The Rate and Extent of Chemical Change

How to calculate mean rate?

quantity of reactant used / time OR

quantity of products formed / time

40cm$^3$ of product was produced in 10 seconds.

Calculate the mean rate of reaction

40 / 10

4cm$^3$ /s

How do you calculate the rate of reaction for a specific time?

Change in amount / change in time

How do you draw a tangent?

Draw a straight line that touches the curve

How do you find the gradient of a tangent?

change in y / change in x

What is collision theory?

Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must react is called the activation energy.

Explain why many collisions do not lead to a reaction

Do not have enough energy

What happens if you increase the amount of collisions in a certain time?

Increases the rate of reaction

What happens if you decrease the energy of collision?

Decreases the rate of reaction

What are the factors that affect the rate of reaction ?

Concentration, Surface Area, Pressure(Gas), Temperature, Catalyst

Why does increasing temperature increase the rate of reaction?

• particles have more energy and move faster

• Particles move frequently

• Particles collide with more energy (more collisions are successful - achieve activation energy)

Why does decreasing temperature decrease the rate of reaction?

• particles have less energy and move slower

• Particles move less frequently

• Particles collide with less energy (less collisions are successful - don’t achieve activation energy)

Why does increasing concentration (or pressure) increases the rate of reaction?

• More particles in the same volume

• Particles collide more frequently

Why does decreasing concentration (or pressure) decrease the rate of reaction?

• Fewer particles in the same volume

• Particles collide less frequently

Why does increasing surface area (small pieces) increase the rate of reaction?

• more particles are exposed

• Particles collide more frequently

Why does decreasing surface area (large pieces) decrease the rate of reaction?

• Fewer particles are exposed

• Particles collide less frequently

Label the rate of reaction graph

Reaction fastest, slows down, stops

the steeper the line…

… the faster the reaction

In terms of particles explain why the rate of reaction is the fastest at the beginning ?

highest concentration (most reactant particles in the volume), so most frequent successoul collision between reactants particles.

In terms of particles explain why the rate of reaction slows down ?

less reactant particles in the same volume (lower concentration , so less frequent successful collision between reactant particles.

In terms of particles explain why the rate of reaction stops at the end?

no reactant particles, so no reaction.

Graphs:

Concentration practical : Method

• measure 50cm3 of sodium thiosulfate using a measuring cylinder and pour it into a conical flask.

• Place the flask on top of a piece paper marked with a black cross

• Measure 10cm3 of dilute hydrochloric acid using a measuring cylinder and pour into the flask

• Start the timer when you add the acid and swirl the mixture

• Look down the flask and stop the timer when you can no longer soo the cross

• Repeat using different concentration of sodium thiosulfate but keep the acid concentration the same

Concentration Practical : Equation

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

Concentration Practical : Balanced equation

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

Concentration Practical : Why does the solution become cloudy?

sulfur is formed as a product

Sulfur is an insoluble solid

Concentration Practical : Variables

Control - same concentration of hydrochloric acid

Independent - concentration of sodium thiosulfate

Dependent - time taken for cross to disappear

Concentration Practical : Conclusion

The higher the concentration of a dissolved reactant the faster the rate of reaction

WHY : At nigher concentration, there are more particles in the same volume. This means that the particles collide more frequently and are therefore more likely to react.

Surface Area Practical : What are marble chips made out of?

Calcium carbonate

Surface Area Practical : What is the product when calcium carbonate react with acid?

Carbon dioxide gas

Surface Area Practical : Equation

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

Surface Area Practical : Balanced equation

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Surface Area Practical : Method

1. Using a measuring cylinder, add 50 cm³ of dilute hydrochloric acid to a conical flask.

2. Add 0.4 g of calcium carbonate to the flask. Immediately connect the gas syringe and start a stop clock.

3. Every 10 seconds, record the volume of gas produced.

4. When the reaction is complete, clean the apparatus as directed by a teacher.

5. Repeat steps 1 to 5 with different concentrations of hydrochloric acid.

Surface Area Practical : Conclusion

smaller marble chips with greater surface area, produced 100cm3 gas is less time than the large marble chips showing they reached faster with more frequent collisions with acid particles.

What is a catalyst?

speeds up à chemical reaction without being used up.

It does this by providing an alternative pathway with a lower activation energy

What are advantages of a catalyst?

Reaction is faster

Reduces the amount of energy needed for a reaction to occur

Reaction can be done at lower temperatures and pressure

Can be reused over and over again

What are disadvantages of a catalyst?

Expensive to buy

May need to buy / design specific catalysts for each reaction

May become poisoned and have to be cleaned before the will work again

May be hazardous to the environment

What is a reversible reaction?

The products of the reaction can react to produce the original reaction.

We represent this using the arrow ⇌

A + B ⇌ C + D

What is the word equation for the reversible reaction of Ammonium Chloride?

ammonium chloride ⇌ ammonia+hydrogen chloride