Rate of Reaction

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Rate of Reaction

Rate of Reaction

The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed.

How can the rate of a chemical reaction be found?

By measuring the quantity of a reactant used or the quantity of product formed over time.

Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without itself being used up in the reaction

How do catalysts work?

• The minimum amount of energy that particles must have to react is called the activation energy

• A catalyst provide an alternative pathway with a lower activation energy

Give a type of catalyst

Enzymes - molecules that act as catalysts in biological systems

Collision Theory

Chemical reaction scan only occur when reacting particles collide with each other and with sufficient energy

What is a collision that causes a reaction called?

A successful collision

Activation Energy

The minimum amount of energy that colliding particles must have for them to react.

Equations for mean rate of reaction

• mean rate of reaction = quantity of reactant used ÷ time taken

• mean rate of reaction = quantity of product formed ÷ time taken

Why is measuring mass useful in a reaction?

This method is useful when eg carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small relative formula mass, M_r.

Units for measuring mass in a reaction

g/s or g/min

Why is measuring volume useful in a reaction?

This method is useful when a gas leaves the reaction container.

How is the volume of a gas measured in a reaction

The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder

Burette

Long glass tube with a tap and marked with volume measurements, used in titrations.

Units for measuring volume in a reaction

cm³ s^-1 or cm³ min^-1

What can the rate of a chemical reaction be measured in

• g/s, cm³/s, mol/s Mol s^-1

How do you use graphs to analyse the rate of reaction?

• by plotting a graph of mass or volume of product formed against time

• The gradient of the line is equal to the rate of reaction:

  • the steeper the line, the greater the rate of reaction

  • fast reactions - seen when the line becomes horizontal - finish sooner than slow reactions

The greater the _____ of ____ ____, the greater the ___ ____.

frequency, successful collisions, rate of reaction

Measuring Rate of Reaction: Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen: Symbol Equation

SE: Mg(s) + 2HCl(aq) → MgCl2(aq) +H2(g)

Measuring Rate of Reaction: Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen: Apparatus

• Delivery Tube

• Clamp

• Measuring cylinder

• trough

• water

• dilute acid + magnesium ribbon

• Conical flask

• bung

Measuring Rate of Reaction: Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen: Method

1. Using a measuring cylinder pour 50cm³ of 1.0mol/dm³ hydrochloric acid into a 100cm³ conical flask

2. Fit the bung and delivery tube to the top of the flask

3. Half fill a trough with water

4. Fill the other measuring cylinder with water and make sure it stays filled when you invert it into the water trough and the delivery tube is positioned correctly

5. Add a single 3cm length of magnesium ribbon to the flask, put the bung back into the flask and start the stop watch

6. Record the volume of hydrogen gas collected every ten seconds

7. Continue timing until the gas does not change

Measuring Rate of Reaction: Magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen: Conclusion

1. Plot a graph with volume of gas, cm on the y-axis and time, s on the x-axis

2. Draw a line of best fit (curved)

3. Draw tangents at different point on the graph and create a right-angled triangle to work out the gradient

4. The steeper the gradient the higher the rate of reaction

How else can you measure the rate of reaction using the volume of gas given off?

By using a conical flask (with the reaction mixture in it), a bung, delivery tube and a gas syringe

Observing the effect of surface area on the rate of reaction: Apparatus

• Dilute acid

• marble chips

• conical flask

• bung

• delivery tube

• water trough

• inverted measuring cylinder

• clamp

Observing the effect of surface area on the rate of reaction: Method

1. Set up apparatus using a 100cm³ measuring cylinder

2. Measure out 25cm³ HCl using a 25cm³ measuring cylinder and pour it into the conical flask

3. Weigh out 2g of marble chips using a weighing boat and a mass balance

4. Add the chips to the acid in the conical flask and start the stop watch immediately

5. Record the volume of water displaced (CO2 is produced) each minute and record in a table

6. Repeat for different sized marble chips

Observing the effect of surface area on the rate of reaction: Conclusion

1. Plot a graph with Volume of CO2 collected, cm³ on the y-axis and time, min on the x-axis

2. Plot two lines - one for small chips and one for large chips

3. The large marble chips should be lower than the small marble chips because the small marble chips should react faster than larger ones

Observing the effect of surface area on the rate of reaction: Important notes

• The steeper the curve, the faster the rate of reaction

• The graph levels off because one of the reactants have been used up and so no more gas was produced

Observing the effect of surface area on the rate of reaction: Variables

• Independent: The size of the marble chips

• Dependent: The volume of gas collected in the given time

• Control: Same mass of marble chip used

• Same volume of hydrochloric acid used

• Same temperature

• Same concentration of hydrochloric acid

Investigating the effect of changing concentration on the rate of reaction: Apparatus

• 40g/dm³ sodium thiosulfate solution

• 2.0 M dilute hydrochloric acid

• 10cm³ measuring cylinder

• 100cm³ measuring cylinder

• 100cm³ conical

• printed black paper cross

• stop clock

Investigating the effect of changing concentration on the rate of reaction: Method

1. Use a measuring cylinder to measure out 10cm³ sodium thiosulfate solution into the conical flask

2. Use the measuring cylinder to add 40cm³ water to dilute the sodium thiosulfate solution to a concentration of 8g/dm³

3. Put 10cm³ of hydrochloric acid into the 10cm³ measuring cylinder

4. Put this acid into the flask and swirl gently

5. Start the stop clock

6. Look down through the top of the flask and stop the clock when you can no longer see the cross

7. Record the time taken for the cross to disappear

8. Repeat steps 1-5 using:

9. 20cm³ sts + 30cm³ water - c16g/dm³

10. 30cm³ sts + 20cm³ water- c24g/dm³

11. 40cm³ sts + 10cm³ water- c32g/dm³

12. 50cm³ sts + no water - c40g/dm³

Investigating the effect of changing concentration on the rate of reaction: Conclusion

• Calculate the mean time for each of the sodium thiosulfate concentrations - leave out anomalous results

• Plot a graph with mean time taken for cross to disappear on the y-axis and sodium thiosulfate concentration in g/dm³ on the x-axis

• Draw a smooth curved line of best fit

Investigating the effect of changing concentration on the rate of reaction: Equations

• Sodium thiosulfate +hydrochloric acid → sodium chloride + sulfur dioxide + sulfur + water

• Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) +SO2(g) + S(s) + H2O(l)

Explain why in the reaction of sodium thiosulfate solution the reaction goes cloudy

Sulfur is produced which is insoluble and this forms a precipitate (an insoluble solid)

in the reaction of sodium thiosulfate which product is a gas

Sulfur dioxide (SO2(g)) is a gas

Why does increasing the concentration increase the rate of reaction?

Increasing the concentration of reactants in the solution results in more reactant particles in a given volume and so this leads to more frequent collisions between reactant particles leading to an increase in rate of reaction

Why does increasing the Pressure of reacting gases increase the rate of reaction?

Increasing the pressure of reacting gases results in more reactant particles in a given volume and this results in more frequent collisions between reactant particles. This leads to an increase in rate of reaction

Why does increasing the temperature increase the rate of reaction?

• Increasing the temperature gives the reacting particles more energy and so they move around faster. This results in more frequent collisions between reacting particles.

• Increasing the temperature also makes the collisions more energetic. A higher proportion of the reacting particles have energy equal to or greater than the activation energy and so a greater proportion of the collisions will result in a reaction taking place

Why does increasing the surface area increase the rate of reaction?

Increasing the surface area of a solid reactant causes more of the solid particles to be exposed and so there will be more frequent collisions between reactant particles. This increases the rate of reaction

Investigating the effect of temperature on the rate of reaction: Method

1. Choose a temperature to investigate and use an ice bath or water bath to get the two solutions to that temperature

2. Use a thermometer to check the temperature of the solutions

3. Measure 5cm³ of hydrochloric acid and add 50cm³ of sodium thiosulfate solution in a sperate clean measuring cylinder

4. Place the flask on the centre of the large cross

5. Add sodium thiosulfate and then hydrochloric acid

6. start the stopwatch and swirl to mix to solutions

7. stop the clock when the cross disappears and note the time

8. repeat the experiment for four more different temperatures

Investigating the effect of temperature on the rate of reaction: Conclusion

1. Plot a graph with temperature on the x-axis and time taken for the cross to disappear on y-axis

2. Plot a line of best fit (curved)

3. As the temperature increases the rate of reaction rapidly increases as the time taken for the cross to disappear decreases

Investigating the effect of temperature on the rate of reaction: Variables

• Independent: temperature

• Dependent: time taken for the cross to disappear

• Control: same volume and concentration of sodium thiosulfate solution

• Same volume and concentration of hydrochloric acid