# Rate of Reaction

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## Rate of Reaction

•  Rate of a chemical reaction is how fast reactants are changed into products
• On a graph, the line becomes less steep as the reactants are used up
•  Mean rate of reaction measured in g/s or cm3/s = quantity of reactant or product used in grams or cm3/ time taken in seconds
• Factors that affect the rate of a reaction:
•  Temperature is increased, the particles all move faster, collide more frequently have more energy so more of the collisions to make the reaction happen.
• More concentrated means more particles knocking about in the same volume of water.
•  The pressure of a gas is increased, means the same number of particles occupy a smaller space.
•  Makes collisions between reactant particles more frequent.
•    Increased surface area means more surface for particles to react, increasing the rate
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## Collision Theory

• Catalyst: A substance that speeds up the reaction without being used up and provides an alternative reaction pathway with a lower activation energy
•  Different catalysts are needed for a different reaction
• A catalyst decreases the activation energy, which is the minimum amount of energy required for the reaction to occur.
• Catalysts are usually transition metals
• Collision theory: Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
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## Equillibrium

• The forward reaction will be going at exactly the same rate as the backward one is when a system is at equilibrium.
• This means concentrations of reactants and products have reached a balance and won't change.
• A closed system, where equilibrium takes place means that none of the reactants and products can escape and nothing else can get in
• The equilibrium lies to the left if the concentration of products is greater than that of the reactants.
• If the equilibrium lies to the right, the concentration of reactants is greater than that of the products.
• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.
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## Le Chatelier's Principle

• Le Chatelier’s Principle is the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change
•  If the temperature of a system at equilibrium is increased:
• The relative amount of products at equilibrium increases for an endothermic reaction
• The relative amount of products at equilibrium decreases for an exothermic reaction
• If the temperature of a system at equilibrium is decreased:
• The relative amount of products at equilibrium decreases for an endothermic reaction
• The relative amount of products at equilibrium increases for an exothermic reaction.
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## Le Chatelier's Principle Continued

• If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
• If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.
• For gaseous reactions at equilibrium:
• An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction
• A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction
• Changing the pressure only affects equilibriums involving gases
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