the side with fewer gas molecules.
Haber process: equation and conditions: N₂ + 3H₂ ⇌ 2NH₃ 450 °C, 200 atm, iron catalyst.
Haber process: sources of nitrogen and hydrogen: Nitrogen: the air. Hydrogen: methane.
Contact process: equation and conditions: 2SO₂ + O₂ ⇌ 2SO₃ 450 °C, 200 kPa (2 atm), vanadium(V) oxide catalyst.
Why is 450 °C used in the Haber process rather than a lower temperature?: It is a compromise: a lower temperature would give a higher yield (the forward reaction is exothermic) but the rate would be too slow to be economic.
Why is 200 atm used in the Haber process rather than a much higher pressure?: Higher pressure would push the equilibrium further towards ammonia, but very high pressures are expensive to generate and maintain and are a safety risk. 200 atm is a compromise.
Contact process: sources of sulfur dioxide and oxygen: Sulfur dioxide: burning sulfur or roasting sulfide ores. Oxygen: the air.

6.4 Redox

Oxidation and reduction (oxygen definition): Oxidation: gain of oxygen. Reduction: loss of oxygen.
Redox reaction: A reaction in which oxidation and reduction happen at the same time.
What does the Roman numeral in iron(III) oxide tell you?: The oxidation number of iron in the compound is +3.
Oxidation and reduction (electron definition): Oxidation is loss of electrons; reduction is gain of electrons. (OIL RIG)
Oxidising agent: Causes oxidation in something else, so is itself reduced and gains electrons.
Reducing agent: Causes reduction in something else, so is itself oxidised and loses electrons.
Oxidation number: what does an increase and a decrease mean?: Increase in oxidation number: oxidation. Decrease: reduction.