Chemical Changes And Structure #1: Rates Of Reaction
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Slow reactions
Large particles (lumps), low concentration (dilution), low temperature
Fast reactions
Small particles (powder), high concentration, high temperature
How do you determine what factor is affecting a chemical reaction?
You must only change one variable at a time
What is a variable?
Something that can be changed in a chemical reaction
What are some variables?
• temperature • particle size • concentration • volume • mass
How do you carry out a fair test?
By only changing one variable at a time
What must happen for a chemical reaction to occur?
The reactants particles must collide
How do collisions work?
The collision provides the energy to break the bonds in the reactant molecules and form the new bonds in the product
Not all collisions result in…
• A chemical reaction • Those that do are said to be successful collisions
What makes a successful collision?
The particles must collide with: - the correct geometry (the right angle) - sufficient energy
Any factor which increases the number of collisions between reactant particles will also increase…
The rate of reaction
What is the effect of particle size on collision theory?
• As the particle size decreases, the surface area increases • As the surface area increases, the area available for collisions to happen increases • This increases the rate of reaction
What is the effect of concentration on collision theory?
• As the concentration of reactants increases, there is more particles and so more collisions • Rate of reaction increases
What is the effect of temperature on collision theory?
As the temperature increases: - the amount of energy a particle has increases - the faster the particle moves around - the more likely particles are to collide the rate of reaction increases
All chemical reactions produce…
New substances
How can the speed of reaction be measured?
By finding how fast the new substance is formed
What are 2 ways we can measure the rate of reaction?
• Measuring a change in mass • Measuring a change in volume
How can we interpret graphs?
• Reactions with more collisions (high conc., high temp., larger surface area / powder) will go to completion/ plateau earlier • This is because, in these cases, the reactants are used up quicker because there’s more collisions
Labelling parts of a graph
A: Reaction rate is fastest. Concentration of reactants is highest. B: Reaction rate decreases. Concentration decreasing. Reactants being used up. C: Reaction is finished. All reactants have been converted into products.
How do we calculate the average rate of reaction?
From knowing the initial and final quantities and the time interval
Average Rate formula
Average rate = change in quantity / change in time
What is a catalyst?
• A substance which speeds up the rate of reaction • Can be recovered chemically unchanged
What are 4 ways to speed up a chemical reaction?
• Increasing concentration • Increasing temperature • Increasing particle size • Addition of a catalyst
Progress of a chemical reaction means
Rate of reaction