GCSE Chemistry: Rates of Reaction (T6)

How does the slope give us an idea of the rate of reaction?

The steeper the slope, the faster the rate of reaction.

using a graph to interpret showing the quantities of reactants or products in a chemical reaction

1. Graph : Quantity vs. Time

2. Steep slope	Fast reaction (lots of change in little time)
   Gentle slope	Slower reaction
   Flat line at end	Reaction has finished (no more change)
   Final height	Total amount of product forme

What is rate of reaction?

the amount of reactant used or product formed per unit time.

Equations for mean rate of reaction

Mean rate of reaction= Quantity of product formed/time taken

Mean rate of reaction= Quantity of reactant used/time taken

units can be in either mol/s g/s or cm³/s (for a gas)

Effect of concentration of reactants and the pressure of reacting gases on the rate of reaction

1. Effect of concentration (for solutions)

   • increasing rate of concentration of reactants increases the rate of reaction

   • Higher concentration means more particles per unit volume

   • This leads to more frequent collisions between reacting particles

   • More frequent successful collisions = faster reaction

2. Effect of pressure (for gases)

   • increasing the pressure of gas increases the rate of reaction

   • Higher pressure → gas particles are closer together

   • Increases the frequency of collisions between particles

   • More frequent successful collisons= faster reaction

Using tangents to determine rate

• rate of reaction at a specific point in time by drawing a tangent to the curve and calculating its gradient.

1: Draw the Tangent

• Choose the point on the curve where you want to find the rate (e.g. 30 seconds).

• Place a ruler so it just touches the curve at that point, without cutting through it.

• Draw a straight line (the tangent).

2: Calculate the Gradient

Use the formula:

Rate of reaction = Change in y / change in x = Ду / Дx

• Choose two clear points on the tangent line.

• Use their coordinates (x₁, y₁) and (x₂, y₂).

• Plug them into the gradient formula.

What is a hypothesis?

• a proposal that could explain a fact or an observation

• In science → must be testable

Required practical : rates of reaction

• investigate how changing the concentration of a reactant affects the rate of a chemical reaction.

1. Method: Disappearing Cross Experiment(Sodium thiosulfate + hydrochloric acid)

2. Chemical reaction :

• sodium thiosulfate + hydrochloric acid → Sulfur (solid)

• Observation: Sulfur forms→ solution becomes cloudy → X disappears

3. Use a measuring cylinder to put 10cm3 of sodium thiosulfate solution into a conical flask

4. Place clonical flask on to a printed black cross→ X

5. Add 10cm3 hydrochloric acid into conical flask

6. Swirl solution and start stopwatch

7. Look down through the top of the flask , after sometime solution will turn cloudy

8. Stop the clock when can no longer see X

9. Carry out experiment again using lower concentration of sodium thiosulfate solution

10. Repeat the whole experiment and calculate mean values for each of concentration of sodium thiosulfate solution → DO NOT include any anomalous results when calculating a mean

11. CONTROL variables:

• volume of hydrochloric acid

• Temp

• Total volume of solution

• Same person judging when cross disappears

Reproducibility and problem with experiment

• a measurment is reproducible if it can be repeated by another person or using a different technique or equipment and still get the same result

Problems:

• diff ppl have diff eyesights → some can see the cross for longer than others → may not get same results

• BUT → since use the same size cross → problem isnt too severe

Required practical: measure the volume of gas produced by a reaction

1. investigate how the rate of reaction changes by measuring the volume of gas produced over timE

2. Common Example: Reaction between hydrochloric acid and magnesium:

3. magnesium (s) + hydrochloric acid (aq )→ magnesium chloride (aq) → magnesium chloride (aq)+ hydrogen (g)

Method :

• Using a measuring cylinder Add 50cm3 of hydrochloric acid into a conical flask

• Attach conical flask to a bung and delivery tube and place the delivery tube into a container filled with water

• Place upturned measuring cylinder also filled with water over the delivery tube

• Add 3cm strip of magnesium to the hydrochloric acid and start a stopwatch

• Reaction produces hydrogen gas which is trapped in the measuring cylinder

• Every 10 seconds measure the volume of hydrogen gas in the measuring cuclinder and continue until no more hydrogen is given off

• Repeat experiment using different concentrations of hydrochloric acid

What do both experiment show?

• the greater the concentration of a chemical in a reaction → faster the reaction takes place

• As the result is shown by 2 different experiments → reproducible

What’s collision theory?

Theory that states that chemical reactions can only take place when the reacting particles collide with each other. These collisions must have sufficient energy.

Effect of surface area of solid reactants on the rate of reaction

Particles in solution can only react with particles on the surface of the solid.

Breaking up the solid into smaller pieces increases the surface area of the solid reactant.

That means for the same volume of the solid, the particles around it will have more surface area to react with, increasing the frequency of collisions, thus increasing the rate of reaction.

Effect of temperature on rate of reaction

When temperature is increased the particles have more energy so they move faster.

Because they’re moving faster, the frequency of collisions increases, thus increasing the rate of reaction.

Also because the particles have more energy, more particles can overcome the activation energy barrier and collide successfully.

Effect of catalyst on the rate of reaction

A catalyst is a substance that increases the rate of reactions, without being used up.

Catalysts increase the rate of reaction by providing a different pathway for the reaction to happen that has a lower activation energy.

When a catalyst is present, particles require less energy to cross the activation energy barrier, which increases the frequency of successful collisions thus increasing the rate of reaction.

What is a reversible reaction

When in a chemical reaction, the products of the reaction can react to produce the original reactant.

Example of reversible reaction (thermal decomposition)

hydrated copper sulfate ⇌ anhydrous copper sulfate + water

CuSO₄.5H₂O(s) ⇌ CuSO₄(s) + 5H2O(l)

Key points about reversible reaction

• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.

• The same amount of energy is transferred in each case.

What is equilibrium?

The point at which the rates of the forward and backwards reactions in a reversible reaction are the same, and so the amounts of reactants and products in the reaction container don’t change.

How can equllibrium be achieved?

If the reaction takes place in a closed system.

What is L e Chatelier’s Principle

If a system is at equilibrium and a change is made to the conditions, then the system responds to counteract the change.

Effect of changing the concentration on a reversible reaction at equilibrium

• If you change the concentration of the reactants or products then the system is no longer at equilibrium.

• This means that the concentrations of all the substances will change until equilibrium is reached.

• If you increase the concentration of reactants, more products will be formed to reduce the amount of reactant-until equilibrium is reached again. (shifts to right)

• If you decrease the concentration of products, more reactants will react to increase the amount of product-until equilibrium is reached again.(shifts to right)

Effect of changing the temperature on a reversible reaction at equilibrium

• When the forward reaction is exothermic and the reverse reaction is endothermic

• If you increase the temperature of the system, then the equilibrium shifts to the left to reduce the temperature.

• That is because the reverse reaction is endothermic, so energy is taken in, causing the temperature to fall.

• This would decrease the amount of product and increase the amount of reactant.

• If you decrease the temperature of the system, then the equilibrium shifts to the right to increase the temperature.

• That is because the forward reaction is exothermic, so energy is released, causing the temperature to increase.

• This would increase the amount of product and decrease the amount of reactant.

Effect of changing the pressure on a reversible reaction at equilibrium (only for gases)

• If you increase the pressure on a reversible reaction at equilibrium, then the position of equilibrium shifts to the side with the smaller number of molecules.

• If you reduce the pressure, then the position of equilibrium shifts to the side with the larger number of molecules.

What does the pressure of gas depend on?

The number of molecules (not only thing though)

What is a hypothesis?

• A proposal that could explain a fact or an observation

• Must be testable

The disappearing cross reaction

sodium thiosulfate solution + hydrochloric acid → sulfur (solid)

What makes the solution go cloudy (turbid) in the disappearing cross reaction

• sulfur

• this turbidity can be used to see how long the reaction takes to finish

Describe the Rates of Reaction practical/disappearing cross

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

2. Place the conical flask onto a printed black cross.

3. Next add 10cm³ of hydrochloric acid into the conical flask. Swirl the solution and start a stopwatch.

4. Carefully observe through the top of the flask. After a certain time the solution will turn cloudy and stop the clock when you can no longer see the cross.

5. Carry out the experiment again using lower concentrations of sodium thiosulfate solution.

6. Then repeat the whole experiment and calculate the mean values for each concentration of sodium thiosulfate solution.

   (REMEMBER TO NOT COUNT ANOMOLIES)

How can a measurement be reproducible?

If it can be repeated by another person or using a different technique or equipment and still get the same result.

Problem with the disappearing cross experiment.

Different people have different eyesights, meaning that some people can see the cross for longer than others so they may not get the same result.

However, because all students use the same size printed cross, hopefully this problem won’t be too great.

Describe the second rate of reaction practical/ marble chips

1. Use a measuring cylinder to place 50cm³ of hydrochloric acid into a conical flask.

2. Then attach the conical flask to a bung and delivery tube.

3. Now place the delivery tube into a container filled with water.

4. Then place an upturned measuring cylinder also filled with water over the delivery tube.

5. Add a 3 cm strip of magnesium to the hydrochloric acid and start a stopwatch.

6. The reaction produces hydrogen gas which is trapped in the measuring cylinder. Every 10 seconds measure the volume of hydrogen gas in the measuring cylinder and do this until no more hydrogen is given off.

7. Repeat the experiment using different concentrations of hydrochloric acid.