Concentration and the Rate of Reaction

L.O: To understand how concentration and pressure affects the

rate of a reaction

Iodine clock reaction

If all of the reactants are the same temperature why is the rate different?

Can you draw particle diagrams to show the how many alcohol particles there are in a vodka and a vodka and coke?

Vodka Vodka and coke

Which one will react with your body quicker and make you drunk?

So is there any truth to drinking lots of water?

Concentration• Particles collide when a reaction happens• The more crowded (concentrated) the solution, the faster the reaction.• In a more concentrated solution there are more acid particles and less

water particles in a given volume. So the chance of a collision with an acid particle is higher (increasing the rate)

Collisions infrequent Collisions frequent

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Concentration and particle collisions

Concentration in an acid

The concentration tells us how many particles of a solute we have dissolved in a certain volume of a solution. The unit of measurement is moles per cubic decimetre (mol/dm3).

Can you put these in order least to most concentrated? And in order of slowest rate of reaction to fastest?

2 mol/dm3 1 mol/dm3 0.5 mol/dm3

Your task is to collect data to show how the amount of product formed increases over time. You will need to graph this.

Acid and metal

magnesium + hydrochloric acid

→ magnesium chloride

+ hydrogen

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

11.8011.7711.74

1. Measure the 20cm3 acid into the conical flask.2. Cut a 6cm strip of magnesium.3. Set you equipment up clamping the gas syringe.4. Add the magnesium, and start the stop clock

connecting your gas syringe to the conical flask.5. Record the volume of gas after 10 secs6. Repeat with a different concentration using the

same amount of magnesium.

Volume of gas collected cm3 in 10sec Rate of reaction

(cm3/sec)

concentration 1 2 3 average

0 .5mol

1mol

1.5mol

etc.

Acid and metal

Work out the rate using the rate equation for each conc

Plot the result on a graph

X axis is concentration

Y axis is rate of reaction

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The effect of concentration on rate

Pressure

• Reactions involving gases are affected by the pressure of the gases present.

• If we cover one end of a bicycle pump and push in the plunger we increase the pressure.

• What we are doing is squeezing the gas molecules closer togethercloser together or making them more concentrated.

• And so - pressure speeds up gas reactions

Can you explain why?

Low pressure

High pressure

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Effect of pressure on rate of reaction

The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.

Why does increasing the pressure of gaseous reactants increase the rate of reaction?

As the pressure increases, the space in which the gas particles are moving becomes smaller.

lower pressure higher pressure