Chem: Rates of Reaction (and reversible reactions)

What is the rate of a chemical reaction?

How fast reactants are changed into products.

What are examples of slow, moderate, and fast chemical reactions?

Slow: rusting of iron; Moderate: magnesium reacting with acid producing bubbles; Fast: burning/explosions.

How do steepness and flat lines on a rate graph indicate reaction speed?

Steeper line = faster rate; line becoming flat = reaction has finished.

How do lines on a rate graph change when a reaction is made faster using the same initial amounts of reactants?

The line becomes steeper initially, but flattens at the same height (same amount of product formed).

What causes a rate graph to show both a faster reaction and a higher total amount of product?

Adding more reactants at the start.

What two main factors determine the rate of a reaction according to collision theory?

1) Collision frequency (how often particles collide); 2) Energy transferred during a collision.

What is activation energy?

The minimum amount of energy that particles must have when they collide in order to react (break bonds and start the reaction).

What four key factors affect the rate of a chemical reaction?

Temperature, concentration of a solution (or pressure of a gas), surface area of a solid, and presence of a catalyst.

Why does increasing temperature increase the rate of reaction?

Particles move faster (increasing collision frequency) and collide with more energy, so a higher proportion of collisions have at least the activation energy.

Why does increasing concentration or gas pressure increase the reaction rate?

More reactant particles exist in the same volume, making collisions between particles more frequent.

Why does breaking a solid reactant into smaller pieces increase the rate of reaction?

It increases the surface area to volume ratio, exposing more solid particles to surrounding reactants and increasing collision frequency.

What is a catalyst?

A substance that increases the rate of a chemical reaction without being chemically changed or used up in the reaction.

Do catalysts change the products or overall yield of a chemical reaction?

No, the products and reaction equation remain identical; you only need a tiny amount to catalyse large amounts.

How do catalysts speed up chemical reactions?

They decrease the activation energy required by providing an alternative reaction pathway with a lower activation energy.

How is activation energy represented on a reaction profile graph?

As the height difference between the energy level of reactants and the highest peak on the curve.

How does adding a catalyst alter a reaction profile graph?

It lowers the peak of the curve, representing a lower activation energy for both exothermic and endothermic reactions.

What formula is used to calculate the mean rate of a chemical reaction?

Rate of Reaction = (Amount of reactant used OR amount of product formed) / Time

How can a precipitation reaction be used to measure the rate of reaction?

Mix two clear solutions in a flask over a marked cross on paper and time how long it takes for the precipitate to obscure the mark.

What is the main limitation of the precipitation method for measuring reaction rates?

The result is subjective because different people may disagree on the exact point the mark 'disappears'.

Describe how to measure reaction rate using a change in mass balance.

Place a flask containing the reaction on a mass balance as a gas is produced; as gas escapes, measure the decrease in mass over time.

What safety precautions are needed when using the mass balance method for gas-producing reactions?

If the gas produced is harmful or toxic, the experiment must be carried out in a fume cupboard.

Why is cotton wool placed in the neck of the flask during mass loss experiments?

It lets gas escape while preventing liquid spray or solid particles from spitting out.

Describe how to measure reaction rate by collecting the volume of gas given off.

Connect a gas syringe to the reaction flask and measure the volume of gas produced at regular time intervals.

What precaution must be taken when selecting a gas syringe for measuring rate?

Use the correct size syringe so that a vigorous reaction does not blow the plunger out of the end.

Describe the experimental setup to test how surface area affects rate using marble chips and HCl.

Add a set mass of marble chips to dilute HCl in a flask, connect to a gas syringe, measure volume of gas at regular time intervals, then repeat with more crunched chips and powdered chalk.

How do lines on a volume vs time graph show that finer particles increase reaction rate?

Finer particles have a larger surface area, so the curve is steeper at the start and flattens out sooner (reaction finishes faster).

How does adding a greater mass of small marble chips alter a rate-of-reaction graph?

It produces a steeper initial slope (faster rate) and a higher final line (more total gas evolved overall).

How do you carry out a practical to test how acid concentration affects reaction rate?

Repeat the marble chip experiment keeping the mass, surface area of chips, and total volume of acid constant, but vary the concentration of acid.

How does higher acid concentration affect the curve on a rate graph?

Higher concentration produces a steeper line that flattens out faster, showing a higher rate of reaction.

Describe the reaction between sodium thiosulfate and hydrochloric acid.

Both reactants are clear solutions that react together to form a yellow precipitate of sulfur.

How do you measure the reaction rate using sodium thiosulfate and hydrochloric acid?

Place the flask over a black cross, mix the solutions, and measure how long it takes for the cloudy sulfur precipitate to obscure the cross.

What safety precaution and heating method should be used when investigating temperature using the sodium thiosulfate reaction?

Do not heat acid directly; use a water bath to heat both solutions to the target temperature before mixing, and ensure liquid depth remains constant each time.

How does temperature affect the time taken for the black cross to disappear in the sodium thiosulfate experiment?

Higher temperatures result in a faster reaction, so it takes less time for the mark to disappear.

What chemical reaction is commonly used to test the effect of different catalysts on reaction rate?

The decomposition of hydrogen peroxide: 2H2O2 -> 2H2O + O2.

Which catalyst is typically used to speed up the decomposition of hydrogen peroxide, and what are two alternatives?

Manganese(IV) oxide (MnO2) is typical; copper(II) oxide (CuO) and zinc oxide (ZnO) are alternatives.

How do you measure the rate of hydrogen peroxide decomposition practically?

Measure the volume of oxygen gas produced at regular time intervals using a gas syringe connected to the flask.

How do better catalysts affect the gas volume vs time graph during hydrogen peroxide decomposition?

A better catalyst creates a steeper graph because the reaction is quicker, and the curve levels off faster.

What control variables must be kept constant when comparing different catalysts in hydrogen peroxide decomposition?

The volume and concentration of hydrogen peroxide, temperature, and the amount/mass of catalyst used.

What is a reversible reaction?

A reaction where the products can react with each other to convert back into the original reactants (can go both forwards and backwards).

Describe the forward and backward reactions for the thermal decomposition of ammonium chloride.

Forward (heating): White solid ammonium chloride decomposes into ammonia and hydrogen chloride gas. Backward (cooling): Ammonia and hydrogen chloride react to re-form ammonium chloride.

What is dynamic equilibrium and what condition is required for it to be reached?

A state reached in a closed system where forward and reverse reactions happen at the exact same rate, keeping reactant and product concentrations constant (overall effect is nil).

What is meant by a 'closed system' in chemistry?

A system where none of the reactants or products can escape into the surroundings.

What is the 'position of equilibrium'?

The relative amounts of reactants and products present in a reversible reaction mixture at equilibrium.

How does raising or lowering temperature affect the position of equilibrium in a reversible reaction?

Raising temperature shifts equilibrium towards the endothermic direction (to absorb heat); lowering temperature shifts it towards the exothermic direction (to release heat).

How does changing pressure affect equilibrium in gaseous reactions with different numbers of moles on each side?

Raising pressure shifts equilibrium towards the side with fewer moles of gas; lowering pressure shifts it towards the side with more moles of gas.

What effect does adding a catalyst have on the position of equilibrium?

It has no effect on the position of equilibrium because it speeds up both the forward and reverse reactions by the exact same amount.