[UNIT 4] CHEM - Conditions

Explain the difference between an ‘open system’, ‘closed system’ and ‘isolated system’

· Explain what is meant by 'collision theory’

· Understand what ‘activation energy’ is and refer to it in a chemical reaction.

· Calculate the rate of reaction using the change in amount of reactant and the time of the reaction.

· Analyze rate of reaction graphs

Determine the rate of reaction when a gas is produced in an open system (idea of conservation of mass)

· Explain how changing the temperature of a chemical system affects the rate of reaction.

· Explain how changing the concentration of a reactant within a chemical system affects the rate of reaction

· Explain how changing the surface area of a reactant within a chemical system affects the rate of reaction

· Explain the role of a catalyst in a chemical reaction.

L1 - System

Describe the difference between different types of chemical systems

What is a chemical system:

  • ​​Chemical system: chemical reaction being studied

  • Chemical reactions undergo changes constantly throughout a reaction

  • All matter external to the system: surroundings


There are three types of systems:

Determine what a chemical reaction needs to take place

  • BOND BREAKING = ENERGY (Endothermic)

  • BONDS FORMING = ENERGY (Exothermic)

  • Signs a chemical reaction has taken place:

    • Change in Temperature

    • Change in Color

    • Odor (smell) (after reaction has begun)

    • Formation of a Precipitate (solid)

    • Formation of Bubbles


  • Describe how a chemical reaction forms new substances

    • Chemical reactions happen when chemical bonds are broken

    • This happens by atoms/molecules colliding into each other with enough activation energy.

    • The atoms can form new bonds with different atoms.

    • A new substance (product) forms.


  • chemical reaction: chemical change

    • (example: burning wood)

  • physical reaction: physical change of state but not a chemical change (stays the same substance).

    • (Example: Freezing water)

L2 - Collision theory and rate of reaction (RoR)

Understand what is meant by the rate of a reaction

  • The rate of a reaction is how fast a reaction happens

    • How quickly the reactants are turned into products


Rusting

Baking

Explosion

Slow

Fast

Very fast

Describe the conditions needed for a successful collision to occur

The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction.


The rate of a reaction depends on two things: 

  • the frequency of collisions between particles

  • the energy with which particles collide


If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.


Example:

  • Paper burns

    • Paper + oxygen → carbon dioxide + water + nitrogen

  • The paper in this room isn’t burning.

    • It doesn’t have enough energy to burn.

    • If we make it hotter it will catch fire.

      • Paper burns on its own at 250 ºC

Explain how reaction rates can be measured

There are two common ways:

  • Measure how fast the products are formed

  • Measure how fast the reactants are used up

Analyse information on a graph to determine the rate of the reaction at different time points during the reaction

  • Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop.

  • As the reaction progresses, the concentration of reactants decreases.

  • This reduces the frequency of collisions between particles and so the reaction slows down.



How can RoR be calculated from a graph?


Practice questions


Answers:

L3 - Rate of reaction (RoR) graphs

  • Plot a rate of reaction graph using data given 

  • Draw tangents on the curve of the rate of reaction graph and measure the rate of reaction using the slope

  • Calculate initial rate of reaction from a graph

What conclusion can you draw about the progress of the reaction based on the shape of the curve?

  • The initial rate from 0 to 2 minutes is the fastest rate of reaction as this is the steepest part of the curve. (1)

  • As time progresses, the steepness of the curve decreases and therefore the rate of reaction is decreasing. (1)

L4 - Temp and surface area’s affect on the rate of reaction

Explain how temperature change of a system affects the rate of reaction

  • Increase in temperature (of the system) = molecules gain kinetic energy

  • Increase in kinetic energy = molecules move more → more successful collisions

  • Increasing temperature = gives molecules enough energy to overcome the activation energy

Explain how the surface area of a reactant affects the rate of reaction

  • Increase in surface area = increase the chance of more successful collisions

  • Increase surface area = increase in the number of particles involved in the reaction

Explain how the concentration of a reactant affects the rate of reaction

  • more particles will be present in the reaction, leading to more successful collisions

L5 - Catalysts

Define what a ‘catalyst’ is in a chemical reaction

  • A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction

  • When the reaction has finished, you would have exactly the same mass of catalyst as you had at the beginning

Explain how catalysts affect the rate of a reaction

  • What does the catalyst do the activation energy?

    • Lowers activation energy

  • What affect does this have on the number of molecules which have enough energy to react?

    • More molecules have enough energy to react

  • How does this affect the rate of reaction?

    • More molecules colliding successfully = increase in rate of reaction


Fact

Reason

Model

Reaction rate increases when a catalyst is used

Catalysts lower the activation energy of a reaction, allowing reactants to form products with lesser required heat energy input than without a catalyst


People dancing at a party are more likely to bump into each other than people sitting down

Reaction rate increases when temperature is increased

If particles move faster due to an increased kinetic energy, there is a higher probability of effective collision resulting in the formation of products

If you have 100 people, they are more likely to bump into each other if in this classroom compared to the field

Reaction rate increases when pressure is increased

When pressure increases, volume decreases and there is more chance of a successful collision resulting in reactants being turned to products

If you lower the bar in a high jump competition, more people can get over it.

Reactions go faster when the surface area is increased

If there are more reacting particles in a given volume, then there is a greater chance of effective collision

A lot of children in a room are more likely to bump into each other than a few adults

Rate of reaction increases when particle size gets smaller

Small particles have a greater surface area than the same amount of larger particles. This means there are more particles exposed so more collisions can happen

Chips fry faster than potatoes because the oil can cover a bigger area  

Reactions speed up when the reactants are more concentrated

If there are more reacting particles in a given volume, then there is a greater chance of effective collision

More dodgems on the track means that you are more likely to bump into someone than if there were only 2 or 3