Standard 8: Reaction Rate Chemistry Ms. Siddall

Reaction rate = speed of reaction Example An explosion happens quickly Rusting happens slowly 8a: Measuring reaction rate Reaction rate = change in concentration time ∆[reactants]or ∆[products] time time ∆ = change [ ] = concentration

Summary 1 What is the rate of reaction when 5 moles of reactant is used in 10 seconds?

Reversible reaction: X  Y [X] [Y] time X  Y Y  X time rate concentration As concentration decreases - Rate decreases As concentration increases - Rate increases When concentration is constant - Rate is constant

Summary 2 Describe how rate changes as concentration changes.

Reaction mechanics A reaction can only occur if: 1.Molecules collide 2.Molecules collide with enough energy 3.Molecules collide with the correct orientation Molecules must hit hard enough in just the right place

Summary 3 What are the conditions at the molecular level for a reaction to occur?

There are 4 factors that affect the rate of a reaction: 1.Temperature: Increasing temperature = more energy = more collisions = more reactions 8b: Factors affecting reaction rate

Summary 4 Why do changes in temperature affect reaction rate?

2.Concentration: Increasing concentration = more particles = more collisions = more reactions low concentrationhigh concentration

Summary 5 What happens to the number of collisions when there is an increase in concentration?

3.Surface Area: Increasing surface area = more places to collide = more collisions = more reactions Reaction occurs only at the surface More surface area = More chance to react

Summary 6 1. Which has more surface area? A 1g sugar cube or 1g of granulated sugar 2. Which would react faster?

4.Pressure (gases only): Increasing pressure = particles closer together = more collisions = more reactions Increased pressure forces particles together low pressurehigher pressure

Summary 7 1. How would you increase the rate of a gas reaction? 2. Why does increasing pressure NOT help increase the rate of a liquid reaction?

energy time reactants products Activation energy Reaction pathway Reactions are not always spontaneous Activation energy = energy needed for a reaction to happen

Summary 8 Paper burns in the presence of oxygen. Explain why this paper does not burst into flames.

Catalyst A catalyst increases the rate of a reaction by lowering ‘activation energy’ A catalyst is not used up in the reaction Example: a catalytic converter allows CO to react more easily with O 2 to make CO 2 standard 8c: catalyst Reaction pathway copy diagram on page 547

Summary 9 1.In terms of ‘activation energy’, how does a catalyst increase the rate of reaction? 2.Does a catalyst affect the energy of reactants or products?

Standard 8d: the definition and role of activation energy in a chemical reaction. Reactants must form an ‘activated transition complex’ This requires energy = activation energy Honors.

energy time reactants products Reaction with catalyst Activated transition complex

Summary Which has a higher energy: Reactants or Products? 2. How does the catalyst affect the energy of the activated transition complex?