Unit 6 - Chemical reactions Flashcards
Physical change
Doesn’t alter what the substance is, it only changes the appearance
Chemical change
When a chemical reaction changes what a substance is, so that the reactants are different from the products
Rate of reaction
The speed at which a reaction takes place
Stages of gradient change in a reaction
1.At the beginning the gradient is steep because many reactant particles are present so the rate is fast
2.The gradient starts to decrease - one of the reactant particles starts to get used up (limiting reagent)
3.At the end there are no more reactant particles so the reaction stops and the line plateaus
Rate of reaction equation to measure appearance of products
Rate = Volume of gas (cm^3) ÷ time (s)
Rate of reaction equation to measure disappearance of reactants
Rate = mass loss (g) ÷ time (s)
Factors that affect rate of reaction
- Concentration - More particles gives an increased chance of successful collisions
- Pressure - more particles in a smaller volume causes more collisions
- Surface area - reaction takes place on surfaces, hence more surface increases the rate of reaction - can be done by decreasing particle size
- Temperature - increased temperature leads to particles gaining more energy and moving faster, hence more collisions
- Use of a catalyst - speed up rates of reactions by lowering activation energy
Collision theory for successful chemical reactions
- Particles must collide
- Particles must collide with correct orientation (head-on)
- Particles must collide with sufficient kinetic energy
Relationship between concentration and rate of reaction
Directly proportional
Dynamic equilibrium
When the rate of the forward reaction is equal to the rate of reverse action and at this point there is no change in the proportion of reactants or products
Conditions for reversible reactions to be at equilibrium
- Rate of the forwards reaction equals the rate of backward reaction
- The concentrations of reactants and products remain constant
3, Reaction occurs in a closed system - no exchange of substances with the surroundings
Effect of changing concentration on equilibrium
- If reactant concentration increases equilibrium shifts to remove the extra reactant by increasing the yield of product
- If product concentration increases equilibrium shifts to remove extra product by decreasing yield of product
Effect of changing pressure on equilibrium
- If pressure is increased equilibrium shifts to the side with fewer moles to reduce overall pressure
- If pressure is decreased equilibrium shifts to the side with more moles to increase overall pressure
Le Châtelier’s principle
When a change is made on a reversible reaction at chemical equilibrium, then the equilibrium moves in the direction that will oppose that change and bring the system back to equilibrium
Effect of changing temperature on equilibrium when forward reaction is exothermic and reverse is endothermic
- Temperature increase leads to equilibrium shift to the endothermic reaction to counteract temperature increase by having less product form
- Temperature decrease leads to equilibrium shifting to exothermic to result in a higher yield of products
