GCSE Chemistry Unit 2 - Rates of reaction

Rates of Reaction, Measuring Rates of Reaction, Collision Theory, Catalysts,

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Rates of Reaction

Reactions have lots of different rates.

Reactions depend on several things;
- Temperature
- Concentration (liquids) or Pressure (gases)
- Catalyst
- Surface area

Graphs showing rate of reaction are often curved, with amount of product on the y-axis, and time on the x-axis. When the graph levels off, the reaction has ended. The steeper the slope, and the quicker it levels off, the faster the reaction. If more product is produced, then there were more reactants to start off with. 

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Measuring rates of reaction

Speed of a reaction is how quickly the reactants are used or how quickly the product is formed.
Rate of reaction = amount of reactant used or amount of product formed/time

Ways to measure reaction rates:

If the product is a precipitate, watch a marker through the solution. The quicker it takes for the marker to disappear, the faster the reaction.
Change in Mass;
If the reaction produces a gas, the mass will change, and can be measured on a mass balance. The quicker the mass drops, the faster the reaction.
Volume of Gas Given Off;
As the gas is given off, it is collected in a gas syringe and measured. The more gas given off in an interval, the faster the reaction. This can't be used if the reaction is very quick.

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Collision Theory

For a reaction to occur, particles have to collide, and they have to do so with enough energy. The more successful collisions per second there are, the faster the reaction.

The same things that increase reaction rate increase collisions;

Increased temperature - particles have more energy and move faster, increasing the number of collisions per second.
Higher Concentration(or Pressure) - there are more reactant particles, (or the particles are closer together), and so there are more collisions. 
Larger Surface Area - more space for the particles to work on, more collisions produced. 
Cataylsts - lower the activation energy, and so there are more successful collisions. 

Activation energy is the initial energy needed to break the initial bonds cause the particles to start colliding

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"A catalyst is a substance which changes the speed of a reaction, without being changed or used up in the reaction."

The activation energy is the lowest amount of energy needed for a reaction to occur.

Catalysts lower the activation. This makes it easier for the reaction to occur, and means a lower temperature can be used.

Catalysts work better with a larger surface area. Catalysts usually come in a powder, pellet or gauze. 

Catalysts reduce costs in industry because: they speed up reactions, increasing the amount of product made in a length of time; they lower the temperature, and the amount of fossil fuels burnt; they are not used up, so last for a long time.

Catalysts can be expensive, and often need to be cleaned. You normally need a different catalyst for each reaction, increasing the cost. 

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