The rate and extent of Chemical change

Rate of a chemical reaction

how fast reactants are changed into products

Graphs - amount of product formed over time

Steeper the line/greater the gradient the faster the rate or reaction

Collision theory

The rate of reaction depends on the frequency of collisions — the more successful collisions there are, the faster the reaction.

Energy is transferred during a collision. Particles must collide with enough energy for the collision to be successful. This minimum energy is called the activation energy — the amount of energy needed to break the bonds in the reactants.

Temperature’s effect on rate of reaction

Increasing the temperature gives reactant particles more kinetic energy, so they move faster. This means reactant particles collide more frequently, and more particles have enough energy to overcome the activation energy. As a result, there are more successful collisions, so the rate of reaction increases.

Concentration’s effect on rate of reaction

NO EFFECT ON ENERGY

Increasing the concentration means the reactant particles are closer together. This causes particles to collide more frequently, leading to more successful collisions. As a result, the rate of reaction increases.

Pressure’s effect on rate of reaction

Increasing the pressure means the reactant particles are closer together because the same number of particles occupy a smaller space. This causes particles to collide more frequently, leading to more successful collisions. As a result, the rate of reaction increases.

Surface area’s effect on rate of reaction

Increasing the surface area exposes more reactant particles. This means reactant particles can collide more frequently, leading to more successful collisions. As a result, the rate of reaction increases.

Break solid into smaller pieces increases the surface area to volume ratio

Catalyst’s effect on rate of reaction

A catalyst speeds up a reaction without being used up. It decreases the activation energy by providing an alternative reaction pathway with a lower activation energy. (chemically unchanged)

More reactant particles have sufficient energy to collide successfully.

Rate of Reaction=

amount of reactant used or product formed / time

How can precipitation and colour change be used to measure rate of reaction?

Observing a visible change, such as a solution becoming cloudy or changing colour.

You can time how long it takes for a cross under the solution to disappear — the faster it disappears, the faster the reaction.

Results can be subjective because different people may judge the change at slightly different times.

How can change in mass be used to measure rate of reaction?

• If a product is a gas, the mass of the reaction mixture decreases as the gas escapes.

• Use a mass balance to measure this.

• The quicker the mass decreases, the faster the reaction.

• Take measurements at regular intervals so you can plot a graph of mass vs. time and determine the rate of reaction.

• This method is very accurate, but some gas may escape into the room.

How can volume of gas be used to measure rate of reaction?

• Use a gas syringe to measure the volume of gas given off.

• The more gas produced in a given time, the faster the reaction.

• Take measurements at regular intervals and plot a graph of gas volume vs. time to find the rate of reaction.

• Gas syringes are accurate, but if the reaction is very vigorous, the plunger may blow off the end of the syringe.

Required practical Mg+ HCl

1. Add a set volume of dilute hydrochloric acid to a conical flask.

2. Add some magnesium ribbon to the acid and plug the flask with cotton wool.

3. Start a stopwatch and record the mass using a balance at regular intervals.

4. Record the readings in a table and calculate the mass lost for each measurement.

5. Plot a graph of mass lost vs. time to determine the rate of reaction.

6. Repeat the experiment with more concentrated acid, keeping the volume of acid and size of magnesium ribbon constant.

Required Practical 2 - disappearing cross - could plot graph of 1/time

1. Add a set volume of sodium thiosulfate solution to a conical flask.

2. Place the flask on a piece of paper with a black cross drawn on it.

3. Add dilute hydrochloric acid to the flask and start a stopwatch.

4. Watch the black cross disappear and record the time it takes.

5. Repeat the experiment using different concentrations of one reactant at a time, keeping the volume and depth of solution the same.

6. Record results in a table and plot a graph of 1/time vs. concentration to show the effect on the rate of reaction.

- could plot graph of 1/time

How to calculate mean reaction rate?

overall change in y divided by total time taken

How to calculate reaction rate?

Tangent

Reversible reactions

• As a reaction proceeds, the concentration of reactants decreases, so the forward reaction slows down.

• At the same time, more products are formed, so the backward reaction speeds up.

• Eventually, the forward and backward reactions occur at the same rate. This is when the system is at equilibrium.

• At equilibrium, reactions are still happening, but there is no overall change in the concentrations of reactants and products. This is called a dynamic equilibrium.

• The concentrations of reactants and products remain constant once equilibrium is reached.

• Equilibrium can only be established in a closed system, where no reactants or products can escape and nothing is added.

Position of Equilibrium

• When a reaction is at equilibrium, this does not mean the amounts of reactants and products are equal.

• If the equilibrium lies to the right, there is a greater amount of product.

• If the equilibrium lies to the left, there is a greater amount of reactant.

• The position of equilibrium depends on the conditions: temperature, pressure (for gases), and concentration.

Reversible reactions endothermic and exothermic

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

• The energy transferred from the surroundings in the endothermic reaction is equal to the energy transferred to the surroundings in the exothermic reaction.

Hydrated copper sulfate⇌Anhydrous copper sulfate+Water

• Heating blue hydrated copper sulfate causes the water to evaporate, leaving white anhydrous copper sulfate powder. This forward reaction is endothermic.

• Adding water to the white powder reforms the blue hydrated copper sulfate crystals. This backward reaction is exothermic.

Le Chatelier’’s Principle

“If the conditions of a dynamic equilibrium change, the position of equilibrium/system will shift to counteract the change.”

This allows you to predict how the position of equilibrium will move.

Changes to temperature

Le Chatelier’’s Principle

The forward reaction is exothermic.

• When the temperature increases, the system counteracts the change by reducing the temperature.

• This causes the position of the equilibrium to shift to the left, favouring the backward reaction.

• As a result, the yield of reactants increases.

Changes to pressure

Le Chatelier’’s Principle

The forward reaction is exothermic. A+ 2B → C+D

• Look at the moles of gas on each side of the reaction. More moles → higher pressure.

• When pressure increases, the system counteracts the change by decreasing pressure.

• This causes the equilibrium to shift toward the side with fewer moles.

• If that is the right side, the forward reaction is favoured, increasing the yield of products.

Changes to concentration

Le Chatelier’’s Principle

The forward reaction is exothermic. A+ 2B → C+D

• When the concentration of A is increased, the system counteracts the change by decreasing the concentration of A.

• This causes the equilibrium to shift to the right, favouring the forward reaction.

• As a result, the yield of C + D (products) increases.

(If you decrease the concentration of products the system tries to increase it by reducing the amount of reactants)

Why does solution go cloudy?

Sulfur is a precipitate/insoluble solid

Why does rate decrease on a graph?

concentration of reactant particles decreases so particles get further apart so les successful /frequent collisions

Graphs - draw a line

When the concentration of the acid is halved, the number of moles of acid particles is halved, so the amount of product collected is also halved.