4.6 The rate and extent of chemical change

"How is the rate of a chemical reaction measured?

By measuring how much of a reactant is used up, or how much product is formed, over a given period of time."

"What is the formula for mean rate of reaction (using a reactant)?

Mean rate of reaction = quantity of reactant used ÷ time taken."

"What is the formula for mean rate of reaction (using a product)?

Mean rate of reaction = quantity of product formed ÷ time taken."

"What units is rate of reaction measured in?

g/s (grams per second) if measuring mass, or cm³/s (cubic centimetres per second) if measuring volume of gas."

"How do you find the rate of reaction at one specific moment from a graph?

Draw a tangent to the curve at that point — the slope (gradient) of the tangent is the rate of reaction at that moment. (You can't just read off the curve — the rate changes during the reaction.)"

"What 5 factors affect the rate of a chemical reaction?

1) Concentration of reactants in solution; 2) Pressure of reacting gases; 3) Surface area of solid reactants; 4) Temperature; 5) Presence of a catalyst."

"What is collision theory?

The idea that chemical reactions can only happen when reacting particles collide with each other AND with enough energy to react."

"What is activation energy?

The minimum amount of energy that reacting particles must have for a successful reaction to occur when they collide."

"Why does increasing concentration increase the rate of reaction?

There are more reactant particles in the same volume, so collisions happen more frequently (more often per second)."

"Why does increasing the pressure of reacting gases increase the rate of reaction?

The gas particles get squeezed closer together, so they collide more frequently."

"Why does increasing the surface area of a solid reactant increase the rate of reaction?

More particles of the solid are exposed at the surface, available to collide with the other reactant. (The particles inside the lump are 'hidden' and can't react until the surface is gone.)"

"Why does increasing temperature increase the rate of reaction?

TWO reasons: particles move faster so collide more frequently, AND each collision is more energetic so a higher proportion have enough energy to overcome the activation energy."

"How does surface area to volume ratio affect rate?

The higher the surface area to volume ratio, the faster the reaction. (Cutting one big lump into smaller pieces increases the ratio — e.g. powder reacts much faster than a single chunk of the same mass.)"

"What is a catalyst?

A substance that increases the rate of a chemical reaction without being used up itself. (It's not a reactant — it can be reused.)"

"How does a catalyst speed up a reaction?

It provides a different reaction pathway with a lower activation energy, so more collisions have enough energy to react."

"Are catalysts shown in chemical equations?

No — because catalysts are not used up they aren't included in the balanced equation. (Sometimes written above the arrow as a label.)"

"What do enzymes do?

Enzymes act as catalysts in biological systems (in living organisms)."

"What is a reversible reaction?

A reaction where the products can react together to re-form the original reactants. Shown with a ⇌ symbol instead of →."

"How can the direction of a reversible reaction be changed?

By changing the conditions, e.g. temperature, pressure, or concentration of reactants/products."

"If a reversible reaction is exothermic in one direction, what is it in the opposite direction?

Endothermic — and the SAME amount of energy is transferred either way."

"What is equilibrium (in a reversible reaction)?

When the forward and reverse reactions are happening at exactly the SAME rate, so the amounts of reactants and products stay constant. (Only happens in a closed system where nothing can escape.)"

"Required practical 5: what does it investigate?

How changes in CONCENTRATION affect the rate of reaction, using two methods: 1) measuring the volume of gas produced; 2) measuring change in colour or turbidity (cloudiness)."

"What does 'turbidity' mean in RP5?

How cloudy a solution becomes. (E.g. in the sodium thiosulfate + acid reaction, a cross under the beaker disappears as sulfur is produced — you time how long that takes.)"

"What does a reaction profile show?

The relative energies of reactants and products, the activation energy (the 'hump'), and the overall energy change of the reaction."

"How does a catalyst change a reaction profile?

The activation energy 'hump' is lower (because the catalysed pathway needs less energy). The overall energy change (start vs end) is the SAME — catalysts don't change the energy of reactants or products."

"Two ways to follow the rate of a reaction that produces a gas?

1) Collect the gas and measure its volume over time; 2) Place the reaction flask on a balance and measure the loss in mass as gas escapes."

"Why does the rate of reaction usually slow down as the reaction proceeds?

Reactants get used up, so their concentrations fall — fewer collisions happen per second."

"Why does a reaction eventually stop?

A reactant is completely used up (the 'limiting' reactant runs out), or — in a closed system — the reaction reaches equilibrium."

"What does 'more frequent collisions' mean?

More collisions happening per second between reacting particles. (Frequency = how often.)"

"What does 'more energetic collisions' mean?

Each collision has more energy, so a higher proportion of collisions have enough energy to overcome the activation energy and react successfully."

"Can a catalyst change the amount of product made?

No. A catalyst only changes the SPEED at which the reaction reaches its end point — it doesn't change the final amount of product. (Common misconception!)"

"Does the same catalyst work for every reaction?

No — different reactions need different specific catalysts."

"What's an everyday example of an endothermic process being used?

Cold packs / sports injury packs — they take in heat from their surroundings to cool an injury."

"What's an everyday example of an exothermic process being used?

Hand warmers, self-heating cans, combustion (burning), neutralisation reactions."

"Why does powdered marble react faster with acid than a marble chip?

The powder has a much higher surface area to volume ratio, so more marble particles are exposed and able to collide with the acid at once."

"In a reaction profile, where on the diagram is the activation energy?

It's the height of the 'hump' — from the energy of the reactants UP to the peak of the curve. (Not from zero to the peak.)"

"In a closed system at equilibrium, are reactions still happening?

Yes — the forward and reverse reactions are both still happening, but at equal rates, so the overall amounts don't change. This is called dynamic equilibrium."