LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- distinguish physical and chemical changes
- explain rate factors using collision theory
- interpret and evaluate rate experiments
- predict equilibrium shifts
- explain Haber and Contact process conditions
- identify oxidation, reduction and redox agents
THE BIG IDEA
Explain reaction rates, equilibrium, industrial conditions and redox using particles and electrons.
Chemical reactions rearrange atoms into new substances. Collision theory explains how quickly they occur, while equilibrium explains reversible reactions in closed systems.
Redox provides another view: electrons or oxidation numbers change as one substance is oxidised and another is reduced.
SECTION 01
Physical and chemical changes
A physical change alters state or form without producing a new substance and is often reversible by physical means.
A chemical change forms new substances and may show colour change, gas, precipitate, light or a temperature change. Observations are evidence, but must be interpreted in context.
SECTION 02
Rate of reaction and collision theory
Reaction rate increases with greater concentration, higher gas pressure, larger solid surface area, higher temperature or a catalyst.
Higher concentration or pressure gives more particles per unit volume and more frequent collisions. Greater surface area exposes more particles.
Higher temperature makes particles move faster, collide more often and gives a larger fraction energy at least Eₐ. A catalyst lowers Eₐ.
KEY IDEAS
- Enzymes are biological catalysts.
- A catalyst is chemically unchanged at the end.
- A steeper graph gradient means a faster rate.
SECTION 03
Investigating rates
Gas production can be followed with a gas syringe or by loss of mass when gas escapes. Choose a method that measures the changing quantity directly and safely.
Change one independent variable, control the others, repeat measurements and compare initial rates or time to a fixed endpoint.
A gas syringe captures gas directly; mass loss can record frequent readings but is unsuitable when the gas is hazardous or when mass change is very small.
SECTION 04
Reversible reactions and equilibrium
A reversible reaction proceeds in both directions and uses ⇌. In a closed system at dynamic equilibrium, forward and reverse rates are equal and concentrations remain constant.
Increasing concentration shifts equilibrium to use the added substance. Increasing pressure shifts a gaseous equilibrium toward fewer gas molecules. Increasing temperature shifts toward the endothermic direction.
A catalyst speeds both directions equally, so equilibrium is reached sooner but its position does not change.
KEY IDEAS
- Hydrated copper(II) sulfate is blue; anhydrous copper(II) sulfate is white.
- Hydrated cobalt(II) chloride is pink; anhydrous cobalt(II) chloride is blue.
SECTION 05
Haber and Contact processes
The Haber process uses nitrogen from air and hydrogen mainly from methane at about 450 °C, 20 000 kPa and an iron catalyst.
The Contact process converts sulfur dioxide from burning sulfur or roasting sulfide ores using oxygen from air at about 450 °C, 200 kPa and vanadium(V) oxide.
Conditions compromise between equilibrium yield, reaction rate, plant cost and safety. Catalysts improve rate without moving equilibrium.
SECTION 06
Redox and oxidation numbers
Redox contains simultaneous oxidation and reduction. Oxidation is oxygen gain, electron loss or oxidation-number increase; reduction is oxygen loss, electron gain or oxidation-number decrease.
An oxidising agent oxidises another substance and is reduced. A reducing agent reduces another substance and is oxidised.
An uncombined element has oxidation number 0; a monatomic ion equals its charge; totals equal zero in a compound or the overall charge in an ion.
KEY IDEAS
- Acidified potassium manganate(VII) changes purple to colourless when reduced.
- An oxidising agent converts iodide ions to iodine, giving a brown solution.
- Roman numerals in names state oxidation number, such as iron(III).
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Collision frequency and energy control reaction rate.
- Catalysts lower activation energy.
- Dynamic equilibrium has equal forward and reverse rates.
- Industrial conditions balance yield, rate, cost and safety.
- Oxidation and reduction always occur together.