How do you read a potential energy diagram?

The activation energy is the climb from the reactants to the peak (the activated complex). The heat of reaction is $\Delta H = \text{PE}_{\text{products}} - \text{PE}_{\text{reactants}}$: products lower than reactants is exothermic (negative $\Delta H$), products higher is endothermic (positive $\Delta H$). A catalyst lowers the activation energy (a lower peak) but leaves $\Delta H$ unchanged because the reactant and product energies do not move.

What is Le Chatelier's principle?

When a system at equilibrium is stressed by a change in concentration, temperature or pressure, it shifts to partially relieve the stress. Adding a substance shifts away from it; removing one shifts toward it. Adding heat shifts an exothermic reaction toward reactants. Increasing the pressure on a gaseous equilibrium shifts it toward the side with fewer gas molecules. Equilibrium itself is dynamic: equal forward and reverse rates with constant concentrations.

What are Arrhenius acids and bases, and what does pH measure?

An Arrhenius acid produces hydrogen ions ($\text{H}^+$) as the only positive ion in aqueous solution; an Arrhenius base produces hydroxide ions ($\text{OH}^-$) as the only negative ion. The pH scale runs $0$ to $14$: below $7$ acidic, $7$ neutral, above $7$ basic. Each pH unit is a tenfold change in hydrogen ion concentration, so a difference of three pH units is a factor of one thousand.

How does a titration calculation work?

A titration reacts a measured volume of one solution with a solution of known concentration until the indicator marks the endpoint (neutralization). For a one-to-one acid-base reaction, Table T gives $M_A V_A = M_B V_B$. Substitute the three known values and solve for the unknown molarity. Volumes can stay in milliliters because they appear on both sides and cancel, but molarity is in mol/L.

New YorkChemistry

Regents Chemistry kinetics, equilibrium and acid-base chemistry: a complete skills guide to rates, energy diagrams, Le Chatelier, pH and titration

Q: What must happen for a reaction to occur according to collision theory?

Reactant particles must collide with enough energy (at least the activation energy) and in the correct orientation; such collisions are effective and lead to a reaction. The rate increases when effective collisions become more frequent. Higher concentration, higher temperature, greater surface area of a solid, and a catalyst all increase the rate, with temperature acting most strongly because it increases both collision frequency and the fraction of energetic collisions.

A deep-dive Regents Chemistry guide to kinetics, equilibrium and acid-base chemistry: collision theory and rate factors, potential energy diagrams, dynamic equilibrium and Le Chatelier's principle, Arrhenius acids and bases with the pH scale, neutralization and salts, and titration calculations, with the Reference Tables and exam technique.

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Jump to a section

Why these topics are grouped
Reaction rates and collision theory
Potential energy diagrams
Equilibrium and Le Chatelier's principle
Acids, bases and pH
Neutralization and titration
Check your knowledge

Why these topics are grouped

Kinetics (how fast), equilibrium (how far) and acid-base chemistry (a major reaction type) are closely linked: collision theory and activation energy explain rate; the same energy ideas underpin equilibrium; and acids and bases are where titration and neutralization put concentration to work. The Regents draws steadily on this group across Parts A, B-1, B-2 and C. This guide ties together the matching dot-point pages, each with its own practice: reaction rates and collision theory, potential energy diagrams, equilibrium and Le Chatelier's principle, acids, bases and the pH scale, neutralization and salts, and titration.

Reaction rates and collision theory

A reaction occurs when particles collide with enough energy (the activation energy) and the right orientation. The rate rises when effective collisions become more frequent: higher concentration, higher temperature, greater surface area of a solid, and a catalyst all increase the rate. Temperature is the strongest lever because it both raises collision frequency and increases the fraction of collisions with enough energy. A catalyst lowers the activation energy and is not consumed.

Potential energy diagrams

These plot potential energy against reaction progress. The activation energy is the climb to the peak (the activated complex); the heat of reaction is products minus reactants. Products lower than reactants is exothermic ( $\Delta H$ negative); products higher is endothermic ( $\Delta H$ positive). A catalyst lowers the peak but not $\Delta H$ .

Equilibrium and Le Chatelier's principle

Dynamic equilibrium is equal forward and reverse rates, with constant concentrations. Le Chatelier's principle predicts shifts under stress:

Stress	Shift
Add a reactant or product	away from the added substance
Remove a substance	toward the removed substance
Add heat (exothermic forward)	toward reactants
Increase pressure (gases)	toward the side with fewer gas molecules

Acids, bases and pH

An Arrhenius acid produces $\text{H}^+$ ; an Arrhenius base produces $\text{OH}^-$ . The pH scale runs $0$ to $14$ : below $7$ acidic, $7$ neutral, above $7$ basic. Each pH unit is a tenfold change in $\text{H}^+$ , so three units is a factor of one thousand. Table K lists common acids and Table L common bases.

Neutralization and titration

Neutralization is acid $+$ base $\rightarrow$ salt $+$ water: $\text{H}^+$ and $\text{OH}^-$ form water, while the acid's anion and base's cation form the salt. A titration uses this to find an unknown concentration; for a one-to-one reaction, Table T gives $M_A V_A = M_B V_B$ .

Collision theory says reactions need collisions with enough energy and the right orientation; concentration, temperature, surface area and a catalyst raise the rate by making effective collisions more frequent or more energetic. Potential energy diagrams show activation energy (the climb to the activated complex) and the heat of reaction (products minus reactants), with a catalyst lowering the activation energy but not $\Delta H$ . Dynamic equilibrium is equal forward and reverse rates, and Le Chatelier's principle predicts shifts to relieve a stress in concentration, temperature or pressure. Arrhenius acids give $\text{H}^+$ and bases give $\text{OH}^-$ ; pH runs $0$ to $14$ with each unit a tenfold change in $\text{H}^+$ . Neutralization gives a salt and water, and titration uses $M_A V_A = M_B V_B$ for a one-to-one reaction.

Check your knowledge

Attempt these under timed conditions, then check the solutions.

State two factors that increase the rate of a reaction, and explain one using collision theory. (2 marks)
On a potential energy diagram, state how to find the heat of reaction. (1 mark)
For the exothermic equilibrium $\text{A}(g) + \text{B}(g) \rightleftharpoons \text{C}(g)$ , state the shift when the temperature is raised. (1 mark)
State the defining ion of an Arrhenius acid and of an Arrhenius base. (2 marks)
How many times more acidic is a solution of pH 2 than one of pH 5? (1 mark)
A $20.0$ mL acid is neutralized by $10.0$ mL of $0.40$ M base (one-to-one). Find the acid molarity. (2 marks)

Solutions

Q1: Any two of: higher concentration, higher temperature, greater surface area, a catalyst. Example: higher concentration puts more particles in a given volume, so effective collisions are more frequent and the rate increases.
Q2: Subtract the potential energy of the reactants from that of the products: $\Delta H = \text{PE}_{\text{products}} - \text{PE}_{\text{reactants}}$ .
Q3: The forward reaction is exothermic (heat is a product), so adding heat shifts the equilibrium toward the reactants.
Q4: Arrhenius acid: hydrogen ion, $\text{H}^+$ . Arrhenius base: hydroxide ion, $\text{OH}^-$ .
Q5: The difference is $3$ pH units, so $10^3 = 1000$ times more acidic.
Q6: $M_A = \dfrac{(0.40)(10.0)}{20.0} = 0.20$ M.

Sources & how we know this

Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)