5. Rates Of Reaction

Physical Reaction Vs Chemical Reaction

Physical : the particles in the substance remains unchanged (only physical state is changed)

Chemical : particles arrangement is different - a new substance has been formed.

Signs of a chemical reaction

1. New product is formed (could be solid, aqueous, gas)

2. Gas is produced - Effervescence

3. Colour change

4. Temperature change (exothermic VS endothermic)

What is ‘rate’

Change in amount of product/reactant per unit of time

Exothermic Reaction

A reaction that releases energy from the system in the form of heat

Endothermic reaction

A reaction that the system absorbed energy from its surrounding in the form of heat

Why is it during an endothermic reaction, the solution’s temperature drops?

The thermal energy absorbed is converted into chemical potential energy and so the environment and the reaction mixture/solution feels colder

How to collect and test gas formed in reaction

Use a gas syringe or place a beaker underwater

Precipitate

Solid particles that separate from a liquid solution during a chemical reaction - is insoluble

Formula for Rate of Reaction

= Production / time (sec)

How to calculate the rate of reaction at a specific time (using a graph)

1. Draw a tangent that touches

2. Create a right angled triangle with points you can easily read on the graph (to reduce errors)

3. Calculate the gradient of the tangent (change in y/change in x)

3 necessary conditions for successful collisions

1. Particles must collide

2. Particles must have sufficient energy (Ea)

3. Particles must have correct orientation

Activation Energy (Ea)

The minimum energy required by particles to collide and have successful collisions

→ the energy needed to break apart bonds to allow the chemical reaction to occur

How to increase rate of reaction

1. Increase temperature

2. Increase concentration/pressure

3. Increase surface area

4. Add a catalyst

How does surface area affect rate of reaction

TSA is increased which exposes more inner particles to the surrounding reactants and therefore, more frequent and successful collisions

Surface area and rate of reaction between calcium carbonate and hydrochloric acid

1. Measure 50cm cubed of hydrochloric acid using a measuring cylinder

2. Pour the acid into a conical flask

3. Collect water trough and half fill with tap water

4. Measure (using a balance) 0.5 grams of small calcium carbonate chips

5. Fill the measuring cylinder with water and submerge in the trough

6. Prepare delivery tube, ensuring the tube is underneath the measuring cylinder to collect the gas

7. Prepare the stop watch and add calcium carbonate chips into conical flask, quickly closing the opening and see how much carbon dioxide is produced

8. Repeat with bigger chips

Conclusion : calcium carbonate chips and hydrochloric acid

Smaller particle size of calcium carbonate has a larger surface area. This means there are more, frequent, successful collisions occuring on the surface of the calcium carbonate chips. Due to more, frequent, successful collisions per unit time, the rate of reaction is faster.

Temperature and rate of reaction practical

1. Measure 15cm of 0.2M sodium thiosulfate using a measuring cylinder and pour it into a boiling tube. Measure 10cm of 1M hydrochloric acid into a boiling tube and place them both in the same water bath (w/ recorded temperature)

2. Allow the contents of the respective tubes reach the correct temperature and use a thermometer to check

3. Place a conical flask on the top of the card with a cross. Add the contents into the flask and swirl and start the timer

4. Record and stop timer when the cross isn’t visible anymore

5. Repeat for different temperatures

Why is sodium thiosulfate and hydrochloride acid changing colour

The sulfur created will precipitate (an insoluble solid formed from the 2 solutions)

Conclusion : sodium thiosulfate and hydrochloric acid

As the temperature increases, the time taken for the cross decreases as at higher temperatures, particles have more thermal/kinetic energy and move around more quickly which allows for more frequent, successful collisions

Calcium carbonate and hydrochloric acid practical

1. Measure 2g of marble chips and place them into a conical flask

2. Place measuring cylinder into water, and fill so there are no bubbles

3. Measure 25cm cubes of hydrochloric acid using measuring cylinder, prepare clock and add the acid and bung onto the conical flask

4. Stop the timer when 50cm cubed of carbon dioxide is given off and record

5. Repeat with different volumes of acid (but add water to ensure mass off water is same)

Conclusion : Calcium carbonate and hydrochloric acid [concentration and rate of reaction]

Increasing concentration increases the number of particles (concentration) per volume. More particles increases the frequency of successful collisions and so the rate of reaction also increases.

High Vs low pressure of gasses

At higher pressure, the particles are closer together

How does pressure (gases) affect rate of reaction

Higher pressure meant a more frequent, successful collisions which results in higher rate of reaction

Catalyst

A substance that can increase rate of a chemical reaction without being used up itself

Homogeneous catalyst

Catalysts in the same state as the reactants in a reaction

Heterogeneous catalyst

Catalysts in different state as the reactants in a reaction

How do catalysts work

Provide an alternative route for the reaction which involves a lower activation energy

Reaction profile diagram

• BLUE : uncatalysed, activation energy is higher

• RED : catalysed, requires much less energy

• Green → Products : ∆H (change in heat) is the same regardless of presence of catalyst

Effects of catalysts on the decomposition of hydrogen peroxide

1. Collect 5 test tubes and measure 10cm cubed of hydrogen peroxide into each tube

2. Add 0.25g of each type of metal catalyst/living enzyme (liver, potato, iron oxide, manganese dioxide, copper oxide)

3. Observe the speed at which bubbles are produced