12.10Chapter 12: Chemical Reactions 3 and Rates

Concentration and reaction rate — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 10 of 13 2 quizzes

📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision

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Lesson summary

12.10 Concentration and reaction rate — Collision Theory — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe and explain how concentration affects reaction rate using collision theory.Collisions — substances can only react when their particles hit each other…

Effective collision
Activation energy
Higher concentration
Speed of particles
Independent variable
Control variables

Part 1 of 6Collision theory

Collisions

Collisions substances can only react when their particles hit each other, or collide.

Effective collision

Effective collision a collision that leads to a reaction.

Not every collision leads to a reaction, because the particles must have enough energy to break bonds.

Activation energy

Activation energy the minimum energy needed for a reaction.

Rate

Rate the more frequent the effective collisions, the faster the rate.

Part 2 of 6Effect of concentration

Effect of concentration

Concentration

Concentration the amount of solute dissolved in a given volume of solution, for example g/dm³.

Higher concentration

Higher concentration more reactant particles are in the same volume, so they are closer together and collide more often.
This gives more effective collisions per second, so the rate increases.

Gases

Gases increasing the pressure has the same effect because it packs particles into a smaller space.

Speed of particles

Speed of particles the speed does not change, only the number of particles in the space.

Rule

Rule for reactions involving solutions, the higher the concentration of solution, the faster the reaction.

Part 3 of 6Planning the investigation

Question

Question in acid reactions, how does the concentration of acid affect reaction rate?

Apparatus

Apparatus a stopwatch to measure time accurately, a measuring cylinder to measure volume accurately, a conical flask, hydrochloric acid and magnesium ribbon.

Independent variable

Independent variable the concentration of acid.

Dependent variable

Dependent variable the time for the magnesium ribbon to finish reacting.

Control variables

Control variables the length of magnesium ribbon, the temperature and the volume of acid.

Range

Range the difference between the lowest and highest values of a variable.

The students used relative concentration values between 1 and 2 and collected data for five values in this range.

Other methods

Other methods react sodium thiosulfate with acid and time how long a cross under the beaker takes to disappear.

Part 4 of 6Presenting and analysing results

Graph

Graph plot a line graph with a line of best fit, because both variables are continuous.

Pattern

Pattern as acid concentration increases, the time for the magnesium to finish reacting decreases, so the reaction rate is faster.

Concentration of acid	Time for ribbon to disappear (s)
0.5	120
1.0	60
2.0	30

Relationship

Relationship doubling the concentration halves the time, so the rate doubles.

Explanation

Explanation a conclusion should include the scientific reason using collision theory.

Different acid

Different acid to check whether results are similar with a different acid, repeat the investigation with another acid and keep the other variables the same.

Part 5 of 6Explaining graphs and results

Volume against time

Volume against time the curve for the highest concentration starts steepest.

Final volume

Final volume all curves level at the same height if the same amount of limiting reactant is used,
because the final volume depends on the amount of reactant, not on the concentration of acid when acid is in excess.

Effect

Effect concentration changes the rate, not the total amount made.

Part 6 of 6Industry and safety

Industry

Industry a medicine company wants to make medicines quickly and cheaply, so it studies conditions that speed up reactions.

Trade-off

Trade-off concentrated solutions or high-pressure gases increase output,
but they cost more and create hazards,
so concentration is chosen to balance cost and safety.

Laboratory

Laboratory dilute solutions are used for safety.

Key terms

Term	Meaning
Collision theory	the idea that reactions need collisions with enough energy.
Concentration	the amount of solute per unit volume.
Activation energy	the minimum energy needed for a reaction.
Effective collision	a collision that leads to a reaction.
Range	the difference between the lowest and highest values of a variable.
Control variable	a variable kept the same in a fair test.